Unit 8 Asma Physiology Of Vision And Vestibular System

Front 1

State the function of the cornea:

Back 1

Because the cornea is as smooth and clear as glass but as strong and durable as plastic, it helps the eye in two ways:
The cornea provides a physical barrier that shields the inside of the eye from germs, dust, and other harmful matter. It shares this protective task with the sclera (the white of the eye).
It acts as the eye's outermost lens. When light strikes the cornea, it bends -- or refracts -- the incoming light onto the crystalline lens. The lens then focuses the light onto the retina, the paper-thin tissue at the back of the eye that starts the translation of light into vision. Most of the bending of light rays (refracting) occurs at the cornea.

Front 2

The cornea provides what % of the eye's power to bend light?

Back 2

65%

Front 3

most of the refracting power of the eye resides where and why?

Back 3

Most of this power resides in the center of the cornea, which is rounder and thinner than the outer part of the tissue and is thus better suited to bend lightwaves.

Front 4

State the function of the lens:

Back 4

The lens is a transparent body. To see clearly at near, one must increase the power of the lens; this is known as accommodation.
The parasympathetic impulse to the cilliary muscle constricts the circular part of the muscle, the zonular fibers then slacken, and the elastic capsule of the lens makes it more spherical thus increasing its dioptric power.

Front 5

State the function of the retina:

Back 5

The retina is the innermost photosensitive layer. The neurosensory retina consists of ten layers. The light sensitive elements are the rods and cones.

Front 6

State the function of the optic disk:

Back 6

Axons from the ganglion cells (retina) exit through the optic disk to the brain.
The optic disk (or blind spot) is located 15-degrees to the fovea and covers an area of 7 degrees in height and 5 degrees in width.

Front 7

State the function of the rod cells:

Back 7

Rod cells are photoreceptor cells in the eye that can function in less intense light can other type of photoreceptors (scotopic vision). Rods are more light-sensitive, and are responsible for night vision. Rods are concentrated at the outer edges of the retina and are used in peripheral vision. There are about 120 million rod cells in the human eye.

Front 8

State the function of cone cells:

Back 8

Cone cells are photoreceptor cells in the eye that best function in relatively bright light (mesopic and photopic vision). There are 4.5-6.0 million cone cells. The cone cells are mainly focused in the fovea centralis.

Front 9

List the physiological steps in the process of dark adaptation:

Back 9

Dark adaptation is the process by which the eye adjusts for maximum efficiency in low illumination. The central area of the retina dark-adapts in about 6-8 minutes (useless for night vision). The peripheral area dark-adapts in about 20-30 min. The peripheral area is not sensitive to dark-red light. Dark adaptation is an independent process in each eye (slow to develop in the dark and quickly lost in the light). Dark adaptation also depends on an adequate supply of vitamin A in the diet.

Front 10

List the effects of glare

Back 10

Glare affects vision quality and eye health. Glare is the excess of light, both visible and invisible ultra-violet radiation. Glare reduces contrast, blurs objects, causes eye fatigue, injures cells, makes for wrinkles, causes cataracts and can be so intense from a reflection that for an instant vision is blinded. All patients should be taught that glare is a problem and the effects of UV are accumulated over a lifetime.

Front 11

List the effects of retinal bleaching

Back 11

Being exposed to bright light can cause retinal bleaching which breaks down rhodopsin into opsin and retinal. Looking at a highly illuminated white light for some time markedly but temporarily reduces vision, closing the eyes regains vision.

Front 12

Describe the term refractive index and how it relates to light passing through a lens:

Back 12

The refractive index (or index of refraction) of a medium is a measure for how much the speed of light (or other waves such as sound waves) is reduced inside the medium. For example, typical glass has a refractive index of 1.5, which means that light travels at 1 / 1.5 = 0.67 times the speed in air or vacuum. Two common properties of glass and other transparent materials are directly related to their refractive index. First, light rays change direction when they cross the interface from air to the material, an effect that is used in lenses and glasses. Second, light reflects partially from surfaces that have a refractive index different from that of their surroundings

Front 13

Describe the function of convex lenses:

Back 13

Convex lens focuses light rays
Convex brings the image from behind the retina (farsighted) to the front on the retina.

Front 14

Describe the function of concave lenses:

Back 14

Concave lens diverges light rays
Concave brings the image that is too far in front of the retina (nearsighted back to the retina.

