EYE PHYSIOLOGY Flashcards
Eye Function
The function of the eye is to transform light energy into nerve signals that can be transmitted to the cerebral cortex for interpretation.
Pupillary reflex tests
Automonic nervous sytems
Cranial nerve II AND III
Control of Pupil Diameter
Stimulation of the parasympathetic nerves EXCITES the pupillary sphincter muscle, thereby decreasing the pupillary aperture (miosis)
Sympathetic stimulation, conversely, dilates the pupil (mydriasis)
Light shine constricts pupils (pupillary light reflex)
Functions to allow adaptation with rapid changes of light
Direct response (pupil illuminated)
The direct response is impaired in lesions of the ipsilateral optic nerve, the pretectal area, the ipsilateral parasympathetics traveling in CN III, or the pupillary constrictor muscle of the iris.
Consensual response (contralateral pupil illuminated
The consensual response is impaired in lesions of the contralateral optic nerve, the pretectal area, the ipsilateral parasympathetics traveling in CN III, or the pupillary constrictor muscle.
Accommodation (response to looking at something moving toward the eye)
Accommodation is impaired in lesions of the ipsilateral optic nerve, the ipsilateral parasympathetics traveling in CN III, or the pupillary constrictor muscle, or in bilateral lesions of the pathways from the optic tracts to the visual cortex. Accommodation is spared in lesions of the pretectal area.
Afferent pupillary defect
In an afferent pupillary defect there is a decreased direct response caused by decreased visual function in one eye.
This can be demonstrated with the swinging flashlight test, in which the light is moved back and forth between the eyes every two to three seconds.
The afferent pupillary defect becomes obvious when the flashlight is moved from the normal to the affected eye, and the affected pupil dilates in response to light.
Under normal conditions, the pupil constricts in response to light. Brief oscillations of pupillary size called hippus occur normally in response to light which should not be confused with an afferent pupillary defect.
CNS disease and Pupillary Reflexes
Some toxins & CNS diseases can block the pupillary reflex… these include but are not limited to:
Alcoholism
Encephalitis
CNS syphilis
Extraocular Muscles:Binocular vision depends on three pairs of extraocular muscles
Medial and lateral recti Move the eye from side to side Superior and inferior recti Move the eye up and down Superior and inferior obliques Rotate the eye around its optical axis
Extraocular muscles are innervated by three pairs of cranial nerves
Occulomotor (CNIII)
Trochlear (CN IV)
Abducens (CN VI)
Oculomotor nerve (CN III)
Innervates the medical rectus Turns the eye medially Innervates the superior rectus Elevates the eye and rolls it upward Innervates the inferior rectus Depresses the eye and rolls it downward Innervates the inferior oblique Elevates the eye and turns it laterally
Trochlear nerve (CN IV)
Innervates the superior oblique and turns the eye downward and laterally
Abducens nerve (CN VI)
Innervates the lateral rectus and moves the eye laterally
Cross Eyes
These folks have superior Superior Oblique and Medial Rectus Control
Full visual function
It is necessary that the two eyes point toward the same fixation point and the two images become focused
Binocular fusion is controlled by ocular reflex mechanisms that adjust the orientation of each eye to produce a single image
Conjugate gaze
Refers to the use of both eyes to look steadily in one direction
Saccadic eye movements
Consists of small jumping movements that represent rapid shift in conjugate gaze orientation
Nystagmus
The sequence of SLOW ocular rotation
LENS
An avascular transparent biconvex body.
Posterior side is more convex than the anterior side.
Elastic capsule holds lens in place, allows lens to change shape.
Sympathetic Activation
Lens is flattened for distant vision. Sympatheticinput relaxes the ciliary muscle, tightening the ciliary zonule, and flattening the lens.
Parasympathetic Activation
Lens bulges for close vision. Parasympathetic
input contracts the ciliary muscle, loosening the
ciliary zonule, allowing the lens to bulge.
Refraction of Light
When light rays strike an interface that is perpendicular to the beam, rays do not deviate from course
When light rays strike an interface that is angulated, the rays bend
The amount the rays bend depend the difference between the refractive indices of the respective mediums
The greater the difference in refractive index, the more the ray will bend
Refractive Lens: Convex
Convex Lens:
At the center, the light ray will strike perpendicular and therefore will NOT diverge
The further from the center, the more the angulation… and therefore with a perfect convex lens all of the rays can be focused (convergence) on one spot (focal point)
Bending occurs both:
As the rays enter the lens
As the rays exit the lens
Refractive Lens: Concave
Concave Lens:
At the center, the light ray will strike perpendicular and therefore will NOT diverge
The further from the center, the more the angulation… and therefore the more the rays spread apart (divergence)
Bending occurs both:
As the rays enter the lens
As the rays exit the lens
Is focal Length the same thing as Focal Point?
No. Focal length is NOT the same thing as Focal point.The retina doesn’t move and therefore the lens must change in order to focus the image with change of distance
Accomodation of Lens
Accommodation is the process by which a clear image is maintained as gaze is shifted from afar to a near object.
