Structure of the Human Eye Cornea protects eye

Structure of the Human Eye Cornea protects eye refracts light Iris colored muscle regulates pupil size Pupil regulates light input Lens focuses images on retina Ciliary Muscles controls shape of lens accomodation Fovea point of central focus contains most cones birds of prey/rodent variation Retina contains photoreceptors

The Retina (make up the optic nerve) Rods Cones • 100 -120 million • sensitive to dim light • black/white discrimination • large numbers on the periphery • 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 Not Active Bipolar cell Active In the Dark: • rods are depolarized • rods release glutamate • glutamate is inhibitory • bipolar cells are inhibited Glutamate (-) In the Light: • rods are hyperpolarized • no glutamate is released • bipolar cells are not inhibited (disinhibition) • bipolar cells undergo spontaneous activity DARK Rod cell LIGHT

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

Light Transduction DARK • trans-retinal transformed to cis-retinal • cis-retinal and opsin form rhodopsin • rhodopsin activates guanylate cyclase (GC) • GC increases the synthesis of c. GMP • c. GMP opens Na+ channels • rod cell depolarizes • increases the release of glutamate • (darkness adjustment–waiting for rhodosin) LIGHT • cis-retinal transformed to trans-retinal • trans-retinal and opsin dissociate • now active opsin activates transducin • transducin activates PDE • PDE breaks down c. GMP to 5’-GMP • 5’GMP closes Na+ channels • rod cell hyperpolarizes • reduces the release of glutamate

Rhodopsin Cascade Rhodopsin molecule LIGHT Rod cell disc Inside Rod cell 1 photon of light can block the entry of 1, 000 Na+ ions Outside