The Lone Ranger
05-22-2011, 12:29 AM
On the Anatomy and Physiology of Sight
It seemed to me that the anatomy and physiology of how we see might be a fun topic to discuss. Please note, though, that this is only a very, very cursory examination of what we know and understand about how vision works. If I were to go into detail, it would quite literally fill a textbook or two.
With your permission, I'll start out with the basics and work up. So please forgive me if it seems a bit slow at first.
[b]The Nature of Atoms and Ions:
Let's start with the atom. I'm sure everyone knows that atoms are the fundamental unit of which normal matter is constructed. That most-definitely includes the human body. Further, I'm sure everyone understands that an atom has three basic components.
Every atom contains one or more positively-charged protons. (Please don't confuse protons with photons; photons are "particles" of light.)
Most atoms also contain one or more uncharged neutrons. The protons and neutrons are bound together in the central region of the atom, known as the nucleus. Surrounding the nucleus are negatively-charged electrons. The electrons essentially "orbit" the nucleus, kind of like tiny planets orbiting the Sun. The electrons are distributed in what are called orbitals or shells, and it's important to keep in mind that each orbital can hold only so many electrons.
Every atom, by definition, has equal numbers of positively-charged protons and negatively-charged electrons, and therefore has no net electrical charge.
http://www.freethought-forum.com/images/anatomy2/carbon_atom.gif
A carbon atom.
Note that every atom has equal numbers of protons and
electrons by definition. The number of neutrons need not
be equal to that of the number of protons/electrons, however.
Under certain conditions, an atom can either gain or lose electrons and thus acquire an electrical charge. Since an atom has no net electrical charge by definition, an electrically-charged atom is no longer known as an "atom"; it is instead known as an ion.
If an atom gains an electron, it acquires a negative charge. This is necessarily so because each electron has a charge of -1 and each proton has a charge of +1. So if the "atom" has more electrons than protons, it will have a net negative charge. That is, if the ion has one more electron than it has protons, it will have a charge of -1. If it has two more electrons than protons, it will have a charge of -2. And so on. A negatively-charged ion is known as an anion.
If an atom loses an electron, it acquires a positive charge, since it now has more positively-charged protons than negatively-charged electrons. A positively-charged ion is known as a cation.
[b]Chemical Bonds:
Under normal circumstances, the electrons in the outermost orbital are the only part of an atom that can interact with other atoms. And as mentioned previously, each orbital can hold only so many electrons. The first orbital can hold up to 2 electrons, but no more. In atoms that have more than 2 electrons, a second orbital exists, which can hold up to 8 electrons. In atoms that have more than 10 electrons, a third orbital exists. In most atoms that are relevant to living things, the third orbital, like the second, can hold up to 8 electrons.
In their interactions with other atoms, atoms typically configure themselves so that their outermost orbitals are full. They can do this either by exchanging electrons or by sharing electrons. The electrons in the outermost orbital are, as mentioned, the only part of an atom that normally interacts with another atom. These electrons are known as the valence electrons, because they determine the valence of the atom -- that is, how many other atoms it can form chemical bonds with.
When atoms come together and chemically bond (by either exchanging or sharing electrons), they form molecules. In ionic bonding, one of the atoms actually takes one or more electrons away from the other. The atom that takes an electron becomes a negatively-charged anion, and the atom that loses an electron becomes a positively-charged cation. (Hence the term "ionic bonding," since the atoms exchange electrons and form ions in the process.) The ions then stick together because of their opposite charges.
http://www.freethought-forum.com/forum/gallery/files/5/0/ionic_bonding_original.jpg
Ionic Bonding to form a Molecule of Sodium Chloride.
After the atoms exchange an electron, the resulting ions are held together by their opposite charges.
Note that each atom fills its outermost orbital when the two of them exchange an electron.
Another way in which atoms can form chemical bonds is by coming together and merging their outermost orbitals, and thus sharing electrons. This is known as covalent bonding.
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Covalent bonding between 4 hydrogen atoms and a single carbon atom to form a molecule of methane.
Note that each atom fills its outermost orbital by sharing electrons with one or more other atoms.
The carbon atom "gains" four electrons and thus fills its outermost orbital by sharing a pair of electrons with each of the four hydrogen atoms.
Each hydrogen atom "gains" a single electron and thus fills its outermost orbital by sharing a pair of electrons with the carbon atom.
Now then, as we've seen, the subunits within an ionic-bonded substance are electrically charged by definition. The subunits within a covalently-bonded substance may or may not have electrical charges, however.
[b]Polar and Nonpolar Covalent Bonds:
In some cases, the atoms within a covalently-bonded molecule share their electrons equally. In that case, the charges balance out, and the molecule has no net electrical charge. This is known as a nonpolar molecule.
But in some covalently-bonded substances, electrons are not shared equally. Because one or more of the atoms in the molecule "hoards" electrons, the charges are unbalanced. In effect, the electrons tend to "cluster" on one side of the molecule. This means that one end of the molecule has a slight negative charge (where the electrons tend to cluster) and the other side has a slight positive charge. This is a polar molecule. Water is a good example of a polar molecule.
http://www.freethought-forum.com/forum/gallery/files/5/0/polar.jpg
Water is a polar molecule because the oxygen atom and the two hydrogen atoms do not share their electrons equally.
The oxygen, in effect, "hoards" the electrons. The “δ” symbol indicates a partial charge.
http://www.freethought-forum.com/forum/gallery/files/5/0/c02water_original.jpg
A molecule of carbon dioxide is about the same size as a molecule of water, but has very different properties.
Because the valence electrons are shared equally in a molecule of carbon dioxide, carbon dioxide is a non-polar substance.
In a molecule of water, electrons are not shared equally, so the molecule is polar.
[b]Hydrogen Bonding between Polar Molecules:
Because each polar molecule has a (slightly) positive end and a (slightly) negative end, polar molecules tend to be attracted to each other. That is, the partially-positive end of one polar molecule is attracted to the partially-negative end of another polar molecule. The weak bonds that thus form between polar molecules are known as hydrogen bonds.
http://www.freethought-forum.com/forum/gallery/files/5/0/hydrogen_bonds_244306_original.jpg
Hydrogen bonding between water molecules.
Because of their charges (either partial in the case of polar molecules or complete in the case of ions), most polar and ionic substances will easily dissolve into water. Such substances are therefore said to be hydrophilic (literally, "water-loving").
Because they won't form hydrogen bonds with polar molecules, non-polar molecules typically will not dissolve into water. They are therefore said to be hydrophobic (literally, "water-fearing"). This is why most oils, for example, will not dissolve into water -- the molecules that make up the oil are non-polar and thus hydrophobic, and so cannot form hydrogen bonds with the water molecules.
http://www.freethought-forum.com/forum/gallery/files/5/0/saltwater_original.jpg
Sodium chloride (table salt) dissolves so well in water because the salt is ionic.
The sodium cations are attracted to the partially-negative regions of the water molecules
and the chloride anions are attracted to the partially-positive regions of the water molecules.
[b]Amphipathic Molecules and Phospholipid Bilayers:
Some larger molecules have regions that are hydrophilic (and will thus dissolve into water) and other regions that are hydrophobic (and thus won't). A good example is a phospholipid molecule. Such a molecule is said to be amphipathic.
A phospholipid molecule has a "head" that is strongly polar (hydrophilic) and so is attracted to water molecules. But it also has 2 nonpolar (hydrophobic) "tails" that are, in effect, repelled by water.
When placed into water, phospholipid molecules will therefore spontaneously organize into a bilayer. The hydrophilic "heads" face outward, toward the water, and the hydrophobic "tails" face inward, away from the water.
You'll doubtless have noticed that this means the central region of a phospholipid bilayer is hydrophobic. Thus most polar and ionic substances cannot cross phospholipid bilayers, because they can't pass through the hydrophobic interior.
http://www.freethought-forum.com/images/anatomy2/phospholipid.gif
A phospholipid molecule. Note the polar (hydrophilic) "head" and the nonpolar (hydrophobic) "tails."
http://www.freethought-forum.com/images/anatomy2/phospholipid_bilayer.jpg
A Phospholipid Bilayer.
When placed into water, phospholipid molecules spontaneously organize into a bilayer, with the hydrophilic "heads"
facing outward, toward the surrounding water and the hydrophobic "tails" facing inward, away from the water.
[b]Cells and Cellular Membranes:
The basic subunits of living things are cells, of course. And every cell is surrounded and held together by a plasma membrane. The plasma membrane is primarily made up of a phospholipid bilayer. Molecules of cholesterol are embedded in the membrane, and help to strengthen it. There are also various proteins embedded in the membrane. The proteins serve various functions, but a very important one is that they help determine what substances can cross the membrane, and under what circumstances.
http://www.freethought-forum.com/forum/gallery/files/5/0/cell.jpg
A generalized animal cell.