Front 15

Describe the optical principles of the eyes that allow for near and distant vision:

Back 15

The lens is held by ligaments attached to the ciliary muscles. When the ciliary muscles contract, the ligaments loosen, releasing tension on the lens; the lens, being elastic, relaxes, assuming a more spherical shape. This decreases the lens’ focal distance. Relaxation of the ciliary muscles pulls on the ligaments, which in turn increases tension from the lens, making it flatter and increasing the lens focal distance. Therefore, in order to form sharp images of distant objects on the retina, the ciliary muscle relax to flatten the lens, for near objects, the muscles contract to increase the lens’ curvature.

Front 16

Describe the fluid system of the eye:

Back 16

The eye is filled with intraocular fluid, which maintains sufficient pressure in the eyeball to keep it distended. The fluid can be divided into two portions-aqueous humor, which lies in front of the lens and the vitreous humor, which is between the posterior surface of the lens and the retina.

Front 17

Name the 10 cellular layers of the retina:

Back 17

Retinal pigment epithelium, photoreceptor layer, outer limiting member, outer nuclear layer, outer plexiform layer, inner nuclear layer, inner plexiform, glial cell layer, nerve fiber layer, and inner limiting membrane

Front 18

Describe the retinal pigment epithelium

Back 18

Retinal Pigment Epithelium-layer of the retina, 1 cell thick, contains melanin; next to choroid;layer derived from outer portion of optic cup; significant clinical importance (albinism, retinitis pigmentosa, retinal detachment)

Front 19

what is the dual role of the retinal pigment epithelium?

Back 19

Dual role:
(a) light absorption: not all of light is absorbed by the rods and cones Þ the remaining photons are absorbed by the melanin to prevent back scattering of light (very important function)
(b) photoreceptor "maintenance": cyclic phagocytosis of photoreceptors outer segments and sloughing off of membrane discs containing visual pigments

Front 20

what is the photoreceptor cleft and where is it found?

Back 20

photoreceptor cleft – cleft exists since different layers derived from outer and inner cups; found on the retinal pigment epithelium of the retina

Front 21

Where does retinal detachment occur?

Back 21

retinal detachment occurs at the photoreceptor cleft

Front 22

Describe the outer limiting membrane

Back 22

Outer Limiting Membrane – layer of the retina, formed by Glial Cells

Front 23

Describe the outer nuclear layer

Back 23

Outer Nuclear Layer – layer of the retina, cell nuclei of rods/cones; inner segment contains the cell nuclei and major organelles; segments linked via connecting stalk

Front 24

Describe the photoreceptor layer

Back 24

Photoreceptor Layer – layer of the retina, contain the sensory cells of retina (photoreceptors) – two types:
(a) rods (1 type) – high sensitivity, high amplification, saturation in daylight, low temporal resolution (much convergence)
(b) cones (3 types: red, green, blue)-low sensitivity, low amplification, saturated only in intense light, high temporal resolution photoreceptors contain parallel membrane folds with visual pigments in membrane discs (transduces light) constantly sloughed off and phagocytosed by retinal pigment epithelium

Front 25

Describe the outer plexiform layer

Back 25

Outer Plexiform Layer - synaptic layer of the retina where photoreceptors synapse bipolar cells; horizontal cells modulate synapse many photoreceptors converge on fewer bipolar cells; cones converge significantly less than rods

Front 26

Describe inner nuclear layer

Back 26

Inner Nuclear Layer – layer of the retina, bipolar cell nuclei

Front 27

What are bipolar cells?

Back 27

Bipolar cells are the site of convergence of rods and cones at the outer plexiform layer

Front 28

Describe inner plexiform layer

Back 28

Inner Plexiform – synaptic layer of the retina where bipolar cells synapse with ganlion cells; amacrine cells modulate synapse

Front 29

Of the layers of the retina, which is the where action potential is generated

Back 29

Inner plexiform layer

Front 30

Do the layers of the retina before the inner plexiform layer involve generation of action potential?

Back 30

No, all layers before this involve hyper- or depolarization

Front 31

Describe the Glial cell layer

Back 31

Glial Cell Layer-layer of the retina that projects axons to optic nerve; axons are not myelinated until they get to the optic disc

Front 32

Describe the nerve fiber layer

Back 32

Nerve Fiber Layer – optic nerve, considered to be a layer of the retina

Front 33

Describe the inner limiting membrane

Back 33

Inner Limiting Membrane - layer of the retina
Retinal Support Network – Müller cells, Astrocytes, Oligodendrocytes
Regional Specialization – at the fovea, retinal layers bend away, reducing barrier to passage of light to photoreceptors

Front 34

The highest desnsity of rods on the retina is how many degrees from the fovea?

