Flashcards in Vision Physiology Deck (78):
What is the function of the eye?
The function of the eye is to transform light energy into nerve signals that can be transmitted to the cerebral cortex for interpretation.
What is the pupillary relfex controlled by?
1. Autonomic nervous system
2. Specifically the Cranial nerve II and III
What is the function of the sphincter pupillae eye?
How does it do this?
constrict pupil (circular muscle) when exposed to
close vision and bright light
What is the funciton of the dilator pupillae?
How does it do this?
pupil dilates (radial muscle) when exposed to
distant vision and dim light
What happens in the pupillary reflex when the parasympatheic nerves are stimulated?
Excites the pupillary sphincter muscle thereby decreasing the pupillary aperture (constricting the pupil) (myosis)
What happens in the pupillary reflex when the sympathetic nerves are stimulated?
Excites the dilator pupillae-conversely, dilates the pupil (mydriasis)
What stimulates constriciton/dilation of the pupil?
Light- pupillary light reflex
What is the function of the pupillary light relfex?
Functions to allow adaption with rapid changes of light
What are we testing with a direct response test?
To see if the illuminated eye constricts normally in response to light
If the direct response test is abnormal what could be wrong?
1. lesions of the ipsilateral optic nerve,
2. Lesions of the pretectal area,
3. lesions of the ipsilateral parasympathetics traveling in CN III,
4. Lesions of the pupillary constrictor muscle of the iris.
What are we testing with a consensual response test?
Since there is crossing over of the nerves in the eye there will be a pupil constriction in the opposite eye as well.
If the consenual response test is abnormal what could be wrong?
1. lesions of the contralateral opic nerve
2. lesions of the pretectal area
3. lesions of the ipsilateral parasympathetics traveling in CNIII
4. lesion of the pupillary restrcitor muscle
What are we testing with a accomodation test?
To see if the pupil constricts when we move an object from far away to very near the eyes.
If the accomodation test is abnormal what could be wrong?
1. lesions of the ipsilateral optic nerve
2. lesions of the the ipsilateral parasympathetics traveling in CN III
3. lesions of the the pupillary constrictor muscles
4. bilateral lesions of the pathways from the optic tracts to the visual cortex
What happens in an afferet pupillary defect?
a decreased direct response caused by decreased visual function in one eye
How do we test for an afferent pupillary defect?
swinging flashlight test (move light back and forth every two/three sec)
-It is a positve test for the disease if the affected pupil dilates in response to light
What are some diseases/toxins that can block the pupillary light reflex?
What is the use of both eyes to look steadily in one direction?
What are saccadic eye movements?
Consists of small jumping movements that represent rapid shift in conjugate gaze orientation
What is the purpose of binocular fusion?
adjustment of the orientation of each eye to produce a single image
What is an avascular transparent biconvex body called?
Which side of the lens of the eye is more convex?
posterior side is more convex than the anterior side
What holds the lens in place and allows it to change shape?
(if you remove it, the lens will look like a golf ball. Needs to change shape to focus)
What is the lens formed by?
Formed from fibers called crystallins arranged in concentric layers like onion rings
Pathology of cataracts
Lens becomes thicker, less flexible and less transparent as you age. Also age-related changes in the tissue cause the lens to break down and clump together (crystallines), clouding small areas in the lens. This clouding becomes denser and involves a greater part of the lens.
Parasympathetic activity of the cillary muscle results in?
contraction of the cililary muscles and lens bulges for close vision
Sympathetic activity of the cillary muscle results in?
relaxation of the cillary muscles and lens flattening for far vision
In a convex lens how will the light refract when it hits the lens?
The light rays will hit the lens and converge at the focal point
Where will the rays bend on a convex lens?
as the rays enter and exit the lens
In a concave lens how will the light refract when it hits the lens?
The light rays will hit the lens and diverge
Where will the rays bend on a concave lens?
as the rays enter and exit the lens
What will making the light rays hit further (not parallel) from the center of the lens cause?
Convex= closer focal point
Convace= greater divergence
How does the strength (ability to bulge) of the lens affect point focus?
the nearer the point focus will be to the lens (lens has more refractive power)
What is the process by which a clear image is maintained as gaze is shifted from afar to a near object?
What part of the CNS is in control of accommodation?
Parasympathetic portion of CNIII
Who does accommodation not occur in?
the totally blind, in sleep or a comatose person because visual function must be present to evaluate and adjust the clarity of the image.
what is the focusing surface of the eye?
The ability to adjust the refractive power of the lens is called?
What is the normal state of the lens?
constantly in tension (suspensory ligaments)
The lens remains relatively flat under normal circumtances
What are the two sets of smooth muscle fibers that make up the cilliary muscle?