Cells contain genetic material and cytoplasm, and are enclosed by a plasma membrane.
http://www.freethought-forum.com/forum/gallery/files/5/0/plasma_membrane_original.jpg
The plasma membrane is primarily a phospholipid bilayer.
Cholesterol molecules help to strengthen the plasma membrane.
Protein molecules embedded in the membrane are essential in determining what can and cannot cross the membrane.
It's essential to keep in mind that the interior of the plasma membrane is hydrophobic. Therefore, most polar and ionic substances cannot cross the plasma membrane of their own accord. Some small ions and polar molecules (such as water, for example) can cross the plasma membrane because they're small-enough to slip between the relatively large phospholipid molecules, though. We'll get back to that point shortly.
[b]Diffusion:
Because of their thermal energy, molecules and ions, when they're free to move, will tend to move from where they're more concentrated to where they're less concentrated. This phenomenon is known as diffusion.
For example, if the concentration of a substance on the outside of a cell is high and the concentration on the inside of the cell is low, molecules will tend to spontaneously move into the cell from the outside. If they can cross the membrane, that is.
The molecules are moving randomly. To understand why they spontaneously move from high concentration to low, imagine that the concentration of molecules on the outside of the cell is 10 times greater than the concentration on the inside of the cell. This means that for every molecule that just happens to move from the inside of the cell to the outside of the cell, ten will be moving into the cell from the outside. So the net movement of molecules is into the cell.
This will continue until a state of equilibrium is reached and the concentration on both sides of the plasma membrane is equal. It's not that the molecules stop moving: it's just that when the concentration is equal on both sides of the membrane, for every molecule that happens to move out of the cell there will be, on average, one molecule moving into the cell. So the net movement is zero.
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Diffusion across a cellular membrane.
When the concentrations on both sides of the membrane are equal, the net molecular motion is zero.
(Hit "refresh" to see the animation.)
[b]Moving Substances Across Cellular Membranes:
Now then, it surely won't surprise you to learn that cells have various mechanisms by which they can move substances across their membranes -- even substances that wouldn't normally be able to cross those membranes. Most of these mechanisms involve those proteins embedded in the membrane.
If the molecules of a substance are hydrophobic (such as most lipid molecules, for instance), they can diffuse through the membrane on their own, since they are not repelled by the hydrophobic membrane interior. Small molecules (like oxygen, for example) can also diffuse across most membranes on their own because they can slip between the phospholipid molecules. When substances diffuse into and out of cells on their own, because they can cross the plasma membrane unhindered, this is known as simple diffusion.
You'll note that the cell does not expend any of its own energy in simple diffusion, since the molecules in question are moving of their own accord.
Some of the proteins in the plasma membrane can temporarily bond to substances and transport them across the membrane from high concentration to low. The proteins that do this are called carrier proteins, since they carry substances across plasma membranes. In this way, carrier proteins allow certain hydrophilic substances to cross plasma membranes, even though those substances would ordinarily be unable to do so. If the carrier proteins allow the substance in question to cross the membrane from high concentration to low, the cell does not have to expend any energy of its own in the process (since the energy is supplied by the moving molecules). This is known as carrier-mediated facilitated diffusion.
In some cases, carrier proteins can actually pull substances across the membrane from low concentration to high. The cell must expend energy in order to do this, however, since it is moving molecules in the opposite direction that they're "trying" to go. This is known as active transport.
Some of the proteins embedded in the plasma membrane of a cell are hollow, and penetrate all the way through. They thus form channels that hydrophilic molecules can pass through to enter or exit the cell. Naturally, these proteins are called channel proteins.
One especially important thing about channel proteins is that some of them can be opened and closed, in order to adjust the movement of specific substances across the plasma membrane, as necessary. These are known as gated channels.
Consider, for example, what will happen if a cell actively transports a hydrophilic substance outside, so that its concentration is greater outside the cell than inside. (It will have to use carrier proteins to do this, of course.) If there are channel proteins in the membrane of the cell through which this substance can pass but they're closed, the cell can make the concentration of the substance in question much higher on the outside of the cell than on the inside. And so long as the gated channels remain closed, this state will persist. But as soon as those gated channels open, there will be a massive rush of the substance into the cell.
Water is a special case, by the way. Water molecules are small-enough that they can "slip between the cracks." Water can therefore diffuse across plasma membranes even though water is (obviously) hydrophilic. But there are gated channels in the membrane called aquaporins. By opening and closing aquaporins, the cell can speed up or slow down the movement of water across its plasma membrane, as necessary. Diffusion of water across a plasma membrane is called osmosis.
http://www.freethought-forum.com/forum/gallery/files/5/0/diffusion_original.jpg
Some of the ways that substances can cross plasma membranes.
Note that none of these illustrated methods require the cell to expend energy.
By the same token, none of these methods can be used to create a difference in concentration on the two sides of the membrane.
The cell can use its carrier proteins to transport substances from low concentration to high and so can create a difference in concentration
on the two sides of the membrane, but it must expend energy to do so.
Some cells, using active transport and gated channels, can move ions across their membranes and so create a situation where the electrical charges on the opposite sides of the membrane are different. When a cell does this, and so has different electrical charges on the two sides of its plasma membrane, it is said to be polarized.
Obviously, a cell that is polarized is in a highly unstable state. But so long as the gated channels remain closed, the ions cannot re-cross the membrane and so the cell will remain polarized. Of course, as soon as the gated channels open, ions will rush across the membrane until the charges on both sides are equalized. When that happens, we say that the cell has depolarized.
[b]Opening and Closing Gated Channel Proteins:
What causes gated ion channels to open or close?
Some are chemically gated ion channels. In this type of channel protein, a region of the protein is known as the receptor. Certain molecules (called ligands) can bond to that receptor. When the appropriate ligand bonds to the receptor, the protein changes shape, causing the channel to open. Two common classes of ligands are neurotransmitters and hormones.
Some ion channels are voltage gated ion channels. They open and close not in response to ligands, but in response to changes in the voltage of the nearby plasma membrane.
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Two types of gated ion channels.
[b]Neurons:
Now let us consider a neuron. A neuron is a highly specialized cell because it can generate and rapidly conduct electrochemical impulses. It can do this because it has specialized proteins in its plasma membrane that can actively transport ions -- especially sodium (Na+) and potassium (K+) ions -- across the membrane. In other words, it can be polarized.
The bulk of a typical neuron is in its central region, called the cell body or soma. Projecting out from the neuron are long, thin projections known as the neural fibers. At one end of a typical neuron are many branching fibers. These are known as the dendrites. A neuron may have as many as 10,000 dendrites. At the other end, there is a single axon. No neuron has more than one axon.
Under normal conditions, a neuron will conduct impulses in only one direction. Dendrites carry impulses toward the cell body, and the axon carries impulses away from the cell body. This is an important factor in how the nervous system works: the nervous system would be a chaotic, function-less mess if neurons conducted impulses in both directions.
Outside of the brain and spinal cord, the axons of neurons are bundled together to form organs called nerves. A neuron that is carrying impulses from a sensory receptor toward the Central Nervous System (that is, the brain or spinal cord) is known as an afferent (or sensory) neuron. A nerve that contains only afferent neurons can therefore conduct impulses only to (not from) the CNS and is known as an afferent or sensory nerve.
A neuron that is carrying impulses from the CNS to an effector (usually a gland or a muscle) is known as an efferent (or motor) neuron. Naturally, a nerve that contains only efferent fibers is known as an efferent (motor) nerve.
Some nerves contain both afferent fibers going to the CNS and efferent fibers coming from the CNS. These are known as mixed nerves.
Nerves that contain only afferent fibers include the olfactory nerve (responsible for the sense of smell) and the optic nerve (responsible for the sense of sight). For comparison, the oculomotor nerve (controls some of the muscles that move the eyes) contains mostly efferent fibers, though it has some afferent fibers as well. All of the nerves that originate in the spinal cord are mixed nerves.
http://www.freethought-forum.com/forum/gallery/files/5/0/neuron_original.jpg
A typical neuron.
Dendrites carry impulses toward the cell body, and the single axon carries impulses away from the cell body.
[b]How a Neuron is Polarized:
Now then: back to neurons.
When it's at rest, a neuron actively transports ions across its membrane. Specifically, it uses transport proteins in its membrane called the Sodium-Potassium pump to transport ions. Here's how it does it.
Potassium ions can diffuse across the plasma membrane. The sodium-potassium pump is an exchange pump. That is, it moves one ion in one direction and another ion in the opposite direction. The Na+-K+ pump consumes energy to actively transport potassium ions to the inside of the cell and sodium ions to the outside of the cell.