Back 34

Highest density at 18° from the fovea

Front 35

Describe the anatomical structures of the photoreceptors:

Back 35

The retina, an evaginated portion of the embryonic brain, consists of an outer layer of pigmented epithelium and an inner layer of neural tissue. Contained within the latter layer are the sensory rod and cone cells, the bipolar and horizontal cells that comprise the intra-retinal afferent pathway from the rods and cones, and the multi-polar ganglion cells, the axons of which are the fibers of the optic nerve.

Front 36

Describe the steps in the photochemical process of vision:

Back 36

Chemical reaction initiated by absorption of energy in the form of visible (light ), ultraviolet , or infrared radiation. Primary photochemical processes occur as an immediate result, and secondary processes may follow. The most important example is photosynthesis

Front 37

Describe the six types of cells that make up the retinal neural network:

Back 37

Ganglion cells: G-cells are the large neurons conducting impulses from the retina to the brain visual centers. Axons of the G-cells make up the optic nerve.
Bipolar Cells: transmit signals from photoreceptor cells to G-cells.
Horizontal and amacrine cells have no axons. Horizontal cells are involved in inhibitory interaction among the photoreceptor cells.
Amacrine cells interact between the G-cells.
Rods and Cones are the photoreceptor cells of the retina.
Pigment Cells: layer just outside the neurosensory retina that nourishes retinal visual cells, and is firmly attached to the underlying choroid and overlying retinal visual cells.

Front 38

Describe the primary neural pathway from the retina to the visual cortex

Back 38

The visual cortex, the retinal image is represented as a more or less point-to-point projection from the lateral geniculate body, which receiveds a similar topographically structured projection from both retinas.

Front 39

Describe the function of the dorsal lateral geniculate nucleus in the neurophysiology of vision

Back 39

The lateral geniculate and the primary visual cortex are thus structurally and functionally suited for the recognition and analysis of visual images. The lateral geniculate nucleus of the thalamus is part of the brain, which is the primary processor of visual information, received from the retina, in the central nervous system.

Front 40

Describe the function of the visual cortex in the neurophysiology of vision

Back 40

The visual cortex is located in and around the calcarine fissure in the occipital lobe. It receives information directly from the lateral geniculate nucleus.

Front 41

Describe the function of the occipital cortex in the neurophysiology of vision

Back 41

The occipital lobe is the visual processing center of the mammalian brain, containing most of the anatomical region of the visual cortex. The primary visual cortex is Brodmann area 17, commonly called V1 (visual one). Human V1 is located on the medial side of the occipital lobe within the calcarine sulcus; the full extent of V1 often continues onto the posterior pole of the occipital lobe. V1 is often also called striate cortex because it can be identified by a large stripe of myelin, the Stria of Gennari. Visually driven regions outside V1 are called extrastriate cortex. There are many extrastriate regions, and these are specialized for different visual tasks, such as visuospatial processing, color discrimination and motion perception

Front 42

Describe the color detection mechanism

Back 42

Three psychologic components to color are hue, saturation, and brightness. Hue is the component denoted by naming a color, such as red, yellow, or orange. This is closely related to the wavelength of the light. Saturation refers to adding white light to the pure color so as to decrease the saturation of this color. For instance, a spectral red becomes pink when it is mixed with white light.

Front 43

What are the 3 types of eye movements?

Back 43

Saccadic, smooth pursuit movements and vergence movements.

Front 44

Describe saccadic eye movements

Back 44

Saccadic: are rapid, ballistic movements of the eyes that abruptly change the point of fixation.

Front 45

Describe smooth pursuit eye movements

Back 45

Smooth pursuit movements are much slower tracking movements of the eyes designed to keep a moving stimulus on the fovea. Such movements are under voluntary control in the sense that the observer can choose whether or not to track a moving stimulus

Front 46

Describe vergence eye movements

Back 46

Vergence movements: align the fovea of each eye with targets located at different distances from the observer. Unlike other types of eye movements in which the two eyes move in the same direction (conjugate eye movements), vergence movements are disconjugate (or disjunctive); they involve either a convergence or divergence of the lines of sight of each eye to see an object that is nearer or farther away.