Requires convergence of the eyes. Pupillary constriction and thickening of the lens through contraction of the ciliary muscle.
Parasympathetic portion of CNIII is in control.
Accommodation does not occur in the totally blind, during sleep or in the comatose person because visual function must be present to evaluate and adjust the clarity of the image.
Accomodation of Lens Cont
Accommodation
The focusing surface of the eye is the retina
It is at a fixed distance from the lens
Adjustability in the refractive power of the lens is needed to keep the image of close objects in focus to the retina
The ability to adjust the refractive power of the lens is… ACCOMMODATION.
Accomodation of lense: How does lense work?
So how does the lens do its business?
If the lens of the eye did not have constant tension, it would assume almost a spherical shape
However, there is a mechanism by which the lens edges are pulled towards the outer circle of the eyeball
This occurs via 70 suspensory ligaments that attach radially around the lens
The constant tension (this is the normal state!) causes the lens to remain relatively flat under normal conditions of the eye
Accomodation of Lense: How lense works continued.
Also located at the lateral attachments of the lens ligaments is the ciliary muscle, which is made up of two separate sets of smooth muscle fibers
Meridional fibers
Circular fibers
… when the ciliary body contracts, the peripheral insertions of the lens ligaments are pulled medially toward the edges of the cornea, thereby RELEASING TENSION ON THE LENS
The end result then is the lens assuming a MORE SPHERICAL SHAPE (because of the natural elasticity of the lens capsule)
What controls the Ciliary Muscle?
The ciliary muscle is controlled almost entirely by parasympathetic nerve signals transmitted to the eye through the third cranial nerve (CN III).
SYMPATHETIC STIMULATION plays only a minimal role on affecting the ciliary muscle! In fact the effect is so weak that it plays ALMOST NO ROLE in the normal accommodation mechanism!
Accommodation of Lense: Flow Chart
Parasympathetic nerve Signal–>Contraction of ciliary muscle fibers–>Relaxation of lense ligaments–>Lense assumes a more spherical shape–>Increased refractive power–>ability to focus on nearer objects
Emmetropia (Normal Vision)
Emmetropia is a fancy word for “normal vision” and is defined as:
When parallel light rays from distant objects are in sharp focus on the retina when the ciliary muscle is completely relaxed
Distant objects can be seen clearly when ciliary muscle relaxed
Eye must contract ciliary muscle to accommodate for objects at close range
Errors of Refraction: Presbyopia (Old Eyes)
Presbyopia (Old eyes):
As a person grows older
Lens grows larger and thicker
Lens becomes far less elastic
Ability of the lens to change shape decreases
Power of accomodation decreases to almost 0 diopters by the age of 70
This results in the eye being focused at an almost constant distance congratulations gramps, you can see neither near nor far, let’s get you some bifocals!
Errors of Refraction: Hyperopia (Farsightedness)
Eyeball is too short so focal point is behind the retina
By using the mechanism of accommodation, these folks are capable of focusing distant objects on the retina
If the person did not have to accommodate much for distant objects, they’ll still have some gumption left over to focus on near objects
Eventually however, the loss of accommodating power does not allow for this person to focus on close objects
“HONEY, I need my reading glasses!”
Once you hit that presbyopia stage this person can’t even accommodate for distant objects back to gramps needing bifocals
Errors of Refraction: Myopia (Nearsightedness):
Eyeball is too long so focal point in front of retina
YOU CAN’T RELAX THE CILIARY MUSCLE ANY MORE TO EXTEND THE FOCAL POINT BACK ANY FURTHER!!
BUT, when an object comes nearer, it finally gets close enough that its image can be focused
Refractive Errors: Astigmatism
Remember how we talked about the importance of the lens being spherical?
Well, all astigmatism is is when the image in one plane focuses at a different distance from that of the plane at right angles
Basically, it’s the golfball (normal lens) versus the egg (astigmatic lens)
***Because accomodation manipulates the entire egg, no degree of accomodation can correct for the refractive error
Rods and Cones
Rods: black and white
Cones: color
Light and dark adaptation
Most people think it just has to do with the amount of light allowed in (this is incorrect)
Cones and Color Sensitivity
Cone receptors that are selectively sensitive to different wavelengths of light provide the basis for color vision.
Three types of cones or cone color systems, respond to the blue, green and red portions of the visible electromagnetic spectrum
The color a person perceives depends on which set of cones or combination of sets of cones is stimulated in given image
Color Blindness
Misnomer for a condition in which persons
Appear to confuse or mismatch colors
Or experience reduced acuity for color discrimination.
Unaware of defect until they attempt to discriminate between red and green traffic lights or show difficulty matching colors.
Genetic defect in one or more of the three color cone mechanisms.
Usually partial but can be complete.
If a person had NO color mechanism, how would they see the world?
Interesting facts about the color-blind
Make color discrimination based on other criteria, such as brightness or position.
Give me an example of position
Males are more frequently affected with red, green, or red-green color-blindness.
Disease of the more peripheral retina affects blue.
