The layers of the eye.txt

• cornea: transparent cover of the front of the eye
• Sclera: white cover over the rest of eyeball

• anterior: iris - muscle that contracts or relaxes to control the amount of light entering the eye
• ciliary body - muscle that controls lens shape
• Posterior: choroid

the sensory tissue

• contains the ganglion cell axons running to the brain
• contains incoming blood vessels that open into the retina to vascularize the retinal lairs

• A transparent body located behind the iris
• Cataract: a lens that loses its transparency
• Cataracts are very common in the elderly and are now easily corrected by surgical removal and replacement with an artificial lens
• Cataracts can also occur in young people

• it is suspended by ligaments (called zone fibers) attached to the ciliary body.
• The lens inverts the image and focuses the image on the retina

• caused by a loss of flexibility within the lens
• it produces difficulty in focusing on close objects
• generally occurs at around age 45

• Anterior chamber: between cornea and iris
• Posterior chamber: between iris, zone fibers and lens

• Vitreous chamber: between the lens and the retina
• Vitreous humor is a gelatinous substance filling the back of the eye

• Capillaries of the choroid layer: supply the outer layer of the retina by diffusion
• Capillaries from the central retinal artery: which enters the eye through the optic papilla, supply the inner layers of the retina

• constituents: neurons, glia, pigmented epithelium, blood vessels
• light passes through the lens, through the inner layer of ganglion cells and bipolar cells to reach the rods and cones

a general term for the back of the eye

the location where axons leave the eye and form the optic nerve. This "blind spot" lacks neurons. It lies nasal to the fovea centralis

• about 3-4 mm in diameter
• contains the fovea
• a yellow pigmentation of the macular area is known as the macula lutea; a short wavelength filter, additional to that provided by the lens
• Age-related macular degeneration is a common retinal problem of the aging eye: A leading cause of blindness in the world - the cones of fovea die causing central visual loss

• 0.5 mm thick
• the photosensors: the rods and cones lie outermost in the retina against the pigment epithelium and choroid
• ganglion cells: (the output neurons of the retina) lie innermost in the retina closest to the lens and front of the eye

• 110,000,000 - 125,000,000 rods/retina
• Rods function under dim conditions but not bright conditions
• 6,400,000 cones/retina
• cones will not function in very dimly lit conditions
• cones mediate color vision

• there are three kinds of cones, with maximum sensitivities to different wavelengths of light
• blue: short wavelength
• green: medium wavelength
• red: long wavelength

• it lies in the center of the macula
• it contains only cones
• the overlying retinal layers are displaced

• the fovea is especially sensitive to color and detail, but functions poorly in badly lit conditions
• when you want to look at something, you move your eye so that the image of the object falls on the fovea

• the periphery has poor resolution of small objects
• the periphery has high sensitivity under dimly-lit conditions

• In the dark, special cation channels in the outer segment membrane are open, allowing influx of positive charge, but in the light, those channels are closed
• the discs in the outer segment of rods contain rhodopsin molecules in their membranes (up to 100 billion molecules per photoreceptor!)
• Rhodopsin: is the "visual pigment" that responds to light: in rods
• Cones contain cone opsins:, that are slightly different from the proteins in rods

• a hereditary disease of the retina for which there is no cure at present
• caused by at least 10 different mutations int he phototransduction cascade
• leads to loss of peripheral vision and can progress to total blindness

• photoreceptors synapse onto many postsynaptic interneurons
• the interneurons synapse onto one another and onto ganglion cells

• there are at least 1 different morphological types of ganglion cell in the human retina
• there are about a million ganglion cells
• some features of ganglion cells are the same at all regions of the retina but some are very different int he fovea vs. the periphery
• understanding these differences helps to explain the consequences of disorders that affect different parts of the retina

• visual field: the whole area seen by the eye
• visual receptive field: refers to the part of the visual field where a visual stimulus affects a particular visual system neuron
• most ganglion cells have center-surround receptive fields: they respond best to bright or dark round spots
• about 50% of those are On-center Off-surround (respond best to bright spots)
• about 50% of those are Off-center On-surround (respond best to dark spots)

• shining light on the center of the field causes depolarization (turns the ganglion cell "ON")
• shining light on the surround part of the field causes hyperpolarization (turns the ganglion cell "OFF")
• this ganglion cell responds best (fires fastest) if there is a small spot of light centered in the receptive field
• It responds less to a larger spot because a larger spot activates inhibitory inputs to the ganglion cell as well as excitatory

• these cells respond best to a small dark spot on a bright background
• neither type responds well to an all-bright or all-dark field

• some respond transiently; some give sustained responses
• an On-center cell can be a transient-type cell or a sustained-type cell
• and OFF-center cell can be a transient-type cell or a sustained-type cell

• some ganglion cells don't have center-surround receptive fields
• much less is known about these cells
• some project to superior colliculus and are involved in eye movement control
• some project to the suprachiasmatic nucleus, a region that controls circadian rhythms

• what makes some receptive fields small and others large? Different amounts of convergence determine different receptive field sizes
• what is the advantage of having lots of convergence in the rod pathway? spacial resolution is degraded byt sensitivity to low levels of light is improved
• Ganglion cells in the fovea have small dendritic arbors and collect input from few interneurons
• ganglion cell centers in the fovea are as small as a fraction of a degree
• Ganglion cell centers in the periphery can be up to 5 degrees in diameter

• Red/green center-surround organization is very common
• other color coding cells use blue-yellow contrast
• the opsins for the L and M (red and green) cones are coded by adjacent regions on X chromosome and are very similar
• they are prone to recombination errors, so red-green color-blindness is very common in males

• typical cell in fovea: sustained, color-selective, some on-center , some off-center; small RF
• typical cell in periphery: transient; broad-band - no color selectivity; some on center, some off center; large RF