Pt9 Vision Flashcards
Describe the visual system of a dog.
-binocular vision & wider periphery
-image of object goes to area of centralis = retinal area that provides the greatest acuity
Describe the choroid.
-loose CT
-pigmented cells
-vasculature
-nutritive
-diurnal animals = melanocytes that absorb light passed by photoreceptors without stimulating them
-nocturnal & domestics = patch of reflective material “tapetum lucidum”
>enhances dark vision under dim light
Describe the iris.
-pigmented structure
-dilator & sphincter muscles
behind iris = lens
Describe the pupillary dilator muscle.
> radially arranged
opposes action of sphincter
part of pigmented anterior epithelial
contraction = dilation “mydriasis”
dilation = sympathetic (pain, fear, anger)
Describe the pupillary sphincter muscle.
-circular arrangement near pupil
-contraction = decrease in pupil size “miosis”
-innervated by parasympathetic fibers
size of pupil regulates amount of light entering eye
Describe the lens of the eye.
-suspended by suspensory ligaments
-fibers attached to ciliary body
-muscle near base of iris
>increase/decrease tension on the lens
>make lens curve more/less convex to focus near/far
Describe the vitreous humor.
-behind lens
-gelatinous fluid = gives spherical shape of eye
-phagocytic cells
-behind vitreous humor is the retina
Describe the retina.
-where light is transformed into electrical activity of neurons
-interrupted where axons of retina ganglion cell layer leave eye on their way to brain = optic disc/blind spot = optic nerve (CN II)
> arteries & veins enter retina at optic disc
> provide nutrition of retina & vessels at choroid
-more axons in both optic nerves than in dorsal root of spinal cord
-specialized sensory epithelium = photoreceptors & other cells arranged in layers
What are the 2 types of photoreceptor cells?
cone & rod
OUTER SEGMENT:
-photosensitive region
-membranous invaginations in cones
-flattened membranous sacs in rods (stack of coins)
-membrane of both contain photopigments
>convert light stimulus to receptor potential
INNER SEGMENT:
-metabolic region
Describe rod cells.
-rhodopsin
>photochemical neurotransmitter for perception of shades of grey
>low threshold of excitability (high sensitivity)
~stimulated by low intensity light
>night vision
>95% of photoreceptor in retina
Vit A imp for formation of rhodopsin
>deficiency = night blindness
Describe cone cells.
color/cone pigments
>photochemical neurotransmitter = perception of color
>requires high intensity light
-3 diff types of cones in primates
each one a diff color pigment & absorptions/wavelengths
>blue, green, red = polychromatic vision
(the visual system mix & contrast effect of each cone cell)
Describe the photoreceptor cells of most mammals.
-2 types of cones = dichromatic
>detect blue & yellow (no reds/oranges)
-birds, lizards, turtles, fish = 4 types
>3 types + UV sensitive cone
>richer color perception
(Color is brains interpretation of differences in wavelengths of light)
What are the 5 layers of the retina?
- Photoreceptor
- Horizontal
- Bipolar
- Amacrine
- Ganglion
all the cells cause distortion of light rays = fovea (area centralis) minimizes distortion
Describe the fovea of the retina.
-center of retina “area centralis”
-acute/detailed vision
-composed of mostly cones
>other cells displaced to allow light to pass unimpeded to cones
Compare rods & cones.
Describe the activation of photoreceptors by light.
-photoreceptors & ganglion = receptor potentials
>retinal neurons conduct visual signals by electric current = electronic conduction
>degree of hyperpolarization in rod & cone flow to cytoplasm to the synaptic body
~neurotransmitter released & transmitted to next cell
-allow graded conduction of signal strength
>strength of hyperpolarizing output signal related to intensity of illumination
>not all or none
Describe the pathway of the photoreceptors in the dark.
not sending signals = depol
1. Dark = cGMP phosphodieserase is inactive (PDE6) enzyme
2. CGMP bind to ligand gated Na/Ca ion channels
3. Leakage of cations into rod = depol
4. Ca transported out by anti porter
5. Energy from Na/K down gradient
6. Force Ca against gradient
Describe visual acuity.
Acuity of visual image reflects many factors
>pop of retinal cells
>ratio of rods to cone photoreceptor cells
>ratio of photoreceptor cells to ganglion cells
Cone cells have better acuity than rods
>100s of rod signals via bipolar cells to a single ganglion
>few cone signals to single ganglion
*more ganglion in area centralis than peripheral portion of retina
>area centralis = highest acuity in retina
(More ganglion = high acuity)
How are images formed?
*eye catches light reflected by objects & guide passage until image is formed
*sequence of transparent media causes light to be refracted (direction change)
1. Air & anterior surface of cornea
2. Posterior surface of cornea & aqueous humor
3. Aqueous humor & anterior surface of lens
4. Posterior surface of lens & vitreous humor
What controls the amount of light that passes through the pupil?
Iris!
-dark = dilated pupil = mydriasis
-light = contracted pupil = miosis
How do the lens work when forming an image?
-converges light rays to a focal point on retina
-light rays = unobstructed access to photoreceptors
-central retina = area of highest visual acuity
-no distinct fovea
How are images on the retina formed?
-real, inverted, & smaller than the object
-distance between the light rays emanating from the top & bottom of tree get closer together as they approach the eye - at some point they cross & reverse
-by reversing & decreasing the image, the eye can visualize objects much larger than it
How are the images formed on the retina transformed?
-by cones & rods into nerve impulses
-visual signals carried by optic nerve fibers that form optic nerve, optic chiasm, & optic tract
-fibers terminate in the thalamus for visual perception (lateral geniculate nucleus) or mid brain for visual reflexes
EX: body & ocular reflex, pupillary constriction
brain processes & reorients image