But the potassium ions can readily diffuse across the membrane, so they don't accumulate inside the cell. The K+ ions diffuse out of the cell as fast as they're pumped in by the Na+-K+ pump.
The sodium ions are much less capable of crossing the plasma membrane, so the cell transports them out at a much faster rate than they can diffuse back in. This means that the concentration of positively-charged sodium ions is much higher on the outside of the neuron's plasma membrane than it is on the inside. So the outside of the cell has a positive charge, compared to the inside of the cell. The charge difference on the two sides of a resting neuron's membrane is known as the resting potential.
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How a neuron establishes and maintains its resting potential.
Because the inside of the cell contains negatively-charged ions and the outside of the cell contains a surplus of positively-charged ions,
there is a charge difference of 70 milliVolts on the two sides of the membrane. This is possible because the Na+-K+ pump
transports Na+ ions out of the cell and K+ ions into the cell. Potassium ions can diffuse back out as fast as they're pumped in.
Sodium ions cannot diffuse back in as fast as they're pumped out, and so accumulate outside the cell.
http://www.freethought-forum.com/forum/gallery/files/5/0/measuring_potential_original.jpg
How to measure the charge difference on the two sides of a resting neuron's plasma membrane.
[b]Depolarization of a Neuron:
The neuron has sodium gates and potassium gates in its membrane. When the cell is polarized, those gates are closed, of course, which prevents sodium from diffusing into the cell as fast as it's being pumped out.
A stimulus is something that causes the sodium gates to open, and therefore causes the neuron to depolarize. Different neurons are specialized to depolarize in response to different stimuli. The chemoreceptor neurons in your respiratory epithelium, for example, depolarize when certain chemicals called odorants bind to receptor proteins in their membranes, causing ion gates to open. The photoreceptors in the retina of your eye depolarize when receptor proteins in their membranes absorb photons of light, causing ion gates to open. (Sort of; I'll get back to that.)
Since the gated ion channels in the plasma membrane of a neuron are voltage-gated channels, if enough of them open, a chain reaction begins. If an appropriate stimulus forces some of the gated channels to open, there is a massive inflow of sodium ions into that part of the neuron and that portion of the neuron depolarizes. If only a few gates open, then they close again almost immediately and the Na+-K+ pumps quickly restore the polarized state -- that is, the affected region of the neuron repolarizes.
But if enough of the gates open, the change in the voltage of the membrane causes nearby gates to open. And as those gates open and sodium ions rush in, gates further down the length of the neural fiber open. And so on, and so on. It's like pushing over a domino: once the first one is knocked over, the rest are knocked over in sequence, and so a "wave" moves down the length of the domino chain.
This is why the neuron will conduct impulses in only one direction -- from the dendrite(s) and cell body toward the axons. The impulse cannot travel backwards, because the only way that could happen would be if a region of the neuron that was already depolarized could somehow be forced to depolarize again. In short, it would be like trying to force a wave to move along a series of dominos that have already been knocked over.
And so, if it receives a sufficiently-strong stimulus of the appropriate type, a wave of depolarization moves rapidly down the length of a neuron. This wave of depolarization is known as the action potential. The action potential is the nerve impulse.
http://www.freethought-forum.com/forum/gallery/files/5/0/propagation_original.jpg
How an action potential is propagated down the length of a neuron.
Because the sodium gates are voltage-gated, when enough of them open, the change in membrane voltage causes gates further along the membrane
to open in sequence, and so a wave of depolarization rapidly moves down the length of the neuron. This is the action potential or nerve impulse.
Note that the action potential can be conducted in only one direction.
[b]Synapses and Neurotransmitters:
Okay, so we know how a neuron generates and propagates an action potential. The logical question to ask at this point is: "How does the action potential get transferred to the next cell along?". (That cell may or may not be another neuron.) I'm glad you asked.
Where the axon of a neuron contacts another cell is known as a synapse. In most cases, the neuron does not actually touch the cell it synapses with. Instead, there's a small gap called the synaptic cleft between the neuron and the next cell.
The end of an axon is swollen, forming what is called the axon terminal. The axon terminal produces and stores chemicals called neurotransmitters.
There are lots of different neurotransmitters, and different kinds of neurons produce different neurotransmitters. This is part of the reason why neurons can affect their target cells in so many different ways.
When an action potential reaches the axon terminal, the axon releases its stored neurotransmitters. The neurotransmitter molecules are then free to diffuse across the synaptic cleft, where they bind to receptor proteins in the membrane of the next cell. (The neuron that's releasing the neurotransmitters is the presynaptic cell, since it's before the synapse. The cell that's receiving the neurotransmitters is the postsynaptic cell, because it's after the synapse.) Note that the neurotransmitter functions as a ligand.
When the neurotransmitter binds to receptors in the membrane of the postsynaptic cell, it may cause gated channels in that cell's membrane to open, and so that cell may depolarize in turn.
Actually, it's a bit more complicated than that. Some neurotransmitters are excitatory and so make the postsynaptic cell more likely to depolarize. (Remember that a critical number of sodium gates must open before the cell will actually depolarize and generate an action potential.)
Some neurotransmitters are inhibitory and so make the postsynaptic cell less likely to depolarize and generate an action potential. That might sound weird, but it's just as important to prevent cells from depolarizing when they shouldn't as it is to cause them to depolarize when they should. Otherwise, there would be so much "noise" due to randomly-depolarizing neurons that the nervous system would function very poorly, if at all. (Inhibition of neuron depolarization is also an important part of how the nervous system processes impulses.)
Note that the neurotransmitter is either reabsorbed or chemically degraded almost immediately in most cases. Otherwise, there would be no way for the postsynaptic cell to repolarize after it generated an action potential.
http://www.freethought-forum.com/forum/gallery/files/5/0/chemical_synapse_original.jpg
How a typical chemical synapse works.
[b]The Structure of the Eye:
Now let's consider the structure of the eye. Don't worry: we'll soon be able to put all of this together.
Broadly speaking, the human eye consists of three distinct layers, called tunics. The tough, outer layer is known as the fibrous tunic. The very tough white portion of the fibrous tunic is known as the sclera; the clear portion that allows light to enter into the eye is the cornea.
The middle portion is known as the vascular tunic. It contains large numbers of blood vessels, as well as a densely-pigmented layer known as the choroid. The blood vessels supply oxygen and nutrients to the other tissues, and the dark pigments of the choroid absorb stray photons of light.
The innermost layer is the neural tunic, because it's made up mostly of neurons. The neural tunic is also known as the retina.
http://www.freethought-forum.com/forum/gallery/files/5/0/eye_541127_original.jpg
The anatomy of the human eye.
The vascular tunic forms a muscular structure called the ciliary body that is attached to a non-living, more or less clear structure called the lens. The muscles of the ciliary body can contract to alter the shape and thickness of the lens.
The vascular tunic also forms a colored muscle that lies just in front of the lens. This muscle is the iris. It has an opening in it known as the pupil. The muscles of the iris can contract and relax as necessary to control the size of the pupil -- and therefore how much light passes into the interior of the eye and ultimately to the retina.
The space between the cornea and the lens is filled with a thin, watery fluid known as the aqueous humor. The space between the lens and the retina is filled with a thicker, jelly-like substance known as the vitreous humor.
[b]The Nature of Light:
Now let us consider the nature of light for a moment. Light consists of moving particles called photons. The various wavelengths of light -- from very high-wavelength radio waves to very low-wavelength gamma rays -- make up the electromagnetic spectrum. There is a direct relationship between the wavelength of light and its energy, by the way. Radio waves carry very little energy. By contrast, x-rays and gamma rays carry a lot of energy -- that's why they'll pass right through most normal matter.
Only a small portion of the electromagnetic spectrum can be absorbed by (and thus detected by) the photoreceptors in our eyes. The wavelengths of light that we can detect with our eyes make up what's called the visible spectrum, because we can see this light. Our brains interpret visible light of different wavelengths as different colors of light.
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The Electromagnetic Spectrum.
Note that there is an inverse relationship between the wavelength of light and its energy.
Long-wavelength light (such as radio waves) carries very little energy, whereas short-wavelength light (such as gamma rays) carries a great deal of energy.
Only a small portion of the electromagnetic spectrum can be detected by the photoreceptors of the eye.
[b]Refraction of Light:
The speed with which light moves is a function of the medium that it's traveling through. The denser the medium, the slower the light moves. So as light passes from the near-vacuum of space into the Earth's atmosphere, it slows down a bit. Similarly, it slows down a bit more as it moves from air into a denser medium, such as water.
If the light passes into a new medium at a distinct angle, then the change in speed causes the rays of light to bend. This is known as refraction. This is why an object looks like it bends when it passes from air into water.
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Refraction of Light.
The spoon looks bent and displaced because the light bends when it passes from the air into the denser water.
