Reading...
Play button
Play button
Use LEFT and RIGHT arrow keys to navigate between flashcards;
Use UP and DOWN arrow keys to flip the card;
H to show hint;
A reads text to speech;
91 Cards in this Set
- Front
- Back
|
Describe the visual system pathway
|
-Retina
-Optic nerve -Optic chiasm (half of info crosses) -Optic tract -Lateral geniculate -Visual/Optic radiations -Primary visual cortex -"Higher" visual cortex |
|
|
How is the visual world represented throughout the visual pathways?
|
Upside down and backwards
|
|
|
What is the sclera?
|
Tough, fibrous, outer white layer of eyeball
-gives it shape and rigidity -continuous with dura of optic nerve posteriorly |
|
|
What is the cornea?
|
Clear first layer of front part of eye (continuous with sclera anteriorly)
-tightly packed lamella structure of collagen gives it transparency and good optical lens properties |
|
|
What is the choroid?
|
Middle layer of eye
-highly vascularized -supplies blood to posterior layer of retina -blood supplied from small ciliary arteries that penetrate globe (branches of ophthlamic artery) |
|
|
What is the retina?
|
Innermost layer of eye that contains the photoreceptors
-Contains three layers of cells (receptor layer with rods & cones, bipolar layer with neurons, ganglion cell layer) -Ganglion cells send axons across the retina in the "nerve fiber layer" to the optic disk, where the axons exit posteriorly to form the optic nerve |
|
|
What is the macula lutea?
|
Central depression in the retina coated with yellow pigment which contains the fovea centralis (zone of tightly packed cones providing highest visual acuity)
|
|
|
What is the central retinal artery?
|
A branch of the ophthalmic artery which enters the optic nerve, runs up the center of the nerve and enters the eye to emerge at the optic disk.
-It divides and branches to supply blood to the neuronal layers of the retina (bipolar/ganglion) |
|
|
What is the optic disk?
|
Contains exiting nerve fibers that form optic nerve, as well as central retinal artery and vein
-produces blind spot in visual field because no photoreceptors in the disk -papilledema is edema of the optic disk (occurs with increased intracranial pressure, increased pressure in CSF surrounding optic nerve, compression of central retinal vein, swelling and edema of disk and veins at optic disk |
|
|
Describe the path of light through the eye
|
-Cornea (fixed refractive power)
-Anterior chamber (filled with aqueous humor -Adjustable pupil of iris -Crystalline lens (adjustable refractive power) -Vitreous body (gel that fills entire eyeball behind lens) -Retinal blood vessels -Nerve fiber layer -Ganglion & bipolar layers -Rod and cones (where light is transduced into receptor potentials) |
|
|
What is the ciliary muscle?
|
A part of the ciliary body that adjusts the tension on the zonule fibers that suspend the lens
-this changes the lens shape to maintain a clearly focused image on the retina |
|
|
What is the refractive power of a lens?
|
The amount it bends parallel light rays to a converging focal point
|
|
|
What does the focal length (F) refer to?
|
The distance from the lens center to the point where parallel light rays are brought to a focus
-A convex lens with a smaller radius of curvature (thicker) has more refractive power and shorter focal length |
|
|
How is the refractive power of a lens measured?
|
In diopters (D) (1/F meters)
-Convex lens cause light rays to converge; measured in + D -Concave lenses cause light rays to diverge; measured in - D |
|
|
What does the lens form?
|
Lens (actually pupil) forms an inverted and reversed image of an object in a focal plane behind the lens at the focal length of the lens
-The image is formed on the retina |
|
|
What is emmetropia?
|
Condition where cornea and lens together have a focal length at rest equal to the length of the eyeball so that light rays are in perfect focus on the retina
|
|
|
What is myopia?
|
"Nearsightedness"
-Occurs when eye is too long in relation to focal length of lens and cornea -Excess refractive power corrected by minus diopter glasses -Excessive close up work leads to and worsens myopia by leading to increased size of eyeball |
|
|
What is hyperopia?
|
"Farsightedness"
-Occurs when the eye is too short in relation to the focal length of lens and cornea so the image is focused behind the retina -Lack of refractive power corrected by adding glasses with plus diopters to shorten the focal length |
|
|
What does accommodation refer to?