Cillary body contracts what happens?
peripheral insertions of the lens ligamnets relax thereby releasing tension on the lens = more spherical shape (which is the natural elasticity of the lens capsule)
What part of the nervous system controls the cillary muscle?
almost entirely by the parasympathetic nerve signals transmitted to the eye through the third cranial nerve (CNIII)
There is almost no sympatheic stimulation with nomral accomodation in the lens (It plays a large role in the fight or flight response)
Name the steps of accomodation of the lens.
1. parasympathetic nerve signal
2. contraction of ciliary muscle fibers
3. relaxation of lens ligaments
4. lens assumes a more spherical shape
5. increased refractive power
6. ABILITY TO FOCUS ON NEARER OBJECTS!
What is emmetropia?
Distant objects can be seen clearly when the cillary muscle is what?
The eye must contract cillary muscle to accomodate for objects at close range
What is presbyopia?
- lens grows larger and thicker
- lens becomes less elastic
-ability to change shape become difficult
Farsighted because the person cant cillary muscles cant affect the shape of the lens as well. Cant see close up things
What is the term for farsightedness?
What is the pathology with hyperopia?
What kind of corrective lens would they need?
eyeball is too short so the focal point is behind the retina
What is the pathology with myopia?
What kind of corrective lens would they need?
eyeball is too long so focal point is in front of the retina
What will lens accomodation in myopia and hyperopia change things?
hyperopia = it will refract the light more and focus the image (better)
Mypoia = will only make things worse
What is the problem with astigmatism?
It is shaped more like a football than a sphere so when light enters the eye it is refracted more in one direction than the other, alllowing only one part of the object to be focused at a time
The color a person perceives is dependant on what?
on which set of cones or combination of sets of cones are stimulated in a given image
Disease of the more peripheral retina affects what color?
Disease of the more central retina affects what color?
red and green (blue is not present in the central fovea)
What is depth perception determined by?
Sizes of the images of known objects
Phenomenon of moving parallax
Phenomenon of stereopsis (binocular vision)
What is moving parallax?
Relative distances of different objects can be determined by the extent with which they move when a person moves his or her head to one side or the other
Images close by? Move rapidly across retina
Images far away? No perceptible movement
(seem to go slower)
What is stereopsis?
binocular vision (the closer an object is the further separated from the retina it is)
(two images to make one central one)
Where does the aqueous humor lie?
in front of the lens
(free flowing fluid)
Where does the vitreous humor lie?
behind the lens
What color is the iris?
brown (blue and green eyes just have less pigment in them)
What is a main difference between aqueous humor and vitreous humor? (besides consistency)
Aqueous is continually being formed and reformed where as the vitreous humor is formed at birth and you have it for life
Follow the flow of aqueous humor through the eye
1. Flows through pupil into anterior chamber
2. Then into the angle between the cornea and iris
3. Through meshwork of trabeculae
4. Enters Canal of Schlemm
5. Empties into extraocular veins
What forms the aqueous humor?
What is average intraocular pressure?
12-20 mm Hg
What is the pressure in the eye determined by?
by resistance to outflow of aqueous humor from the anterior chamber into the canal of Schlemm
(The amount leaving via the canal of Schlemm usually equals the inflow of fluid from the ciliary body)
What is the pathology behind glaucoma?
Disease in which the intraocular pressure becomes pathologically high (sometimes rising to 60-70 mm Hg)
Sometimes low rises up to 25-20mm Hg can lead to loss of vision
Why does the pressure become high in glaucoma?
usually from increased resistance to fluid outflow thorugh the trabecular spaces into the canal of Schlemm
What is the treatment for glaucoma?
1. decrease secretion
2. increase absorption
What meet and fuse at the optic chiasma?
One tract contain fibers from where?
From both eyes. A nasal retina and a temporal retina from each eye so that we can transmit info from the same visual field)
At the chiasm what optic nerve fibers cross over to the other side?
nasal half of the retina
Nerve fibers originating the temporal retina do not cross
What implications would a pituitary tumor have on nerve fibers?
It wouold affect the central most fibers (nasal fibers) thus affecting peripheral vision
What is visual info carried to the brain by?
axons of the retinal ganglion cells, which form the optic nerve.
The optic nerve represents an outgrowth of the brain rather than a peripheral
Trace the path of the optic nerve out of the eye
Exiting the optic globe and the orbit through the optic foremen, traverse the floor of the middle fossa to the optic chiasm at the base of the brain.
Where is the visual cortex located?
What would be a patients visual distrubance if you severed their left optic tract?
the peripheral (temporal) vision of the right eye and the central (nasal) vision of the left eye
What would a patient’s visual disturbance be if you severed their right optic nerve?
The entirity of the right eye