Final answer:
In phototransduction, the biochemical cascade triggered by light exposure ultimately causes sodium ion channels in photoreceptor cell membranes to close, hyperpolarizing the cell and inhibiting neurotransmitter release.
Step-by-step explanation:
The process of phototransduction, which is the conversion of light energy into neural signals, triggers a biochemical cascade that ultimately causes sodium ion channels in photoreceptor cell membranes to close. When light hits a photoreceptor, it causes a shape change in the retinal molecule, altering it from its bent (cis) form to a linear (trans) isomer. This change activates the photopigment rhodopsin, which then starts a cascade of events involving the G-protein transducin and the enzyme phosphodiesterase.
The phosphodiesterase converts cGMP to GMP, leading to the closing of sodium channels, thereby hyperpolarizing the photoreceptor cell's membrane and inhibiting the release of the neurotransmitter glutamate. In contrast to most sensory neurons which depolarize upon stimulus exposure, photoreceptors hyperpolarize, moving further away from the threshold for activation.