Intro to Anatomy 6: Tissues, Membranes, Organs

Nervous Tissue:

Nervous tissue is specialized for the rapid transmission of messages throughout the body. It makes up the brain, the spinal cord, and the various peripheral nerves that extend throughout the body and relay information to and from the brain, spinal cord, and sense organs.

Broadly speaking, neural tissue contains two major types of cells: neurons and neuroglial cells. Neurons transmit electrochemical impulses along their length, and so allow rapid communication between different parts of the body. Neuroglial cells, by contrast, typically provide support for neurons.

Neurons:

The great bulk of a neuron, containing the cell’s nucleus and its various organelles makes up the cell body. Numerous thin, branching structures (up to 10,000 or so) called dendrites extend from one end of the cell body. The dendrites transmit electrochemical impulses from sensory cells or other neurons toward the cell body of the neuron. On the other side, a single axon (it may be branched, however) extends. The axon transmits electrochemical impulses away from the cell body of the neuron, to the next neuron or to a muscle cell or a gland. Because electrochemical impulses always travel from dendrites through the cell body and out through an axon, any given neuron can transmit information in one direction only.

Dendrites and axons are very long and thin compared to the cell body, and they’re collectively called nerve fibers. Outside of the central nervous system (the brain and the spinal cord), the nerve fibers are covered by a white substance called myelin, which both insulates the nerve fibers and greatly increases the speed with which they can transmit impulses.

A single nerve fiber can be three feet long or so (!). The fibers of the sciatic nerve, for instance, extend from the spinal cord down through the leg and all the way to the toes. Nerve fibers carrying impulses to or from a particular part of the body are typically bound together by connective tissue to form nerves.

Afferent (sensory or receptor) neurons have long dendrites and short axons. The elongated dendrites of afferent neurons carry impulses from sensory organs toward the central nervous system. If numerous afferent fibers are bound together to form a nerve, it is known as an afferent or sensory nerve, since it contains only afferent fibers. The optic nerve, which transmits information from the eye to the brain, is an example of an afferent nerve.

Efferent (motor or effector) neurons have short dendrites and long axons. The elongated axons carry impulses away from the central nervous system, to muscles or glands. Of course, if numerous efferent fibers are bound together to form a nerve, it is known as an efferent or motor nerve. The oculomotor nerve is an example of an efferent nerve; it transmits impulses from the brain to the muscles that move the eye.

Interneurons (relay neurons or association neurons) are found only in the central nervous system, and they connect only to other neurons. Interneurons bridge efferent and afferent neurons, allowing for reflex arcs, and they allow communication within the central nervous system. Because they’re typically unmyelinated, interneurons are generally gray in color. This is the distinction between the “gray matter” and the myelinated “white matter” in the nervous sytem.

Some nerves contain both afferent and efferent fibers, and so can transmit messages both to and from the central nervous system. (No individual fiber can do this, of course, since a neuron will transmit impulses in one direction only.) These nerves are known as mixed nerves. The facial nerve is an example of a mixed nerve; it contains afferent fibers that allow you to taste food and it contains efferent fibers that control your facial muscles.

A Typical Neuron (This is a Motor Neuron)
Note that it has many dendrites but only a single axon.

Neuroglial Cells:

Neuroglial Cells and a Neuron

Neuroglial cells (glial cells or neuroglia) are non-neurons in nervous tissue that provide support for the neurons. (In the human brain, neuroglial cells are estimated to outnumber neurons by 50 to 1 or so.) Some neuroglia are phagocytic and protect neural tissues from infection. Other neuroglial cells form “bridges” between neurons and blood vessels, allowing neurons to rapidly absorb oxygen and nutrients from the blood. Yet other neuroglial cells produce the myelin sheath that covers neural fibers. It has recently been discovered that some neuroglial cells play a role in signal transmission, so it appears that neurons aren’t the only cells in nervous tissue that can transmit information after all.

Think of it: there are something like 100,000,000,000 neurons in the adult human brain, and each of them is connected by its dendrites to perhaps 10,000 others. The number of possible brain states in a human brain is truly staggering. If neuroglial cells play a major role in signal processing, this may raise the possible number of brain states by a factor of 10 or more.

Neural Tissue
Several relatively large neurons can be seen,
as well as many relatively tiny neuroglial cells.

Incidentally, the brain works somewhat like a muscle in that it grows “stronger” with use. Studies with mice have shown that if they’re raised in intellectually challenging environments (where they’re forced to solve puzzles to get food, for instance), they have something like 10 times the density of dendritic connections in their brains as do mice raised in environments where they’re simply provided with food and water and aren’t given any intellectual stimulation in the form of toys.

Autopsies of human brains have provided convincing evidence that people who lead richer intellectual lives (people who read a lot, and otherwise keep mentally active) have more dendritic connections in their brains than do those who don’t read or otherwise keep mentally active. Not only does frequent mental stimulation make it easier to learn new things, people with active mental lives seem to be less susceptible to degenerative brain disorders such as Parkinson’s and Alzheimer’s – possibly because their brains, having more dendritic connections, are generally more “robust.”