[b]Lenses:
So, if an object is transparent (that is, if light can pass through it) and it is of a different density than the medium that surrounds it, then light will bend as it passes into the object. If the object in question is properly shaped, it can bend the incoming rays of light so that they come together (focus) and form an image. Such an object is called a lens.
The eye works because it focuses incoming light to form an image that is projected onto the retina.
As light passes into the cornea and the aqueous humor from the air, the light is refracted. As the light continues into the lens, it is further refracted, so that an image is projected onto the retina. (Contrary to what most people think, most of the focusing of light is done by the cornea and the aqueous humor; the lens merely "fine tunes" the focus.)
Because of the ciliary muscles, the lens' shape can be adjusted to ensure that the light is properly focused onto the retina. This is why we can clearly see objects at different distances.
http://www.freethought-forum.com/forum/gallery/files/5/0/focusing_original.jpg
The eye focuses incoming light to form an image that is projected onto the retina.
Note that the image projected onto the retina is upside-down and reversed. The brain reverses and flips the image in order to interpret it.
[b]The Retina:
Now let's look at the retina itself, where the focused image is projected. The retina contains three layers of neurons. The deepest layer (ironically), is the one where the photoreceptors are located. These neurons contain light-absorbing pigments. The middle layer is made up of neurons known as bipolar cells. The outermost layer (ironically, the layer where light first strikes) is made up of neurons known as ganglion cells.
You may have noticed, by the way that there are a number of specialized neurons in the retina other than just the photoreceptors, the bipolar cells, and the ganglion cells. These neurons can receive and process impulses coming from the other neurons. In other words, there is quite a lot of processing of visual impulses before they even reach the optic nerve, much less the brain itself.
http://www.freethought-forum.com/forum/gallery/files/5/0/retina_1_original.jpg
The retina of the eye.
Light passes through the ganglion cells and then the bipolar cells to strike the photoreceptors (the rods and the cones) in the deepest layer.
[b]Photoreceptors in the Retina:
Broadly speaking, the all-important photoreceptors come in two types. Some are more or less rod-shaped and so are somewhat unimaginatively called rods. The rods are quite sensitive to visible light within a broad range of wavelengths. What they don't do is discriminate between the different wavelengths of light that they can detect. So they relay information on the intensity of light, but not its wavelength (that is, they do not convey color information).
The other type of photoreceptor cell is somewhat cone-shaped, and so these cells are called cones. There are actually three different kinds of cones in (most) people's eyes, and each of the different kinds of cones is most sensitive within a fairly narrow range of wavelengths. Since different wavelengths of light stimulate the three different kinds of cones with different intensities, the brain can interpret the impulses coming in from the cones to determine the wavelengths of light that are being received by the retina. This is interpreted by the brain as the color of the light, as mentioned.
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The electromagnetic spectrum and the sensitivities of the four different types of photoreceptor cells to different wavelengths of visible light.
Because each cell type is most sensitive to different wavelengths, this gives the brain the ability to determine both intensity (brightness)
and wavelength (color) of incoming light. Rods are more sensitive at low light levels than are cones.
[b]Transduction of Light by Photoreceptors in the Retina:
The photoreceptors (rods and cones) contain pigment molecules that can absorb light. The process by which a stimulus causes a neuron to depolarize and generate an action potential is known as transduction, so the absorption of light by photoreceptors and the resulting generation of action potentials is known as phototransduction.
The photoreceptors synapse with the bipolar cells, so the impulses generated when the photoreceptors depolarize are conveyed to the bipolar cells. The bipolar cells, in turn, synapse with the ganglion cells, and so convey impulses from the photoreceptor cells to the ganglion cells.
The axons of the ganglion cells come together to form the afferent (sensory) fibers that will ultimately convey these impulses to the brain as the optic nerve. As you recall, neurons will normally conduct impulses in only one direction, and the human optic nerve contains afferent fibers only. So it can conduct impulses only toward the brain. No evidence of any kind has ever been found that impulses of any kind can travel from the brain to the eye via the optic nerve. (Indeed, such a thing could only be possible if neurons function very differently from the way that they actually do.)
Our eyes are "backwards" in a sense, since the ganglion cells are the outermost layer of neurons in the retina. This means that where their axons come together to form the optic nerve, they must penetrate into the retina. So where the axons of the ganglion cells come together to form the optic nerve, there can be no photoreceptors.
This region of the retina, where there are no photoreceptors and therefore there is no vision, is known as the optic disc. Since there are no photoreceptors in the optic disc, each of us has a blind spot in each eye. We don't normally notice, because the brain automatically fills in information from the surroundings, creating the illusion that there is no blind spot. (This is one of the reasons why fighter pilots are taught to constantly swivel their heads. If an enemy plane is approaching from within your blind spot, you won't see it coming. Nor will you notice anything amiss, since you're not normally aware that you even have a blind spot in each eye.)
As an aside, a squid's eye is organized much more sensibly than is the eye of a vertebrate such as ourselves. In a squid, the ganglion cells are behind the photoreceptors, so there's no blind spot.
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Where the axons of the ganglion cells come together to form the optic nerve is the optic disc.
Because there are no photoreceptors in the optic disc, each of us has a blind spot in each eye.
The brain has various means of compensating, so we don't normally notice.
[b]Depolarization and Hyperpolarization of Retinal Photoreceptor Cells:
The photoreceptors in the retina work backwards from the way that you might expect. When they are not receiving light, they depolarize. When a photoreceptor depolarizes, it generates an action potential which causes it to release neurotransmitters at its synapse with a bipolar cell. These neurotransmitters are inhibitory, and so the bipolar cell does not depolarize when it receives neurotransmitters from its associated photoreceptors.
So, when a photoreceptor cell is not receiving light, it depolarizes. Because the photoreceptor releases inhibitory neurotransmitters, the bipolar cell associated with the photoreceptor does not depolarize when the photoreceptor depolarizes. So when a photoreceptor is not receiving light of a wavelength that it can absorb, the bipolar cell does not depolarize. And so, no impulse is relayed by the bipolar cell to the ganglion cell.
This mechanism might seem backwards at first, but it's actually very efficient. Because the photoreceptors actually prevent the bipolar cells from depolarizing when they're receiving no light (and, therefore, the ganglion cells don't depolarize either), this reduces the incidence of spontaneous depolarization in retinal cells. This has the effect of greatly reducing the rates of spontaneous depolarization and so reduces the "noise" that would otherwise plague the visual system.
The photoreceptors, you remember, contain light-absorbing pigment molecules. When a photon of the appropriate wavelength strikes a pigment molecule, it does not cause the neuron to depolarize. Instead, the photoreceptor does exactly the opposite. When pigment molecules in a photoreceptor cell absorb a photon of light, sodium gates in the membrane of the cell close, and the neuron becomes hyperpolarized.
This means that the photoreceptor is no longer generating an action potential, and so it is not delivering inhibitory neurotransmitters to the bipolar cell(s) it synapses with. Since the bipolar cells are no longer receiving inhibitory neurotransmitters from the photoreceptors, they depolarize and generate action potentials. The neurotransmitters released by the bipolar cells are excitatory and so cause the ganglion cells they synapse with to depolarize and generate action potentials of their own.
And since the optic nerve is just the axons of the ganglion cells, the impulses are relayed to the brain.
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When it is not receiving photons that it can absorb, a photoreceptor cell is depolarized. (On the left.)
The depolarized cell releases inhibitory neurotransmitters that prevent the bipolar cells from depolarizing.
Thus the ganglion cells do not depolarize and so do not generate action potentials.
When it is receiving photons that can be absorbed by its pigments, a photoreceptor cell hyperpolarizes
and therefore does not generate an action potential. (On the right).
Since it is not receiving inhibitory neurotransmitters, the bipolar cell depolarizes and
generates an action potential. This, in turn, causes the ganglion cell to depolarize and generate an
action potential. The impulses are relayed via the axons of the ganglion cells (the optic nerve to the brain.)
[b]The Visual Pathway:
The impulses relayed by the optic nerves go ultimately to the brain. Interestingly, the two optic nerves come together at the optic chiasma. Here, many of the fibers "cross over." That is, many of the fibers from the right optic nerve cross over and ultimately go to the left side of the brain. Similarly, many of the fibers from the left optic nerve cross over and ultimately go to the right side of the brain.
Some of the fibers from the optic nerve synapse with a brain region known as the pretectum or pretectal nucleus. Based upon input from the optic nerve, this brain region sends impulses out to the [different] nerves that control the muscles that adjust the size of the pupil. This is why our pupils contract when we're exposed to bright light and why they dilate in dim light. So long as the visual pathway is intact up to the pretectum, a person's pupils will contract and dilate in response to changing light levels, even if the person is completely blind.
