CBNS 120 Sensory Transduction III: Photoreception (Vision)

The five principal cell types in the retina are:

Photoreceptors (rods/cones), bipolar cells, ganglion cells, horizontal cells, and amacrine cells.

Light passes through which retinal layers first?

Ganglion cell layer, then bipolar cell layer, then photoreceptor layer (back of retina).

The two types of vertebrate photoreceptors are:

Rods (scotopic, low-light vision) and cones (photopic, color vision).

Intrinsically photosensitive ganglion cells (ipRGCs) are involved in:

Entrainment of circadian rhythms (not vision; discovered by Berson et al., 2002).

Rods are specialized for:

High light sensitivity, peripheral vision, slow responses, low flicker resolution (12 Hz).

Cones are specialized for:

Color vision (trichromatic: S, M, L), high acuity (fovea), fast responses, high flicker resolution (55 Hz).

The photopigment in rods is:

Rhodopsin (chromophore = 11-cis-retinal; opsin protein).

The three types of cone photopigments are:

S-cone (blue, 455 nm), M-cone (green, 530 nm), L-cone (red, 625 nm).

All photopigments use the same chromophore (11-cis-retinal) but:

Have different opsin proteins that confer different wavelength sensitivity.

The outer segment of a rod contains:

Membrane disks (80% of disk protein = rhodopsin) where phototransduction occurs.

Synaptic ribbons are found in:

Photoreceptors, bipolar cells, and hair cells; they tether vesicles for continuous release.

In the dark, photoreceptors are:

Depolarized (-30 to -40 mV) due to "dark current" (Na+ influx through cGMP-gated channels in outer segment).

Light causes photoreceptors to:

Hyperpolarize (closing Na+ channels, decreasing glutamate release).

Phototransduction steps:

Light → 11-cis-retinal isomerizes to all-trans-retinal → activates meta-rhodopsin → activates transducin (Gt) → activates PDE → decreases cGMP → cGMP-gated Na+ channels close → hyperpolarization.

In the dark, cGMP levels are:

High (keeping cGMP-gated Na+ channels open).

In light, cGMP levels:

Decrease (due to PDE activation), closing Na+ channels.

George Wald discovered:

The light-induced isomerization of retinal (11-cis → all-trans), Nobel Prize 1967.

Transducin (Gt) is:

A G protein in rod outer segments with 80% sequence homology to gustducin (taste G protein).

The ratio of rhodopsin:transducin:PDE in rod outer segments is approximately:

1000 : 100 : 4 (amplification cascade).

Ca2+ regulates adaptation in photoreceptors by:

Inhibiting guanylate cyclase (GC). In light, [Ca2+]i decreases → GC activity increases → cGMP increases → Na+ channels reopen (restores dark current).

A single photon can close: ~

300 cGMP-gated channels (3-5% of dark current; ~0.5 pA current reduction).

Rhodopsin regeneration (all-trans → 11-cis) requires:

Interaction with the pigment epithelium.

Retinal is derived from:

Vitamin A ("Eat your carrots, or you'll go blind!").

Ribbon synapses in photoreceptors release:

Glutamate continuously (modulated by graded changes in membrane potential, not action potentials).

OFF-center bipolar cells:

Depolarize to light offset (glutamate ionotropic receptors).

ON-center bipolar cells:

Hyperpolarize to light offset (glutamate metabotropic receptors).

Dichromatic color vision (e.g., horses) results from:

Having only two types of cones (vs. three in trichromatic humans). Protanopia = no L cones; deuteranopia = no M cones; tritanopia = no S cones.