Compare rods and cones. Describe the mechanism of phototransduction and dark adaptation.

Rods and Cones Compared

• Optic disc — no receptors at all → the blind spot
• At the fovea the inner retinal layers are swept aside so light reaches the cones directly — the point of greatest acuity

Phototransduction

Light strikes rhodopsin → 11-cis retinal → all-trans retinal (isomerisation) → Rhodopsin → metarhodopsin II → Activates transducin (a G protein) → Activates phosphodiesterase → ↓ cGMP → Na+ channels close → hyperpolarisation → ↓ Glutamate release

• Rhodopsin = opsin (scotopsin) + 11-cis retinal, which is derived from vitamin A
• Regeneration of rhodopsin requires vitamin A and takes minutes — the basis of dark adaptation
• Enormous amplification — one photon → hundreds of transducin → hydrolysis of a million cGMP

Retinal Circuitry

Photoreceptor → Bipolar cell → Ganglion cell → Optic nerve

• Horizontal and amacrine cells provide lateral inhibition → sharpen contrast and edges
• Only the ganglion cell fires action potentials — all other retinal neurones respond with graded potentials
• Light is refracted through all retinal layers before reaching the receptors, since the retina is inverted

Dark Adaptation

• Mechanism — regeneration of rhodopsin, plus pupillary dilatation and increased neural convergence
• Light adaptation is far faster — complete in about 5 minutes
• Measured by the adaptometer

Photopic, Scotopic and Mesopic Vision

• Purkinje shift — the peak of sensitivity moves toward blue in dim light, so at dusk red flowers look black while blue ones stay bright
• Mesopic vision uses both systems, as at twilight

Electroretinogram

• A wave — negative; from the photoreceptors
• B wave — positive; from bipolar and Müller cells
• C wave — from the pigment epithelium
• Used to assess retinal function when the media are opaque, and in retinitis pigmentosa

Visual Pigments and the Retinal Pigment Epithelium

• Retinal pigment epithelium — stores vitamin A, regenerates 11-cis retinal, phagocytoses shed outer segments, and absorbs stray light
• It also forms part of the blood–retinal barrier
• Retinal detachment separates the photoreceptors from this epithelium → rapid degeneration unless reattached

Applied Aspects

• Vitamin A deficiency → night blindness (nyctalopia) — delayed and incomplete dark adaptation; the earliest sign of xerophthalmia, and an important public health problem in India
• Retinitis pigmentosa — progressive rod degeneration → night blindness with tunnel vision
• Red goggles are worn by pilots and radiologists before night duty — red light stimulates cones but does not bleach rhodopsin, so rod adaptation is preserved
• To see a dim star, look slightly to one side of it — peripheral rods are far more sensitive than the rod-free fovea
• Age-related macular degeneration — the commonest cause of central visual loss in the elderly; peripheral vision is preserved