Some of the fibers of the optic nerve continue on to synapse with a brain region known as the superior colliculus or tectum. Among other things, the superior colliculi are responsible for some of our reflexive responses to movement within the visual field. (Again, no information is relayed to the eyes via the optic nerve from the superior colliculi.)
The ultimate destination for the remaining fibers of the optic nerves is the visual cortex in the occipital lobe of the brain. This is where the visual impulses are interpreted.
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The visual pathway.
This is, as mentioned, a very abbreviated summary of what we know about the anatomy and physiology of sight. I've left out a lot for the sake of clarity and brevity. But I hope that it's of use.
It seemed to me that the anatomy and physiology of how we see might be a fun topic to discuss. Please note, though, that this is only a very, very cursory examination of what we know and understand about how vision works. If I were to go into detail, it would quite literally fill a textbook or two.
With your permission, I'll start out with the basics and work up. So please forgive me if it seems a bit slow at first.
[b]The Nature of Atoms and Ions:
Let's start with the atom. I'm sure everyone knows that atoms are the fundamental unit of which normal matter is constructed. That most-definitely includes the human body. Further, I'm sure everyone understands that an atom has three basic components.
Every atom contains one or more positively-charged protons. (Please don't confuse protons with photons; photons are "particles" of light.)
Most atoms also contain one or more uncharged neutrons. The protons and neutrons are bound together in the central region of the atom, known as the nucleus. Surrounding the nucleus are negatively-charged electrons. The electrons essentially "orbit" the nucleus, kind of like tiny planets orbiting the Sun. The electrons are distributed in what are called orbitals or shells, and it's important to keep in mind that each orbital can hold only so many electrons.
Every atom, by definition, has equal numbers of positively-charged protons and negatively-charged electrons, and therefore has no net electrical charge.
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A carbon atom.
Note that every atom has equal numbers of protons and
electrons by definition. The number of neutrons need not
be equal to that of the number of protons/electrons, however.
Under certain conditions, an atom can either gain or lose electrons and thus acquire an electrical charge. Since an atom has no net electrical charge by definition, an electrically-charged atom is no longer known as an "atom"; it is instead known as an ion.
If an atom gains an electron, it acquires a negative charge. This is necessarily so because each electron has a charge of -1 and each proton has a charge of +1. So if the "atom" has more electrons than protons, it will have a net negative charge. That is, if the ion has one more electron than it has protons, it will have a charge of -1. If it has two more electrons than protons, it will have a charge of -2. And so on. A negatively-charged ion is known as an anion.
If an atom loses an electron, it acquires a positive charge, since it now has more positively-charged protons than negatively-charged electrons. A positively-charged ion is known as a cation.
[b]Chemical Bonds:
Under normal circumstances, the electrons in the outermost orbital are the only part of an atom that can interact with other atoms. And as mentioned previously, each orbital can hold only so many electrons. The first orbital can hold up to 2 electrons, but no more. In atoms that have more than 2 electrons, a second orbital exists, which can hold up to 8 electrons. In atoms that have more than 10 electrons, a third orbital exists. In most atoms that are relevant to living things, the third orbital, like the second, can hold up to 8 electrons.
In their interactions with other atoms, atoms typically configure themselves so that their outermost orbitals are full. They can do this either by exchanging electrons or by sharing electrons. The electrons in the outermost orbital are, as mentioned, the only part of an atom that normally interacts with another atom. These electrons are known as the valence electrons, because they determine the valence of the atom -- that is, how many other atoms it can form chemical bonds with.
When atoms come together and chemically bond (by either exchanging or sharing electrons), they form molecules. In ionic bonding, one of the atoms actually takes one or more electrons away from the other. The atom that takes an electron becomes a negatively-charged anion, and the atom that loses an electron becomes a positively-charged cation. (Hence the term "ionic bonding," since the atoms exchange electrons and form ions in the process.) The ions then stick together because of their opposite charges.
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Ionic Bonding to form a Molecule of Sodium Chloride.
After the atoms exchange an electron, the resulting ions are held together by their opposite charges.
Note that each atom fills its outermost orbital when the two of them exchange an electron.
Another way in which atoms can form chemical bonds is by coming together and merging their outermost orbitals, and thus sharing electrons. This is known as covalent bonding.
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Covalent bonding between 4 hydrogen atoms and a single carbon atom to form a molecule of methane.
Note that each atom fills its outermost orbital by sharing electrons with one or more other atoms.
The carbon atom "gains" four electrons and thus fills its outermost orbital by sharing a pair of electrons with each of the four hydrogen atoms.
Each hydrogen atom "gains" a single electron and thus fills its outermost orbital by sharing a pair of electrons with the carbon atom.
Now then, as we've seen, the subunits within an ionic-bonded substance are electrically charged by definition. The subunits within a covalently-bonded substance may or may not have electrical charges, however.
[b]Polar and Nonpolar Covalent Bonds:
In some cases, the atoms within a covalently-bonded molecule share their electrons equally. In that case, the charges balance out, and the molecule has no net electrical charge. This is known as a nonpolar molecule.
But in some covalently-bonded substances, electrons are not shared equally. Because one or more of the atoms in the molecule "hoards" electrons, the charges are unbalanced. In effect, the electrons tend to "cluster" on one side of the molecule. This means that one end of the molecule has a slight negative charge (where the electrons tend to cluster) and the other side has a slight positive charge. This is a polar molecule. Water is a good example of a polar molecule.
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Water is a polar molecule because the oxygen atom and the two hydrogen atoms do not share their electrons equally.
The oxygen, in effect, "hoards" the electrons. The “δ” symbol indicates a partial charge.
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A molecule of carbon dioxide is about the same size as a molecule of water, but has very different properties.
Because the valence electrons are shared equally in a molecule of carbon dioxide, carbon dioxide is a non-polar substance.
In a molecule of water, electrons are not shared equally, so the molecule is polar.
[b]Hydrogen Bonding between Polar Molecules:
Because each polar molecule has a (slightly) positive end and a (slightly) negative end, polar molecules tend to be attracted to each other. That is, the partially-positive end of one polar molecule is attracted to the partially-negative end of another polar molecule. The weak bonds that thus form between polar molecules are known as hydrogen bonds.
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Hydrogen bonding between water molecules.
Because of their charges (either partial in the case of polar molecules or complete in the case of ions), most polar and ionic substances will easily dissolve into water. Such substances are therefore said to be hydrophilic (literally, "water-loving").
Because they won't form hydrogen bonds with polar molecules, non-polar molecules typically will not dissolve into water. They are therefore said to be hydrophobic (literally, "water-fearing"). This is why most oils, for example, will not dissolve into water -- the molecules that make up the oil are non-polar and thus hydrophobic, and so cannot form hydrogen bonds with the water molecules.
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Sodium chloride (table salt) dissolves so well in water because the salt is ionic.
The sodium cations are attracted to the partially-negative regions of the water molecules
and the chloride anions are attracted to the partially-positive regions of the water molecules.
[b]Amphipathic Molecules and Phospholipid Bilayers:
Some larger molecules have regions that are hydrophilic (and will thus dissolve into water) and other regions that are hydrophobic (and thus won't). A good example is a phospholipid molecule. Such a molecule is said to be amphipathic.
A phospholipid molecule has a "head" that is strongly polar (hydrophilic) and so is attracted to water molecules. But it also has 2 nonpolar (hydrophobic) "tails" that are, in effect, repelled by water.
When placed into water, phospholipid molecules will therefore spontaneously organize into a bilayer. The hydrophilic "heads" face outward, toward the water, and the hydrophobic "tails" face inward, away from the water.
You'll doubtless have noticed that this means the central region of a phospholipid bilayer is hydrophobic. Thus most polar and ionic substances cannot cross phospholipid bilayers, because they can't pass through the hydrophobic interior.
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A phospholipid molecule. Note the polar (hydrophilic) "head" and the nonpolar (hydrophobic) "tails."
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A Phospholipid Bilayer.
When placed into water, phospholipid molecules spontaneously organize into a bilayer, with the hydrophilic "heads"
facing outward, toward the surrounding water and the hydrophobic "tails" facing inward, away from the water.
[b]Cells and Cellular Membranes:
The basic subunits of living things are cells, of course. And every cell is surrounded and held together by a plasma membrane. The plasma membrane is primarily made up of a phospholipid bilayer. Molecules of cholesterol are embedded in the membrane, and help to strengthen it. There are also various proteins embedded in the membrane. The proteins serve various functions, but a very important one is that they help determine what substances can cross the membrane, and under what circumstances.
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A generalized animal cell.
Cells contain genetic material and cytoplasm, and are enclosed by a plasma membrane.
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The plasma membrane is primarily a phospholipid bilayer.
Cholesterol molecules help to strengthen the plasma membrane.
