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