|
The lens is able to change its shape when the ciliary muscle contracts to aid in near vision
-Refractive power may be increased by 10-12 D -When object is near need to increase power (no extra lens power needed for distance vision) |
|
|
Describe how accommodation works
|
-When ciliary muscle is relaxed (for distance vision), zonule fibers are tight, stretching lens into thin shape
-When ciliary muscle contracts and moves inward toward lens, relases tension on the zonule fibers, lens balloons out through its natural elasticity to become more curved (decrease its radius of curvature?) and increase its refractive power -Increased refractive power enables focusing of diverging rays from nearby objects on retina -"Near point" is the closest point of clear focus during maximum accommodation |
|
|
What is presbyopia?
|
With advancing age, the lens loses its elasticity and its ability to increase refractive power during accommodation
-Corrected by + diopter lenses during near vision (e.g., readers) -The "near point" increases with age |
|
|
What happens during the near response?
|
When vision is shifted to a nearer object, a triad of events takes place simultaneously to bring object into clear view
1. Lens accommodates: gets fatter, decreases readius of curvature, more light bending power to maintain image on retinal focal plane (bilateral E-W nucleus to ciliary ganglion and muscle) 2. Pupil constricts to help increase depth of field and decrease blur (Bilateral E-W nucleus to ciliary ganglion to iris) 3. Eyes converge to keep image on corresponding points of retinas (typically fovea) which prevents double vision (Bilateral nerve III to medial recti muscle) |
|
|
Describe the near response reflex arc
|
-Occipital cortex perceives onset of double vision
-pathway to accomodation center in midbrain -to Edinger Westphal nucleus for accommodation and constriction of pupils -to oculomotor nucleus for convergence of globes |
|
|
Describe the pupillary light reflex
|
-Light in one eye
-Optic nerve -Optic chiasm -Optic tract -Pretectal region near superior colliculus -Bilateral E-W nuclei |
|
|
What will happen with a lesion in the pretectal region?
|
Abolish the pupillary light reflex but leave contriction during the near response intact (light-near dissociation)
-This is part of dorsal midbrain syndrome |
|
|
What happens to the pupillary light reflexes if there is a cortical lesion or lateral geniculate lesion?
|
Blindness with preserved pupillary light reflexes
|
|
|
What is the Snellen test?
|
Test of visual acuity by reading letters of graded size
-distance tested at 20ft so accommodation is minimized -"Normal" visual acuity: ability to read block letters five minutes of visual angle in height with line widths of one minute -Rating of 20/40 indicates that individual can read at 20ft same size letter as "normal" person can read at 40ft |
|
|
What does amaurosis mean?
|
Blindness of any cause
|
|
|
What does amblyopia mean?
|
Decreased visual acuity due to disuse of the eye
|
|
|
Where is visual acuity strongest?
|
Fovea (falls off rapidly in the peripheral retina)
-Fovea contains only cones -Each cone has its own private line (connection with one bipolar cell and then to one ganglion cell) thus preserving fine grained acuity in the central visual projections (in peripheral retinal, convergence of many receptors upon each bipolar cell -- fine grain is lost) -Foveal acuity further enhanced by the displacement of retinal blood vessels and neurons out of the light path -Since cones are less sensitive to light than rods, full foveal acuity requires good illumination |
|
|
What are rods?
|
Photoreceptor that has pigment rhodopsin
-only one kind of rod -absorbs light best at blue-green wavelengths (505 nm) -super-sensitive for dim lights -more sensitive than cones -no color perception -absent at fovea -more numerous than cones (20:1) |
|
|
What are cones?
|
Photoreceptor
-Three classes of cones each having a different pigment and making different connections -Blue (440 nm), green (535 nm), red (565 nm) -Dominant receptor in bright light (the rods are saturated and no longer signal differences) -highest concentration in fovea -brain calculates ratio of activity among the three cone systems to produce color sensation |
|
|
How does light adaptation work?
|
-As illumination increases, more pigment is bleached, and receptor potentials increase momentarily
-After several seconds, receptor becomes less sensitive to light and further increments of light elicit a smaller response |
|
|
How does dark adaptation work?
|
-Recovery of sensitivity upon entering the dark takes place gradually over many minutes as pigments are regenerated
|
|
|
How do red glasses preserve night vision?
|
Eyes just use the cones -- don't use the rods at all so don't need to wait for them to become adapted
-Good night vision is thus preserved for immediate use without waiting for rods to dark-adapt |
|
|
How does the absorption of light lead to an action potential?