Protein molecules embedded in the membrane are essential in determining what can and cannot cross the membrane.
It's essential to keep in mind that the interior of the plasma membrane is hydrophobic. Therefore, most polar and ionic substances cannot cross the plasma membrane of their own accord. Some small ions and polar molecules (such as water, for example) can cross the plasma membrane because they're small-enough to slip between the relatively large phospholipid molecules, though. We'll get back to that point shortly.
[b]Diffusion:
Because of their thermal energy, molecules and ions, when they're free to move, will tend to move from where they're more concentrated to where they're less concentrated. This phenomenon is known as diffusion.
For example, if the concentration of a substance on the outside of a cell is high and the concentration on the inside of the cell is low, molecules will tend to spontaneously move into the cell from the outside. If they can cross the membrane, that is.
The molecules are moving randomly. To understand why they spontaneously move from high concentration to low, imagine that the concentration of molecules on the outside of the cell is 10 times greater than the concentration on the inside of the cell. This means that for every molecule that just happens to move from the inside of the cell to the outside of the cell, ten will be moving into the cell from the outside. So the net movement of molecules is into the cell.
This will continue until a state of equilibrium is reached and the concentration on both sides of the plasma membrane is equal. It's not that the molecules stop moving: it's just that when the concentration is equal on both sides of the membrane, for every molecule that happens to move out of the cell there will be, on average, one molecule moving into the cell. So the net movement is zero.
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Diffusion across a cellular membrane.
When the concentrations on both sides of the membrane are equal, the net molecular motion is zero.
(Hit "refresh" to see the animation.)
[b]Moving Substances Across Cellular Membranes:
Now then, it surely won't surprise you to learn that cells have various mechanisms by which they can move substances across their membranes -- even substances that wouldn't normally be able to cross those membranes. Most of these mechanisms involve those proteins embedded in the membrane.
If the molecules of a substance are hydrophobic (such as most lipid molecules, for instance), they can diffuse through the membrane on their own, since they are not repelled by the hydrophobic membrane interior. Small molecules (like oxygen, for example) can also diffuse across most membranes on their own because they can slip between the phospholipid molecules. When substances diffuse into and out of cells on their own, because they can cross the plasma membrane unhindered, this is known as simple diffusion.
You'll note that the cell does not expend any of its own energy in simple diffusion, since the molecules in question are moving of their own accord.
Some of the proteins in the plasma membrane can temporarily bond to substances and transport them across the membrane from high concentration to low. The proteins that do this are called carrier proteins, since they carry substances across plasma membranes. In this way, carrier proteins allow certain hydrophilic substances to cross plasma membranes, even though those substances would ordinarily be unable to do so. If the carrier proteins allow the substance in question to cross the membrane from high concentration to low, the cell does not have to expend any energy of its own in the process (since the energy is supplied by the moving molecules). This is known as carrier-mediated facilitated diffusion.
In some cases, carrier proteins can actually pull substances across the membrane from low concentration to high. The cell must expend energy in order to do this, however, since it is moving molecules in the opposite direction that they're "trying" to go. This is known as active transport.
Some of the proteins embedded in the plasma membrane of a cell are hollow, and penetrate all the way through. They thus form channels that hydrophilic molecules can pass through to enter or exit the cell. Naturally, these proteins are called channel proteins.
One especially important thing about channel proteins is that some of them can be opened and closed, in order to adjust the movement of specific substances across the plasma membrane, as necessary. These are known as gated channels.
Consider, for example, what will happen if a cell actively transports a hydrophilic substance outside, so that its concentration is greater outside the cell than inside. (It will have to use carrier proteins to do this, of course.) If there are channel proteins in the membrane of the cell through which this substance can pass but they're closed, the cell can make the concentration of the substance in question much higher on the outside of the cell than on the inside. And so long as the gated channels remain closed, this state will persist. But as soon as those gated channels open, there will be a massive rush of the substance into the cell.
Water is a special case, by the way. Water molecules are small-enough that they can "slip between the cracks." Water can therefore diffuse across plasma membranes even though water is (obviously) hydrophilic. But there are gated channels in the membrane called aquaporins. By opening and closing aquaporins, the cell can speed up or slow down the movement of water across its plasma membrane, as necessary. Diffusion of water across a plasma membrane is called osmosis.
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Some of the ways that substances can cross plasma membranes.
Note that none of these illustrated methods require the cell to expend energy.
By the same token, none of these methods can be used to create a difference in concentration on the two sides of the membrane.
The cell can use its carrier proteins to transport substances from low concentration to high and so can create a difference in concentration
on the two sides of the membrane, but it must expend energy to do so.
Some cells, using active transport and gated channels, can move ions across their membranes and so create a situation where the electrical charges on the opposite sides of the membrane are different. When a cell does this, and so has different electrical charges on the two sides of its plasma membrane, it is said to be polarized.
Obviously, a cell that is polarized is in a highly unstable state. But so long as the gated channels remain closed, the ions cannot re-cross the membrane and so the cell will remain polarized. Of course, as soon as the gated channels open, ions will rush across the membrane until the charges on both sides are equalized. When that happens, we say that the cell has depolarized.
[b]Opening and Closing Gated Channel Proteins:
What causes gated ion channels to open or close?
Some are chemically gated ion channels. In this type of channel protein, a region of the protein is known as the receptor. Certain molecules (called ligands) can bond to that receptor. When the appropriate ligand bonds to the receptor, the protein changes shape, causing the channel to open. Two common classes of ligands are neurotransmitters and hormones.
Some ion channels are voltage gated ion channels. They open and close not in response to ligands, but in response to changes in the voltage of the nearby plasma membrane.
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Two types of gated ion channels.
[b]Neurons:
Now let us consider a neuron. A neuron is a highly specialized cell because it can generate and rapidly conduct electrochemical impulses. It can do this because it has specialized proteins in its plasma membrane that can actively transport ions -- especially sodium (Na+) and potassium (K+) ions -- across the membrane. In other words, it can be polarized.
The bulk of a typical neuron is in its central region, called the cell body or soma. Projecting out from the neuron are long, thin projections known as the neural fibers. At one end of a typical neuron are many branching fibers. These are known as the dendrites. A neuron may have as many as 10,000 dendrites. At the other end, there is a single axon. No neuron has more than one axon.
Under normal conditions, a neuron will conduct impulses in only one direction. Dendrites carry impulses toward the cell body, and the axon carries impulses away from the cell body. This is an important factor in how the nervous system works: the nervous system would be a chaotic, function-less mess if neurons conducted impulses in both directions.
Outside of the brain and spinal cord, the axons of neurons are bundled together to form organs called nerves. A neuron that is carrying impulses from a sensory receptor toward the Central Nervous System (that is, the brain or spinal cord) is known as an afferent (or sensory) neuron. A nerve that contains only afferent neurons can therefore conduct impulses only to (not from) the CNS and is known as an afferent or sensory nerve.
A neuron that is carrying impulses from the CNS to an effector (usually a gland or a muscle) is known as an efferent (or motor) neuron. Naturally, a nerve that contains only efferent fibers is known as an efferent (motor) nerve.
Some nerves contain both afferent fibers going to the CNS and efferent fibers coming from the CNS. These are known as mixed nerves.
Nerves that contain only afferent fibers include the olfactory nerve (responsible for the sense of smell) and the optic nerve (responsible for the sense of sight). For comparison, the oculomotor nerve (controls some of the muscles that move the eyes) contains mostly efferent fibers, though it has some afferent fibers as well. All of the nerves that originate in the spinal cord are mixed nerves.
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A typical neuron.
Dendrites carry impulses toward the cell body, and the single axon carries impulses away from the cell body.
[b]How a Neuron is Polarized:
Now then: back to neurons.
When it's at rest, a neuron actively transports ions across its membrane. Specifically, it uses transport proteins in its membrane called the Sodium-Potassium pump to transport ions. Here's how it does it.
Potassium ions can diffuse across the plasma membrane. The sodium-potassium pump is an exchange pump. That is, it moves one ion in one direction and another ion in the opposite direction. The Na+-K+ pump consumes energy to actively transport potassium ions to the inside of the cell and sodium ions to the outside of the cell.
But the potassium ions can readily diffuse across the membrane, so they don't accumulate inside the cell. The K+ ions diffuse out of the cell as fast as they're pumped in by the Na+-K+ pump.
The sodium ions are much less capable of crossing the plasma membrane, so the cell transports them out at a much faster rate than they can diffuse back in. This means that the concentration of positively-charged sodium ions is much higher on the outside of the neuron's plasma membrane than it is on the inside. So the outside of the cell has a positive charge, compared to the inside of the cell. The charge difference on the two sides of a resting neuron's membrane is known as the resting potential.
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How a neuron establishes and maintains its resting potential.
