Structure of the Human EyeCornea protects eye refracts lightIris colored muscle regulates pupil sizePupil regulates light inputLens focuses images on retinaCiliary Muscles controls shape of lens accomodationFovea point of central focus contains most cones birds of prey/rodent variationRetina contains photoreceptors

The Retina

(make up the optic nerve)

Rods•100-120 million•sensitive to dim light•black/white discrimination•large numbers on the periphery

Cones•4-6 million•used for color vision•located near the fovea•red, green, and blue cones

Visual Pathway

Light to rods/cones to bipolar cells to ganglion cells to LGN cells to Visual Cortex

Photoreceptor Action

In the Dark:•rods are depolarized•rods release glutamate•glutamate is inhibitory•bipolar cells are inhibited

In the Light:•rods are hyperpolarized•no glutamate is released•bipolar cells are not inhibited (disinhibition)•bipolar cells undergo spontaneous activity

Glutamate(-)

Bipolarcell

Rodcell

DARK LIGHT

Not Active Active

Rhodopsin Photopigment

Rhodopsin: made up of retinal and opsin spans the disc membrane acts as a G-protein

Light Transduction

DARK LIGHT

•cis-retinal transformed to trans-retinal•trans-retinal and opsin dissociate•now active opsin activates transducin•transducin activates PDE•PDE breaks down cGMP to 5’-GMP•5’GMP closes Na+ channels•rod cell hyperpolarizes•reduces the release of glutamate

•trans-retinal transformed to cis-retinal•cis-retinal and opsin form rhodopsin•rhodopsin activates guanylate cyclase (GC)•GC increases the synthesis of cGMP•cGMP opens Na+ channels•rod cell depolarizes•increases the release of glutamate•(darkness adjustment–waiting for rhodosin)

Rhodopsin Cascade

Rod celldisc

Inside Rod cell

Outside

Rhodopsin molecule

LIGHT

1 photon of light can block the entry of 1,000,000 Na+ ions