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.
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.)