Front 47

What do vestibulo-ocular movements do?

Back 47

Vestibulo-ocular movements stabilize the eyes relative to the external world, thus compensating for head movements. These reflex responses prevent visual images from “slipping” on the surface of the retina as head position varies.

Front 48

Describe the mechanisms for visual accommodation (focusing)

Back 48

The lens is composed of a strong elastic capsule filled with viscous, proteinaceous, but transparent fluid. When the lens is in a relaxed state with no tension on its capsule, it assumes an almost spherical shape, owing mainly to the elastic retraction of the lens capsule. However, about 70 suspensory ligaments attach radially around the lens, pulling the lens edges toward the outer circle of the eyeball. These ligaments are constantly tensed by their attachements at the anterior border of the choroid and retina. The tension on the ligaments cause the lens to remain relatively flat under normal conditions of the eye.

Front 49

List physiological/Psychological concerns specific to the focal mode of visual processing

Back 49

Vision is a complex physiologic and psychologic process that necessitates a decoding or interpretation of signals coming from the sensor (the eye) to the brain. Environmental stresses may disrupt the delicate physiologic balance necessary for maintaining clear vision and are discussed in ensuing sections

Front 50

List physiological/Psychological concerns specific to the Ambient Mode of visual processing

Back 50

Vision is a complex physiologic and psychologic process that necessitates a decoding or interpretation of signals coming from the sensor (the eye) to the brain. Environmental stresses may disrupt the delicate physiologic balance necessary for maintaining clear vision and are discussed in ensuing sections.

Front 51

Describe the anatomical components of the middle ear

Back 51

The middle ear (ME) and contents serve to transmit external sound energy to the components of the inner ear. Sound pressure applied to the ear drum (tympanum) creates motion which is transmitted through the ossicles to the oval window. The oval window membrane is the interface with the inner ear. The inner ear is where motion is translated to neural signals.The cochlea is a system of coiled tubes. It consists of three tubes coiled side by side

Front 52

List the 3 parts of the functional anatomy of the vestibular apparatus

Back 52

Semicircular canals, utricle, and saccule

Front 53

Describe the semicircular canals

Back 53

Semicircular canals: The 3 semicircular canals are small ringlike structures: lateral or horizontal, superior or anterior, and posterior or inferior. They are oriented at right angles to each other and are situated so that the superior and posterior canals are at 45° angles to the sagittal plane, and the horizontal canal is 30&3176; to the axial plane. Each canal is maximally responsive to angular motion in the plane in which it is situated and is paired with a canal on the contralateral sized so that stimuli that are excitatory to one are inhibitory to the other.

Front 54

Describe the utricle

Back 54

Utricle: The utricle is larger than the saccule and lies posterosuperiorly to it in the elliptical recess of the medial wall of the vestibule. It is connected anteriorly via the utriculosaccular duct to the endolymphatic duct. The 3 semicircular canals open into it by means of 5 openings; the posterior and the superior semicircular canals share one opening at the crus commune

Front 55

Describe the saccule

Back 55

Saccule: The saccule is an almost globular-shaped sac that lies in the spherical recess on the medial wall of the vestibule. It is connected anteriorly to the cochlear duct by the ductus reuniens and posteriorly to the endolymphatic duct via the utriculosaccular duct. The saccular macula is an elliptical thickened area of sensory epithelium that lies on the anterior vertical wall of the saccule

Front 56

Describe the mechanism for sensory production of the saccule and utricle

Back 56

In the vestibule lie the two otolith organs, the utricle and the saccule. They translate gravitational and inertial forces into spatial orientation information – specifically, information about the angular position (tilt) and linear motion of the head. They are in effect, linear accelerometers.

Front 57

Describe the mechanism for sensory function of the semi-circular canals

Back 57

The semicircular ducts, contained in the semicircular canals, convert inertial torques into information about angular motion of the head. They function as angular accelerometers

Front 58

Describe vestibular reflex action

Back 58

In humans, the retinal image is stabilized mainly by vestibuloocular reflexes, primarily those of semicircular-duct origin

Front 59

Describe the practical relationship between the semicircular canals and orientation

Back 59

Semi-circular canals convert inertial torques into information about angular motion of the head. They function as angular accelerometers.

Front 60

Describe the practical relationship between the otilith organs and orientation

Back 60

Otilith organs function as liner accelerometers.