vision physiology Flashcards
Pupillary reflex
Automonic nervous sytems
Cranial nerve II AND III
What causes miosis (decreasing the pupillary aperture?
Stimulation of the parasympathetic nerves EXCITES the pupillary sphincter muscle
What causes mydriasis (dilation of the pupil)
Sympathetic stimulation
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.
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
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 laterall
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
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
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
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
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)
Is Focal length the same thing as Focal point?
No
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.
Accommodation
The focusing surface of the eye is the retina
It is at a fixed distance from the lens
The ability to adjust the refractive power of the lens is… ACCOMMODATION.
Emmetropia
normal vision
Presbyopia
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
Hyperopia
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
Myopia
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!!
Astigmatism
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
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
Depth Perception
Sizes of the images of known objects
Phenomenon of moving parallax
Phenomenon of stereopsis (binocular vision)
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
Aqueous Humor
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
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
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.
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
