Light ––> Rhodopsin: weak strong
Meta II ––> Transducin (––>PDE): weak strong
PDE ––> cGMP (––>NaV): weak strong
NaV ––> Vm: weak strong
Visual transduction: direct metabolic
Notes:The photoreceptor uses a visual pigment embedded in the bi-lipid membranous discs that make up the outer segment to detect light. The photoreceptor consists of 1) an outer segment, filled with stacks of membranes (like a stack of poker chips) containing the visual pigment molecules such as rhodopsins, 2) an inner segment containing mitochondria, ribosomes and membranes where opsin molecules are assembled and passed to be part of the outer segment discs, 3) a cell body containing the nucleus of the photoreceptor cell and 4) a synaptic terminal where neurotransmission to second order neurons occurs.
Rhodopsin consists of two components, a protein (opsin) and a covalently-bound cofactor called retinal. Opsin is a light-sensitive G protein coupled receptor. Thousands of opsin molecules are found in each outer segment disc of the host cell. Retinal is produced in the retina from vitamin A, from dietary beta-carotene. Isomerization of 11-cis-retinal into all-trans-retinal by light sets off a series of conformational changes ('bleaching') in the opsin, eventually leading it to a form called metarhodopsin II (Meta II), which activates an associated G protein, transducin. This is the first amplification step – each photoactivated rhodopsin triggers activation of about 100 transducins. Transducin activates cGMP phosphodiesterase, PDE then catalyzes the hydrolysis of cGMP to 5' GMP. This is the second amplification step, where a single PDE hydrolyses about 1000 cGMP molecules.
Rhodopsin pigment must be regenerated for further phototransduction to occur. This means replacing all-trans-retinal with 11-cis-retinal and the decay of Meta II to Meta III. In the rod outer segment, Meta III decays into separate all-trans-retinal and opsin.
Similar to other cells, K+ channels maintain the resting membrane potential of the cell. The outward potassium current tends to hyperpolarize the photoreceptor at around -60 mV. cGMP molecules gate activation of an inward sodium current. This so-called 'dark current' depolarizes the cell to about -30 mV. A high density of Na+-K+ pumps enables the photoreceptor to maintain a steady intracellular concentration of Na+ and K+.
In the dark cGMP levels are high and keep cGMP-gated sodium channels open allowing a steady inward current, called the dark current. This dark current keeps the cell depolarized around -30 mV, leading to glutamate release.
Light leads to reduced cGMP levels and closure of cGMP-gated sodium channels. Stopping the influx of Na+ ions effectively switches off the dark current. Reducing this dark current causes the photoreceptor to hyperpolarise, which reduces glutamate release.https://en.wikipedia.org/wiki/Visual_phototransduction https://webvision.med.utah.edu/book/part-ii-anatomy-and-physiology-of-the-retina/photoreceptors/ Composed by Alon Poleg-Polsky, email@example.com