Because the inside of the cell contains negatively-charged ions and the outside of the cell contains a surplus of positively-charged ions,
there is a charge difference of 70 milliVolts on the two sides of the membrane. This is possible because the Na+-K+ pump
transports Na+ ions out of the cell and K+ ions into the cell. Potassium ions can diffuse back out as fast as they're pumped in.
Sodium ions cannot diffuse back in as fast as they're pumped out, and so accumulate outside the cell.
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How to measure the charge difference on the two sides of a resting neuron's plasma membrane.
[b]Depolarization of a Neuron:
The neuron has sodium gates and potassium gates in its membrane. When the cell is polarized, those gates are closed, of course, which prevents sodium from diffusing into the cell as fast as it's being pumped out.
A stimulus is something that causes the sodium gates to open, and therefore causes the neuron to depolarize. Different neurons are specialized to depolarize in response to different stimuli. The chemoreceptor neurons in your respiratory epithelium, for example, depolarize when certain chemicals called odorants bind to receptor proteins in their membranes, causing ion gates to open. The photoreceptors in the retina of your eye depolarize when receptor proteins in their membranes absorb photons of light, causing ion gates to open. (Sort of; I'll get back to that.)
Since the gated ion channels in the plasma membrane of a neuron are voltage-gated channels, if enough of them open, a chain reaction begins. If an appropriate stimulus forces some of the gated channels to open, there is a massive inflow of sodium ions into that part of the neuron and that portion of the neuron depolarizes. If only a few gates open, then they close again almost immediately and the Na+-K+ pumps quickly restore the polarized state -- that is, the affected region of the neuron repolarizes.
But if enough of the gates open, the change in the voltage of the membrane causes nearby gates to open. And as those gates open and sodium ions rush in, gates further down the length of the neural fiber open. And so on, and so on. It's like pushing over a domino: once the first one is knocked over, the rest are knocked over in sequence, and so a "wave" moves down the length of the domino chain.
This is why the neuron will conduct impulses in only one direction -- from the dendrite(s) and cell body toward the axons. The impulse cannot travel backwards, because the only way that could happen would be if a region of the neuron that was already depolarized could somehow be forced to depolarize again. In short, it would be like trying to force a wave to move along a series of dominos that have already been knocked over.
And so, if it receives a sufficiently-strong stimulus of the appropriate type, a wave of depolarization moves rapidly down the length of a neuron. This wave of depolarization is known as the action potential. The action potential is the nerve impulse.
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How an action potential is propagated down the length of a neuron.
Because the sodium gates are voltage-gated, when enough of them open, the change in membrane voltage causes gates further along the membrane
to open in sequence, and so a wave of depolarization rapidly moves down the length of the neuron. This is the action potential or nerve impulse.
Note that the action potential can be conducted in only one direction.
[b]Synapses and Neurotransmitters:
Okay, so we know how a neuron generates and propagates an action potential. The logical question to ask at this point is: "How does the action potential get transferred to the next cell along?". (That cell may or may not be another neuron.) I'm glad you asked.
Where the axon of a neuron contacts another cell is known as a synapse. In most cases, the neuron does not actually touch the cell it synapses with. Instead, there's a small gap called the synaptic cleft between the neuron and the next cell.
The end of an axon is swollen, forming what is called the axon terminal. The axon terminal produces and stores chemicals called neurotransmitters.
There are lots of different neurotransmitters, and different kinds of neurons produce different neurotransmitters. This is part of the reason why neurons can affect their target cells in so many different ways.
When an action potential reaches the axon terminal, the axon releases its stored neurotransmitters. The neurotransmitter molecules are then free to diffuse across the synaptic cleft, where they bind to receptor proteins in the membrane of the next cell. (The neuron that's releasing the neurotransmitters is the presynaptic cell, since it's before the synapse. The cell that's receiving the neurotransmitters is the postsynaptic cell, because it's after the synapse.) Note that the neurotransmitter functions as a ligand.
When the neurotransmitter binds to receptors in the membrane of the postsynaptic cell, it may cause gated channels in that cell's membrane to open, and so that cell may depolarize in turn.
Actually, it's a bit more complicated than that. Some neurotransmitters are excitatory and so make the postsynaptic cell more likely to depolarize. (Remember that a critical number of sodium gates must open before the cell will actually depolarize and generate an action potential.)
Some neurotransmitters are inhibitory and so make the postsynaptic cell less likely to depolarize and generate an action potential. That might sound weird, but it's just as important to prevent cells from depolarizing when they shouldn't as it is to cause them to depolarize when they should. Otherwise, there would be so much "noise" due to randomly-depolarizing neurons that the nervous system would function very poorly, if at all. (Inhibition of neuron depolarization is also an important part of how the nervous system processes impulses.)
Note that the neurotransmitter is either reabsorbed or chemically degraded almost immediately in most cases. Otherwise, there would be no way for the postsynaptic cell to repolarize after it generated an action potential.
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How a typical chemical synapse works.
[b]The Structure of the Eye:
Now let's consider the structure of the eye. Don't worry: we'll soon be able to put all of this together.
Broadly speaking, the human eye consists of three distinct layers, called tunics. The tough, outer layer is known as the fibrous tunic. The very tough white portion of the fibrous tunic is known as the sclera; the clear portion that allows light to enter into the eye is the cornea.
The middle portion is known as the vascular tunic. It contains large numbers of blood vessels, as well as a densely-pigmented layer known as the choroid. The blood vessels supply oxygen and nutrients to the other tissues, and the dark pigments of the choroid absorb stray photons of light.
The innermost layer is the neural tunic, because it's made up mostly of neurons. The neural tunic is also known as the retina.
http://www.freethought-forum.com/forum/gallery/files/5/0/eye_541127_original.jpg
The anatomy of the human eye.
The vascular tunic forms a muscular structure called the ciliary body that is attached to a non-living, more or less clear structure called the lens. The muscles of the ciliary body can contract to alter the shape and thickness of the lens.
The vascular tunic also forms a colored muscle that lies just in front of the lens. This muscle is the iris. It has an opening in it known as the pupil. The muscles of the iris can contract and relax as necessary to control the size of the pupil -- and therefore how much light passes into the interior of the eye and ultimately to the retina.
The space between the cornea and the lens is filled with a thin, watery fluid known as the aqueous humor. The space between the lens and the retina is filled with a thicker, jelly-like substance known as the vitreous humor.
[b]The Nature of Light:
Now let us consider the nature of light for a moment. Light consists of moving particles called photons. The various wavelengths of light -- from very high-wavelength radio waves to very low-wavelength gamma rays -- make up the electromagnetic spectrum. There is a direct relationship between the wavelength of light and its energy, by the way. Radio waves carry very little energy. By contrast, x-rays and gamma rays carry a lot of energy -- that's why they'll pass right through most normal matter.
Only a small portion of the electromagnetic spectrum can be absorbed by (and thus detected by) the photoreceptors in our eyes. The wavelengths of light that we can detect with our eyes make up what's called the visible spectrum, because we can see this light. Our brains interpret visible light of different wavelengths as different colors of light.
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The Electromagnetic Spectrum.
Note that there is an inverse relationship between the wavelength of light and its energy.
Long-wavelength light (such as radio waves) carries very little energy, whereas short-wavelength light (such as gamma rays) carries a great deal of energy.
Only a small portion of the electromagnetic spectrum can be detected by the photoreceptors of the eye.
[b]Refraction of Light:
The speed with which light moves is a function of the medium that it's traveling through. The denser the medium, the slower the light moves. So as light passes from the near-vacuum of space into the Earth's atmosphere, it slows down a bit. Similarly, it slows down a bit more as it moves from air into a denser medium, such as water.
If the light passes into a new medium at a distinct angle, then the change in speed causes the rays of light to bend. This is known as refraction. This is why an object looks like it bends when it passes from air into water.
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Refraction of Light.
The spoon looks bent and displaced because the light bends when it passes from the air into the denser water.
[b]Lenses:
So, if an object is transparent (that is, if light can pass through it) and it is of a different density than the medium that surrounds it, then light will bend as it passes into the object. If the object in question is properly shaped, it can bend the incoming rays of light so that they come together (focus) and form an image. Such an object is called a lens.
The eye works because it focuses incoming light to form an image that is projected onto the retina.
As light passes into the cornea and the aqueous humor from the air, the light is refracted. As the light continues into the lens, it is further refracted, so that an image is projected onto the retina. (Contrary to what most people think, most of the focusing of light is done by the cornea and the aqueous humor; the lens merely "fine tunes" the focus.)
Because of the ciliary muscles, the lens' shape can be adjusted to ensure that the light is properly focused onto the retina. This is why we can clearly see objects at different distances.
http://www.freethought-forum.com/forum/gallery/files/5/0/focusing_original.jpg
The eye focuses incoming light to form an image that is projected onto the retina.