|
-Stimulation of the receptor by light results in receptor hyperpolarization and reduced transmitter release
-Activates bipolar cell -Activates ganglion cell -Activates optic nerve (Note: receptors, H cells, and B cells do not generate spikes but use graded changes in membrane potentials. G cells are spiking neurons). |
|
|
How does the receptive field of bipolar and ganglion cells influence signals to the brain?
|
Bipolar and ganglion cells have "on-center, off-surround" or "off-center, on-surround" organization to their receptive fields (the brain is informed about light-dark boundaries that differentially stimulate the center and surround of a G cell receptive field)
-If light shines on center, sends excitatory signal to cell -if light shines on surround, sense inhibitory signal to cell (this is done by horizontal cells) -if light is diffuse (activates center and surround), there is no response (e.g., no signal to cell) |
|
|
How is a specific color perceived?
|
The entire range of color sensations can be duplicated by mixing three primary colors and adjusting their intensities. The perceived color is determined the the relative proportions of excitation among the three cone systems
-This is achieved in the retina by G cells that take the difference between the outputs of the red and green cones -A second system of opponent process cells computes the difference between the blue cone activity and the long wavelength (red + green) cone activity |
|
|
What is the most common type of color blindness?
|
Red-green (difficulty discrimination among wavelengths in the red-green range)
|
|
|
What is a dichromat? What are the two types?
|
Individual who lacks entirely one of the cone pigments
1. Protanopia: lack of red pigment -does not see long wavelength reds at all, also can't discriminate reds from greens well because lack the red-green opponent system 2. Deuteranopia: lack of green pigment -Can't see medium wavelengths; also can't discriminate reds from greens well because lack the red-green opponent system |
|
|
What are color anomalies?
|
Produce less severe red-green color blindness, due to deficient or spectrally shifted red/green pigment
1. Protanomaly: deficient red pigment 2. Deuteranomaly: deficient green pigrment -Both do not experience hues in red-green range as intensely as normal and discriminate poorly between these hues |
|
|
What is developmental amblyopia?
|
Severe astigmatism in childhood that leads to modifications of visual neurons so that visual acuity is poor for detecting lines oriented along the axis of astigmatism
-environmental experience can modify the orientation preference of cortical cells |
|
|
What is the purpose of the optic chiasm?
|
To bring the two separate "mappings" of the binocular visual field (one in each eye) into superimposition and create a single "fused" retinotopic map of visual space in the visual cortex
-this is achieved by the crossing in the optic chiasm of the optic nerve fibers coming from the nasal half of each retina -Fibers from the right halves of the two retinas (from left half of visual field) are brought together and combined in the right optic tract -posterior to the OC the visual path contains a single map of the contralateral visual half-field |
|
|
What are corresponding points?
|
Pairs of points, one on each retina, which are seen as a single fused image
-pairs of corresponding retinal points send their projections (via LGN) to exactly the same point on the cortical "map" and synapse upon the same neurons in the primary visual cortex -this is the neurological basis of binocular fusion and single vision -if different optical images fall on corresponding points, they will be seen as superimposed and will alternately be suppressed (binocular rivarly)d |
|
|
What is diplopia?
|
Double-vision
-occurs if the same optical image falls on noncorresponding points in the two eyes -frequently happens when eyes are not properly aligned |
|
|
How does the visual cortex represent the actual visual environment?
|
Visual cortex map contains an upside-down and backwards representation of the external visual environment
-input is from contralateral half-field (e.g., right visual field projects to left side of each retina, end up in left cortex) -upper quadrant of contralateral field projects to lower visual cortex (below calcarine fissure) and vice versa -Fovea projects to posterior striate cortex and has a much larger portion of the cortex devoted to its inputs |
|
|
How is a patient's visual field examined?
|
Confrontation testing
-patient looks at examiner's nose with one eye to count the number of fingers presented briefly in each quadrant of the visual field (upper right, lower right, upper left, lower left) -patient asked to detect small movements of examiner's finger at the very periphery of the visual field in each of the four quadrants |
|
|
How are visual field defects analyzed?
|
By a test called perimetry in which a target spot is moved systematically through the visual field of each eye separately and patient reports whether they see the target
|
|
|
What happens when a patient gets a lesion before the optic chiasm?
|
Visual defect in one eye only
-may also be decreased acuity and color vision in that eye -optic disk may appear pale |
|
|
What happens when a patient gets a lesion in the optic chiasm?
|
Selective destruction of crossing fibers coming from nasal half of retinas
-bitemporal hemianopia: eliminates vision in temporal visual half-fields of both eyes -because you lose fibers from the medial retina which represents the lateral visual field |
|
|
What happens if a patient has a lesion after the optic chiasm?