Disease of the more central retina affects red and green because blue cones are not present in the central fovea.
Depth Perception: Determined by 3 factors
Determined by three major means:
Sizes of the images of known objects
Phenomenon of moving parallax
Phenomenon of stereopsis (binocular vision)
Depth Perception Cont: Sizes of the images of known objects
Sizes of the images of known objects
If you know the person is 6 feet tall, you can tell how far away they are simply by the size of that person on the retina
The brain just calculates this automatically
This is great for a person with unilateral blindness!
Depth Perception: Moving Parallax
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
Depth Perception: Stereopsis
Stereopsis – Binocular Vision
The distance of an object will determine the relative locations on the retina for each eye
The closer, the further separated on the retina
Fluid System of the Eye: Aqueous + Vitreous Humor
Eye is filled with intraocular fluid
Maintains sufficient pressure to keep eyeball distended
Two portions:
Aqueous humor: lies in front of the lens
Freely flowing fluid
Vitreous humor: lies between posterior surface of the lens and the retina
Gelatinous mass (water and dissolved substances can diffuse very slowly in the vitreous humor, but there is little flow of fluid)
Aqueous Humor
Continually being formed and reabsorbed
***Balance between formation and reabsorption of aqueous humor regulates the total volume and pressure of the intraocular fluid
Formation
Ciliary body forms aqueous humor
2-3 microliters per minute
Outflow
Flows through pupil into anterior chamber
Then into the angle between the cornea and iris
Through meshwork of trabeculae
Enters Canal of Schlemm
Empties into extraocular veins
Intraocular Pressure
Average normal intraocular pressure is about 12-20mm Hg
Pressure remains constant in the normal eye
***Pressure determined mainly 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
Glaucoma
One of the most common causes of blindness
Disease in which the intraocular pressure becomes pathologically high (sometimes rising to 60-70 mm Hg)
Pressures of 25-30 mm Hg for long periods can lead to loss of vision however
In most cases, this results from increased resistance to fluid outflow through the trabecular spaces into the canal of Schlemm
Treatment: decrease secretion or increase absorption (if this fails there are surgical techniques to facilitate fluid flow)
Tonometry
As you can see in this picture the PA misplaced the tonometer and applied an inappropriate amount of force
Extraocular Muscles: 3 Muscles for Binocular Vision
Binocular vision depends on three pairs of extraocular muscles
Medial and lateral recti Move the eye from side to side Superior and inferior recti Move the eye up and down Superior and inferior obliques Rotate the eye around its optical axis
EO Muscles:
Medial and lateral recti movements
Move the eye from side to side
EO Muscles: Superior and inferior recti movement
Move the eye up and down
EO Muscles: Superior and inferior obliques
movement
Rotate the eye around its optical axis
3 Cranial Nerves for Extraocular Muscles
Occulomotor (CNIII)
Trochlear (CN IV)
Abducens (CN VI)
Occulomotor Nerve CN III
Innervates the medical rectus Turns the eye medially Innervates the superior rectus Elevates the eye and rolls it upward Innervates the inferior rectus Depresses the eye and rolls it downward Innervates the inferior oblique Elevates the eye and turns it laterally
Trochlear nerve (CN IV)
Innervates the superior oblique and turns the eye downward and laterally
Abducens nerve (CN VI)
Innervates the lateral rectus and moves the eye laterally
Full visual function
It is necessary that the two eyes point toward the same fixation point and the two images become focused
Binocular fusion is controlled by ocular reflex mechanisms that adjust the orientation of each eye to produce a single image
Conjugate gaze
Refers to the use of both eyes to look steadily in one direction
Saccadic eye movements
Consists of small jumping movements that represent rapid shift in conjugate gaze orientation
Nystagmus
The sequence of SLOW eye movement, followed by a rapid, saccadic, movement in the opposite direction.
Optic Chiasm
The two optic nerves meet and fuse at the optic chiasm.
Axons from the nasal retina of each eye cross to the opposite side and join the axons of the temporal retina of the contralateral eye to form the optic tracts.
One tract contains fibers from both eyes that transmit info from the same visual field.
Optic Chiasm
Optic nerve travels to the optic chiasm located just in from and below the pituitary gland.
At the chiasm optic nerve fibers from the nasal half of each retina cross over to the other side.
Nerve fibers originating the temporal retina do not cross over.
What implications would a pituitary tumor have?
Nerve pathways: What is required for perfect sight
Full visual function requires the normally developed brain-related photoreaction and the pupillary reflex.
Functions depend on the integrity of all visual pathways, including retinal circuitry and the pathway from the optic nerve to the visual cortex and other visual regions of the brain and brainstem.
Nerve pathways
Visual info is 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 nerve
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.
Visual fields of 2 eyes
The visual fields of the two eyes overlap considerably.
Note that fibers from the lateral portion of each retinal field do
not cross at the optic chiasma.
Visual cortex
Located in the occipital lobe
How is this information disseminated?
Physical separation of the info from the left and right visual fields is maintained in the visual cortex.
