3/3/99
Physiology II
Unit V—Special Senses
Questions to ask for each part of this unit.
1. Can the signal reach the receptor
· Far sighted—long
· Near sighted—short
· Hearing—ossicle damage
2. Is the receptor functional and the present
3. Is the pathway intact
4. Cortical area functioning
Optics of vision
· Eye is the equivalent of a camera
· Lens, variable aperture, dark interior, energy, light
· Melanin is like black paint
· Albinism (lack of melanin)
· Dark colors absorb light; light colors reflect light
· Retinitis pigmentosia—melanin goes over the rods and cones and loses field of vision (too much melanin)
· Delivery signal to the eye
· As bring signal in, content w/refraction
· Structure of the lens
· Suspensory ligaments to ciliary muscles
· Ciliary muscles are circular—contract and constrict the circle
· Accommodate (convex on up)
· Under resting conditions, pull maximally on the lens
· Round up lens have more refraction (shorter the focal point, deeper the angle)
· Far sighted—they can accommodate
· Near sighted—cannot accommodate
· Refractive errors—can't deliver signal to retina appropriately (lens misshapen, cornea, eye)
· Cornea could be curved too much
· Cataract—opacities in the lens (coagulation of protein and/or CHO)
· Bifocals—fixed lens—can't accommodate in either direction—presbiopia (old eyes)
· Emmotropia—normal vision
· Myopia—shorten focal length; near sightedness
· Farsightedness—focus beyond the retina—hyperopia
· Stigmatism—potential defects; corneal is asymmetrical which refracts at odd angles, lens out of shape, eyeball is asymmetrical
· Never achieve a focal point
· Similar to albinism
· Rays come in
· Can't accommodate—ciliary muscles contract symmetrically
· Glaucoma
· Lens if fine
· Receptor is fine
· Optic nerve can't transmit the signal
· Brain corrects for upside down and backwards
· Getting through the lens; if not refractive error
· Retina
· Fovea centralis—very rich in cone receptors; area where you want to focus light
· Sits in the macula
· Cone receptors—color, depth, visual acuity, distance
· Macular degeneration—more difficult to focus an image
· Depth is determined by variance of color
· Peripheral vision—less cones and not see as well
· Rods—shape distinction; black and white vision
· Stereopsis—two eyes help determine distance
· Size of object can help determine distance of an object
· Stereopsis good up to about 200ft; triangulating two images—register
· Double vision
· Cross eyed—strobismis (torsional (looking up), looking towards the center)
Visual acuity
· 20/100—poor vision—what an average person can see at 100ft, you can only see at 20ft
· 20/15—what an average person sees at 15 ft you can see at 20ft—better than average vision
· visional transmission process goes from photons to chemical signals
· rod and cones
· rod—rhodopsin; 1 type; feed into bipolar cells;
· cones—scotopsin; 3 different kinds; have 1:1 ratio of cones to bipolar cells; more specific signal by conversion
· collateral involvement
· horizontal cells—lateral inhibition
· Amacrine cell—excitatory or inhibition
· Horizontal and Amacrine go w/rods
· Cones are the more evolutionary pathway
· Rods are what the lower animals have
· Less stuff covering over the cones; more efficient stimulation of the pigments
· Outer segment—contains the pigment
· Inner segment—mitochondria
· Contact w/down stream cell—cell body
· Propagation of signal
· Reverse of normal neural transfer of signal; sends depolarized signal and sends the signal
· Never start at zero; basal tone
· Visual inputs are instantaneous
· Only have to pull the brakes off; already stimulated
· Bipolar cells are spastic little things—always excited
· Under normal circumstances the rods and cones release the neurotransmitter glutamate that slows down or stops the excitation of the bipolar
· In darkness, Na+ is going in and Na+ is being pumped out
· Channel closes, Na+ going in and still being pumped out
· Allows for no delay in vision
· Conversion of vit A in the outer segment how long the cones can be hyperpolarized
· Camera flash, sponts, lack of vit A, eventually goes back to normal
Wavelengths
· Absorption and reflection of light
· Green—absorpting everything except green; green is reflected
· Blue—max at 445nm
· 505—rhodopsin—visual purple
· green—535nm
· red—570nm
· deficiency parts of the cone population
· genetic; passed through mother
· protenopes—lack red cones; shortened spectrum; green compensates
· duderope—lack green cones; virtually normal vision
· lack red and green cones—color blindnesss
· blue weakness; blue/yellow color blindness—under representation of blue cones
· adaptation—lose visual acuity in adaptation b/c cones adapt faster than rods
· -log[P]
· similar to camera flash
· 100million rods
· 3million cones
· 1.3million ganglion
· photo bleaching
Old Vision Pathway
· light and dark
· a lot of cells involved
New Vision Pathway
· direct pathway
· cone to bipolar
· 3 kinds of ganglia (W, X, Y)
· transmitter
· glutamate—rods and cones
· lysine
· dopamine
· indolamine
· Ach—bipolar
· GABA
· 23 types of cells
· Amacrin only affects the rods
· Convergence—how we process signal
· Amacrine inhibition on bipolar cells
· 100:1 convergence
· in the fovea the relationship is 1:1 of cones to bipolar cells
· W cells—responsible for rod vision, 40%, small, slow, excited exclusively by rods; affected by amacrine and bipolar cells; large dendritic field; most important under dark conditions; tract image in a direction
· X cells—55%, most predominant, medium diameter, 14msec, small dendritic field, service cones, for every X ganglion there is at least 1 cone making contact, better acuity
· Y cells—big cells, small number of them, fastest 50m/sec, huge dendritic fields, pick up rod, convergence, see and process rapidly changing images, and report abnormal events w/o; allows us to watch movies
· Color constancy—research; Edwin Land; polarid camera film