|
Homonymous hemianopia:
Visual field defects in the contralateral visual half-fields of both eyes (e.g., lesion on left side post-chiasm will create right homonymous hemianopia) |
|
|
What do the optic radiations that pass through the temporal lobe represent?
|
Input from the upper visual field going to lower visual cortex
(I think these are optic radiations from lateral retina) -lesion in temporal lobe produce superior quadrantanopia |
|
|
What doe the optic radiations that pass through the parietal lobe represent?
|
Input from lower visual field going to upper visual cortex (I think these are the optic radiations from medial retina)
-lesion in parietal lobe produce inferior quadrantanopia |
|
|
What happens if a patient has a lesion above the calcarine fissure in the primary visual cortex?
|
Homonymous hemianopia of lower contralateral visual field quadrant
|
|
|
What happens if a patient has a lesion below the calcarine fissure in the primary visual cortex?
|
Homonymous hemianopia of upper contralateral visual field quadrant
|
|
|
What happens if a patient has a lesion in the posterior part of the primary visual cortex?
|
Homonymous hemianopia in macular zone
|
|
|
What happens in the macula in general lesions of the primary visual cortex?
|
The macular zone may be spared in an otherwise complete HH
-because some of the central foveal region still intact |
|
|
What happens when a patient has an anterior lesion of one primary cortex?
|
Causes a contralateral field defect that is present only in the visual field of the contralateral eye
-this "temporal crescent" (the most lateral portion of the visual field) is only seen by the ipsilateral eye |
|
|
What happens when a patient has a lesion of the left optic tract?
|
Complete right homonymous hemianopia
|
|
|
What happens when a patient has a lesion of the lateral optic radiations?
|
Right homonymous hemianopia of superior visual field
|
|
|
What happens when a patient has a lesion of the left medial optic radiations?
|
Right homonymous hemianopia of the inferior visual field
|
|
|
How can you tell if a lesion is in the primary visual cortex or in the tract/radiations?
|
-In primary visual cortex, corresponding retinal points have been fused into a single map so lesions always produce visual field defects that are identical in the two eyes (congruous)
-In OT, LGN, OR, fibers from corresponding retinal points of the two eyes are separate so lesion could influence fibers differently and produce field defects that are similar but not identical in the two eyes (incongruous field defect) |
|
|
In cases of total blindness, how can you distinguish a cortical lesion from a lesion at retinal, ON, or OT levels?
|
Cortical lesion will have preservation of the pupillary light reflex
|
|
|
What visual defect is present in glaucoma?
|
Arch shaped scotoma follows path of nerve fibers across retina from ganglion cells to optic disk
|
|
|
What kind of tumor often causes an optic chiasm lesion?
|
pituitary tumor
|
|
|
What visual system is in the parietal lobe?
|
the "where" system (spatial vision)
|
|
|
What visual system is in the temporal lobe?
|
the "what" system (object recognition
|
|
|
What happens when a patient has a pituitary lesion that grows and pushes on bottom of the optic chiasm?
|
Bitemporal hemianopia of the superior visual field
|
|
|
What happens when a patient has a hypothamic lesion that grows and pushes on the top of the optic chiasm?
|
Bitemporal hemianopia of the inferior visual field
|
|
|
What type of visual field loss is associated with a parietal lobe lesion?
|
inferior field
|
|
|
What type of visual field loss is associated with a temporal lobe lesion?
|
superior field
|
|
|
How many extraocular muscles move each eye?
|
6 (LR, MR, SR, IR, IO, SO)
-each extraocular muscle responsible for moving the eye into each cardinal position of gaze |
|
|
All eye movement control systems activate the same cell bodies located in the ?
|
Left and right oculomotor (III), trochlear (IV), and abducens (VI) nuclei
-these neurons innervate the extraocular muscles, but they are not "lower motor neurons" because they are innervated by the 7 eye movement control systems (rather than upper motor neurons) |
|
|
What is the purpose of the vestibular eye movements (e.g., VOR)?
|
To hold an image steady on the retina during brief head rotations
-when a head rotation is prolonged (20-30 sec), the VOR adapts as the stimulation from the semicircular canal ceases -involuntary, but can be suppressed |
|
|
What is the purpose of the smooth pursuit eye movement?
