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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.