Note that the image projected onto the retina is upside-down and reversed. The brain reverses and flips the image in order to interpret it.
[b]The Retina:
Now let's look at the retina itself, where the focused image is projected. The retina contains three layers of neurons. The deepest layer (ironically), is the one where the photoreceptors are located. These neurons contain light-absorbing pigments. The middle layer is made up of neurons known as bipolar cells. The outermost layer (ironically, the layer where light first strikes) is made up of neurons known as ganglion cells.
You may have noticed, by the way that there are a number of specialized neurons in the retina other than just the photoreceptors, the bipolar cells, and the ganglion cells. These neurons can receive and process impulses coming from the other neurons. In other words, there is quite a lot of processing of visual impulses before they even reach the optic nerve, much less the brain itself.
http://www.freethought-forum.com/forum/gallery/files/5/0/retina_1_original.jpg
The retina of the eye.
Light passes through the ganglion cells and then the bipolar cells to strike the photoreceptors (the rods and the cones) in the deepest layer.
[b]Photoreceptors in the Retina:
Broadly speaking, the all-important photoreceptors come in two types. Some are more or less rod-shaped and so are somewhat unimaginatively called rods. The rods are quite sensitive to visible light within a broad range of wavelengths. What they don't do is discriminate between the different wavelengths of light that they can detect. So they relay information on the intensity of light, but not its wavelength (that is, they do not convey color information).
The other type of photoreceptor cell is somewhat cone-shaped, and so these cells are called cones. There are actually three different kinds of cones in (most) people's eyes, and each of the different kinds of cones is most sensitive within a fairly narrow range of wavelengths. Since different wavelengths of light stimulate the three different kinds of cones with different intensities, the brain can interpret the impulses coming in from the cones to determine the wavelengths of light that are being received by the retina. This is interpreted by the brain as the color of the light, as mentioned.
http://www.freethought-forum.com/forum/gallery/files/5/0/spectrum_and_sensitivity_original.jpg
The electromagnetic spectrum and the sensitivities of the four different types of photoreceptor cells to different wavelengths of visible light.
Because each cell type is most sensitive to different wavelengths, this gives the brain the ability to determine both intensity (brightness)
and wavelength (color) of incoming light. Rods are more sensitive at low light levels than are cones.
[b]Transduction of Light by Photoreceptors in the Retina:
The photoreceptors (rods and cones) contain pigment molecules that can absorb light. The process by which a stimulus causes a neuron to depolarize and generate an action potential is known as transduction, so the absorption of light by photoreceptors and the resulting generation of action potentials is known as phototransduction.
The photoreceptors synapse with the bipolar cells, so the impulses generated when the photoreceptors depolarize are conveyed to the bipolar cells. The bipolar cells, in turn, synapse with the ganglion cells, and so convey impulses from the photoreceptor cells to the ganglion cells.
The axons of the ganglion cells come together to form the afferent (sensory) fibers that will ultimately convey these impulses to the brain as the optic nerve. As you recall, neurons will normally conduct impulses in only one direction, and the human optic nerve contains afferent fibers only. So it can conduct impulses only toward the brain. No evidence of any kind has ever been found that impulses of any kind can travel from the brain to the eye via the optic nerve. (Indeed, such a thing could only be possible if neurons function very differently from the way that they actually do.)
Our eyes are "backwards" in a sense, since the ganglion cells are the outermost layer of neurons in the retina. This means that where their axons come together to form the optic nerve, they must penetrate into the retina. So where the axons of the ganglion cells come together to form the optic nerve, there can be no photoreceptors.
This region of the retina, where there are no photoreceptors and therefore there is no vision, is known as the optic disc. Since there are no photoreceptors in the optic disc, each of us has a blind spot in each eye. We don't normally notice, because the brain automatically fills in information from the surroundings, creating the illusion that there is no blind spot. (This is one of the reasons why fighter pilots are taught to constantly swivel their heads. If an enemy plane is approaching from within your blind spot, you won't see it coming. Nor will you notice anything amiss, since you're not normally aware that you even have a blind spot in each eye.)
As an aside, a squid's eye is organized much more sensibly than is the eye of a vertebrate such as ourselves. In a squid, the ganglion cells are behind the photoreceptors, so there's no blind spot.
http://www.freethought-forum.com/forum/gallery/files/5/0/retina_2_original.jpg
Where the axons of the ganglion cells come together to form the optic nerve is the optic disc.
Because there are no photoreceptors in the optic disc, each of us has a blind spot in each eye.
The brain has various means of compensating, so we don't normally notice.
[b]Depolarization and Hyperpolarization of Retinal Photoreceptor Cells:
The photoreceptors in the retina work backwards from the way that you might expect. When they are not receiving light, they depolarize. When a photoreceptor depolarizes, it generates an action potential which causes it to release neurotransmitters at its synapse with a bipolar cell. These neurotransmitters are inhibitory, and so the bipolar cell does not depolarize when it receives neurotransmitters from its associated photoreceptors.
So, when a photoreceptor cell is not receiving light, it depolarizes. Because the photoreceptor releases inhibitory neurotransmitters, the bipolar cell associated with the photoreceptor does not depolarize when the photoreceptor depolarizes. So when a photoreceptor is not receiving light of a wavelength that it can absorb, the bipolar cell does not depolarize. And so, no impulse is relayed by the bipolar cell to the ganglion cell.
This mechanism might seem backwards at first, but it's actually very efficient. Because the photoreceptors actually prevent the bipolar cells from depolarizing when they're receiving no light (and, therefore, the ganglion cells don't depolarize either), this reduces the incidence of spontaneous depolarization in retinal cells. This has the effect of greatly reducing the rates of spontaneous depolarization and so reduces the "noise" that would otherwise plague the visual system.
The photoreceptors, you remember, contain light-absorbing pigment molecules. When a photon of the appropriate wavelength strikes a pigment molecule, it does not cause the neuron to depolarize. Instead, the photoreceptor does exactly the opposite. When pigment molecules in a photoreceptor cell absorb a photon of light, sodium gates in the membrane of the cell close, and the neuron becomes hyperpolarized.
This means that the photoreceptor is no longer generating an action potential, and so it is not delivering inhibitory neurotransmitters to the bipolar cell(s) it synapses with. Since the bipolar cells are no longer receiving inhibitory neurotransmitters from the photoreceptors, they depolarize and generate action potentials. The neurotransmitters released by the bipolar cells are excitatory and so cause the ganglion cells they synapse with to depolarize and generate action potentials of their own.
And since the optic nerve is just the axons of the ganglion cells, the impulses are relayed to the brain.
http://www.freethought-forum.com/forum/gallery/files/5/0/signalling_original.jpg
When it is not receiving photons that it can absorb, a photoreceptor cell is depolarized. (On the left.)
The depolarized cell releases inhibitory neurotransmitters that prevent the bipolar cells from depolarizing.
Thus the ganglion cells do not depolarize and so do not generate action potentials.
When it is receiving photons that can be absorbed by its pigments, a photoreceptor cell hyperpolarizes
and therefore does not generate an action potential. (On the right).
Since it is not receiving inhibitory neurotransmitters, the bipolar cell depolarizes and
generates an action potential. This, in turn, causes the ganglion cell to depolarize and generate an
action potential. The impulses are relayed via the axons of the ganglion cells (the optic nerve to the brain.)
[b]The Visual Pathway:
The impulses relayed by the optic nerves go ultimately to the brain. Interestingly, the two optic nerves come together at the optic chiasma. Here, many of the fibers "cross over." That is, many of the fibers from the right optic nerve cross over and ultimately go to the left side of the brain. Similarly, many of the fibers from the left optic nerve cross over and ultimately go to the right side of the brain.
Some of the fibers from the optic nerve synapse with a brain region known as the pretectum or pretectal nucleus. Based upon input from the optic nerve, this brain region sends impulses out to the [different] nerves that control the muscles that adjust the size of the pupil. This is why our pupils contract when we're exposed to bright light and why they dilate in dim light. So long as the visual pathway is intact up to the pretectum, a person's pupils will contract and dilate in response to changing light levels, even if the person is completely blind.
Some of the fibers of the optic nerve continue on to synapse with a brain region known as the superior colliculus or tectum. Among other things, the superior colliculi are responsible for some of our reflexive responses to movement within the visual field. (Again, no information is relayed to the eyes via the optic nerve from the superior colliculi.)
The ultimate destination for the remaining fibers of the optic nerves is the visual cortex in the occipital lobe of the brain. This is where the visual impulses are interpreted.
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The visual pathway.
This is, as mentioned, a very abbreviated summary of what we know about the anatomy and physiology of sight. I've left out a lot for the sake of clarity and brevity. But I hope that it's of use.