|
To hold the image of a moving target on the fovea
-voluntary, but only moves the eyes to follow an object whose image is moving off the fovea -horizontal or vertical |
|
|
What is the purpose of the saccadic eye movements?
|
To place the image of an object of interest on the fovea
-voluntary, discrete movements -only one speed (very fast) -vision is temporarily suppressed during a saccade (acuity way down) -only system that can be used voluntarily to look at a particular point in space |
|
|
What is the purpose of vergence eye movements?
|
To keep the images of a single object simultaneously on both fovea
-accomplished by crossing the eyes (converging) to look at a nearby object and uncrossing the eyes (diverging) to look at a distant object -only eye movement that is not conjugate (eyes don't stay perfectly aligned) |
|
|
What is the purpose of the nystagmus quick phase eye movement?
|
To reset the eyes toward the oncoming visual scene during prolonged head rotation
|
|
|
Describe the anatomical pathways of the vestibular and optokinetic systems
|
-VOR is activated by vestibular system input and optokinetic movements activated by retinal input that reaches the vestibular nuclei via cortical and subcortical pathways
-effected by neurons with cell bodies in the vestibular nuclei -excitatory and inhibitory projections course (through MLF) to the III, IV, and VI nuclei and to gaze holding centers |
|
|
Describe the anatomical pathways of the nystagmus quick phase system
|
-center is in the paramedian reticular formation of the pons and midbrain
-easily abolished in comatose patients |
|
|
Describe the anatomical pathway of saccades
|
Horizontal saccades:
-Right "frontal eye field" -Left PPRF (pontine paramedian reticular formation) -Left abducens nucleus -Horizontal saccade to left Vertical saccades: -bilateral frontal eye fields -rostral midbrain reticular formation -bilateral oculomotor and trochlear nuclei *Does not use vestibular nuclei because has to move fast *Each hemisphere has a frontal eye field located in lateral frontal lobe that is important for initiating saccades to contralateral space *Caudate is also involved in generating saccades in all directions |
|
|
Describe the anatomical pathway of the vergence system
|
Vergence center located in the midbrain reticular formation and presumably projects to both oculomotor nuclei
|
|
|
What is the final common pathway for all eye movement control systems?
|
Abducens nucleus is the center for conjugate horizontal eye movements
-receives input from each eye movement control system that can effect conjugate gaze -contains neurons that project to the ipsilateral lateral rectus muscle and to the contralateral oculomotor nucleus (via MLF) |
|
|
What does a lesion of the MLF cause?
|
Internuclear ophthalmoplegia (INO)
-ipsilateral (e.g., if right eye not moving well, lesion in right MLF, and syndrome is right INO) -Say "look left", left eye moves over fine, but right eye is stuck in middle (same findings for VOR, saccade, smooth pursuit) -occurs between the 3rd and 6th nuclei -Can test to make sure the problem isn't with the medial rectus by asking the patient to cross their eyes (if cross fine, problem not in medial rectus or oculomotor nerve) |
|
|
What does a lesion of one abducens nucleus or the contralateral frontal eye field do?
|
-if right abducens nucleus destroyed, the unopposed input from the left abducens nucleus will move both eyes to the left
-if left frontal eye field destroyed, unopposed right frontal eye field will move eyes to the left -Both lesions lead to identical symptom |
|
|
How can you distinguish an abducens nucleus lesion from a frontal eye field lesion?
|
1. Right pontine lesion (that includes the abducens nucleus) will produce a left hemiparesis and the left frontal lesion will produce a right hemiparesis
2. Right pontine lesion destroys abducens nucleus so no other eye movement system can move eyes to the right. If the lesion is actually in the left frontal eye field, putting ice water in the right ear (stimulating VOR) will produce a VOR (because the abducens nucleus still works) and move the eyes to the right |
|
|
If someone can smooth pursue and VOR both ways, but can't saccade left and right, where is the lesion?
|
PPRF (it just generates horizontal saccades)
|
|
|
What eye movements are typically seen as one of the early signs in Huntington's disease?
|
Very slow saccades (both horizontal and vertical) due to degeneration of the caudate nucleus
|
|
|
Where is the lesion if someone can't smooth pursue one way?
|
On one side in the cerebellumw
|
|
|
What is the pathway of the smooth pursue eye movement system?
|
-L primary visual cortex
-L cerebellum -R vestibular nuclei -Then, the right vestibular nucleus would effect the smooth pursuit eye movement as if it were executing a VOR to the left |
|
|
What is achieved by the MLF?
|
horizontal conjugate gaze
|
