Vision AV

Photopigment Physiology

* Contain membrane-bound proteins called opsins * Opsins surround and bind a chromophore molecule (differs in each of the 4 photopigments) * Chromophore is retinal (derivative of vitamin A) - part of the photoreceptor that is light-sensitive

Photopigments

* Contained in photoreceptors * Absorb light * 4 different kinds in the retina * Rhodopsin (in rods) and three others in cones

Light Adaptation

* Dark - light movement * Initially, the eye is extremely sensitive to light, and the visual image is too bright and has poor contrast. * However, the rhodopsin is soon used up ("bleached" by the bright light), and the rods become unresponsive so that only the less-sensitive cones are operating and the image becomes less bright.

Photoreceptors & Dark

* Only type of sensory cell that is depolarized when it is at rest (i.e., in the dark), and hyperpolarized in response to its adequate stimulus NO LIGHT: * Action of the membrane-bound enzyme guanylyl cyclase converts GTP into a high intracellular concentration of the second messenger molecule, cyclic GMP (cGMP). * The cGMP maintains the ligand- gated cation channels in the outer segment membrane in the open state * A persistent influx of sodium and calcium results. * In the dark, cGMP concentrations are high, and the photoreceptor cell is maintained in a relatively depolarized state.

Eyes

* Optical portion: focuses the visual image on the receptor cells * Neural component: transforms the visual image into a pattern of graded and action potentials.

Photoreceptor Cells

* Outer segment: composed of stacked layers of membrane called discs * Discs: hold the chemical substances that respond to light * Inner segment: contains nucleus, mitochondria

Neural Pathways of Vision

* Photoreceptor and bipolar cells only undergo graded responses (lack voltage-gated ion channels) * Ganglion cells are the first cells in the pathway where action potentials can be initiated * Photoreceptors interact with bipolar and ganglion cells in two distinct ways * In both types, photoreceptors are depolarized in the absence of light, causing the neurotransmitter glutamate to be released onto bipolar cells. Light striking either pathway hyperpolarizes the photoreceptors, resulting in a decrease in glutamate release onto bipolar cells. * KEY DIFFERENCE: lies in the type of glutamate receptors

Neural Processing

* Photoreceptors, bipolar cells, and ganglion cells are interconnected by horizontal cells and amacrine cells, which pass information between adjacent areas of the retina. * The retina is characterized by its very great amount of convergence * There can be "ON center/OFF surround" or "OFF center/ON sur- round" cells, so named because the responses are either depolarization (ON) or hyperpolarization (OFF) in the two areas of the field. The usefulness of this organization is that the existence of a clear edge between the "ON" and "OFF" areas of the receptive field increases the contrast between the area that is receiving light and the area around it, increasing visual * The axons of the ganglion cells form the output from the retina—the optic nerve, which is cranial nerve II. The two optic nerves meet at the base of the brain to form the optic chiasm, where some of the fibers cross and travel within optic tracts to the opposite side of the brain, providing both cerebral hemispheres with input from each eye.

Choroid & Pigment Epithelium

* Two pigmented layers are at the back of the retina * Absorb light that has bypassed the photoreceptors * Prevents its reflection and scattering back through the rods and cones

Rods vs. Cones

* Two types of photoreceptors * Cones: less sensitive and respond only when the light is bright * Rods: extremely sensitive and respond to low levels of light

Photoreceptors & Light

* When light shines on the photoreceptors, a cascade of events leads to hyperpolarization of the photoreceptor cell membrane. * Retinal molecules in the disc membrane assume a new conformation induced by the absorption of energy from photons. * This stimulates an interaction between associated opsins and another protein that is a member of the G-protein family called transducin. * Transducin activates the enzyme phosphodiesterase, which rapidly degrades cGMP. * The decrease in cytoplasmic cGMP concentration allows the cation channels to close, and the loss of depolarizing current allows the membrane potential to hyperpolarize. * After its activation by light, retinal changes back to its resting shape by way of several mechanisms that do not depend on light, but are enzyme-mediated.

Dark Adaptation

* Bright light - no light: temporary "blindness" occurs until the photoreceptors can undergo dark adaptation. * During the exposure to bright light, however, the rods' rhodopsin has been completely activated, making the rods insensitive to light. * Rhodopsin cannot respond fully again until it is restored to its resting state * Vitamin A is necessary for night vision because it is required for the synthesis of the retinal portion of the rhodopsin

Phototransduction - Light Events

1) Light is absorbed by photopigment 2) Retinal and opsin dissociate 3) Transducin is activated 4) Photodiesterase is activated 5) cGMP levels in cytosol decrease 6) Sodium channels close 7) With less sodium entering the cell, the cell hyperpolarizes 8) Calcium channels close 9) Transmitter release is decreased 10) Graded potential in bipolar cells gets smaller

Phototransduction - Dark Events

1) cGMP levels in the cytosol are high 2) Sodium channels are open 3) Sodium enters the cell, causing a depolarization that spreads from the outer segment to the terminal 4) Calcium channels open in response to depolarization 5) Calcium enters the cell, triggering exocytosis of transmitter 6) Transmitter causes graded action potential

OFF-Pathway

NO LIGHT * OFF-pathway bipolar cells have ionotropic glutamate receptors that are nonselective cation channels, that depolarize the bipolar cells when glutamate binds. * Depolarization of these bipolar cells stimulates them to release excitatory neurotransmitter onto their associated ganglion cells, stimulating them to fire action potentials. * Thus, the OFF-pathway generates action potentials in the absence of light, and reversal of these processes inhibits action potentials when light does strike the photoreceptors. * The co-existence of these ON and OFF pathways in each region of the retina greatly improves image resolution by increasing the brain's ability to perceive contrast at edges or borders.

ON-Pathway

NO LIGHT * Glutamate released onto ON-pathway bipolar cells binds to metabotropic receptors that cause enzymatic breakdown of cGMP * This hyperpolarizes the bipolar cells by a mechanism similar to that occurring when light strikes a photoreceptor cell. * When the bipolar cells are hyperpolarized, they are prevented from releasing excitatory neurotransmitter onto their associated ganglion cells. * Thus, in the absence of light, ganglion cells of the ON-pathway are not stimulated to fire action potentials. WITH LIGHT * When light strikes the photoreceptors: glutamate release from photoreceptors is reduced, ON-bipolar cells depolarize, excitatory neurotransmitter is released, the ganglion cells are depolarized, and action potentials propagate to the brain.

Summary ON/OFF Pathways

ON PATHWAY * Glutamate receptors: inhibitory * Bipolar cells spontaneously depolarize in the absence of input * Metabotropic receptors * Excitatory transmitter is released at a synapse with ganglion cell - in LIGHT OFF PATHWAY * Glutamate receptors: excitatory * Bipolar cells hyperpolarize in the absence of input * Ionotropic receptors * Release excitatory neurotransmitter to ganglion cell - in DARK