2-Minute Neuroscience: Phototransduction


Welcome to 2 minute neuroscience, where I
explain neuroscience topics in 2 minutes or less. In this installment I will discuss phototransduction. Phototransduction is the process that occurs
in the retina where light is converted into electrical signals that can be understood
by the nervous system. It primarily takes place in photoreceptor
cells, of which there are two main types: rods and cones. I will discuss phototransduction in rods,
although the process is similar in cones. In the dark, positively charged sodium ions
flow into rod cells through ion channels that are activated by a substance called cyclic
guanosine monophosphate, or cGMP. This influx of positively charged ions causes
cells to remain in a depolarized state, leading to the continuous release of the neurotransmitter
glutamate. Inside the rod cell there is a substance called
rhodopsin, which is made up of a protein called opsin and a molecule called retinal, which
is capable of absorbing light. When retinal absorbs light, its configuration
changes, an event that prompts opsin to activate a protein called transducin. Transducin activates a type of enzyme known
as a phosphodiesterase, which begins breaking down cGMP. As cGMP levels fall, the ion channels that
are opened by cGMP begin to close. Thus, less sodium enters the cell and the
cell becomes hyperpolarized due to potassium ions that regularly leave the cell. Consequently, glutamate release decreases. Strangely enough, this decrease in neurotransmitter
release acts as a signal that a light stimulus is present. The rod cell returns to its normal state quickly
when activated rhodopsin is inactivated, and a protein called arrestin subsequently binds
to it. Arrestin blocks the ability of rhodopsin to
activate transducin, which makes the cascade unable to continue. A complex process then restores retinal to
its original configuration, making it ready to absorb light once again.

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