Light striking a photoreceptor causes the isomerization of retinene from the 11-cis to the all-trans form. This leads to breakdown of the photopigment rhodopsin. Rhodopsin is formed from a serpentine "receptor" protein (opsin) embedded in the membranes of the discs of the rods, combined with the 11-cis retinene (retinal).

The process of breakdown activates the receptor protein which in turn, becomes able to activate a G_t protein, transducin, bound to the inner face of the membranes of the photoreceptor discs.

The activated G-protein releases the G-alpha subunit, to activate a cGMP phosphodiesterase, that breaks down cGMP. A constitutive guanylate cyclase in the cell, constantly produces cGMP, which when bound to the inner, cytoplasmic face of the Na⁺ channels in the membrane of he outer segment, causes them to open. The breakdown of cGMP in light, leads to a fall in cGMP concentration, closing of Na⁺ channels, and hyper-polarisation of the receptor, due to the continued activity of the Na⁺/K⁺-ATPase pumps in the inner (proximal) segment.

Illustration of the "dark current" constantly flowing in the photoreceptor not exposed to light. Decreased permeability to Na+ due to closure of distal channels, along with continued activity of the proximal sodium pumps, leads to hyperpolarisation of the receptor on exposure to light.

Exposure to bright light leads to bleaching of the photopigment, and reduction of the sensitivity of the receptors. Rods are very sensitve to light, and at high light intensities are readily bleached and rendered non-functional. Cones are less sensitive, and function well at high light intensities. In both cases however, bleached pigment is constantly resynthesized, and the standing concentration depends on the balance between the rate of degradation and the rate of resynthesis. This process of constant breakdown and resynthesis by re-isomerization of the retinene, is called the rhodopsin cycle, and is illustrated here.

The rate of resynthesis after bleaching determines the rate of recovery of visual sensitivity after exposure to bright light. Cones, being less sensitive, are not extensively bleached in bright light, and recover rapidly. Because they are not very sensitive, their recovery leads to a fast, but not very large fall in visual threshold (minimum detectable light intensity). The rods are extensively bleached in bright light, and recover more slowly, but due to their greater sensitivity, lead to a much more profound fall in threshold. The recovery of sensitivity to light after exposure to bright light, therefore, is a biphasic process.

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