Download homework for eye anatomy

HANDOUT FOR EYE ANATOMY
The visual pathway begins with stimulation of the eyeball and ends in the striate cortex
(visual cortex in occipital lobe).
Figure 4 Page 6
The eyeball is connected to the orbital cavity by six external muscles, which provide rotary
movement of the eye and support.
Figure 1 Page 4
The three layers of the eye consist of the 1) outer (white of eye) which consists of the
sclera and cornea, 2) center, (vascular pigmented layer) which consists of choroid, the
ciliary body and the iris (eye color) the pupil is in the iris and permits light to enter the eye,
and 3) inner (retina)
Figure 2 Page 4
The three cell layers of the retina are first, second and third order neurons. Figure 3 P5
The retina, organized into 10 layers, contains two types of photoreceptors (rods & cones)
and four types of neurons (bipolar cells, ganglion cells, horizontal cells, and amacrine
cells).
Figure 3 Page 5
Transmission from photoreceptors (rods & cones, first-order sensory neurons), to bipolar
cells (second-order sensory neurons), and then to ganglion cells (third-order sensory
neurons) is modified by horizontal cells and amacrine cells.
Figure 3 Page 5
The rods and cones comprise the outermost layer of the retina.
Rods- are more numerous than cones in the retina, are sensitive to low intensity light and
provide visual input when illumination is dim, eg, at twilight and at night.
Cones- are stimulated by relatively high intensity light: they are responsible for sharp
vision and color discrimination.
Impairments of Vison
Emmetropic- a normal eye that readily focuses on objects
Presbyopia- with age (40s-50s) each eye is permanently focused at a constant distance and
reading becomes difficult.
Myopia- (nearsightedness) the refracting system is too powerful for the length of the
eyeball, causing the image of a distant object to focus in front of instead of at the retina
Hyperopia- (farsightedness) the refracting power is too weak for the length of the eyeball
causing the image to appear on the retina before it focuses
Scotomas- abnormal blind spots in the visual fields of varying size and shape
Nystagmus- rapid involuntary oscillation (swing to and fro) of the eyeball. This will
diminish the P100 amplitude but the latency will be OK.
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Papilledema- swelling of the optic disk, usually a symptom of increased intracranial
pressure caused by a mass (tumor)
Amblyopia- reduction or dimness of vision with no apparent pathologic condition of the
eye.
Anatomy Vocabulary
The retinal area for central fixated vision during good light is the macula lutea (lutea is
Latin for yellow spot). The macula is a high sensitivity area opposite the pupil at the back
of the retina and contains only specialized cone cells. The macula lies in a shallow pit
known as the fovea. The fovea of the retina is responsible for fine detail discrimination
and full color distinction. Nasal and temporal fibers have crossed by the time they reach
the retina.
Figure 2
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Optic nerve- (before chiasm, n before t) conveys visual impulses, consists of about a
million nerve fibers & contains axons arising from the inner ganglion cell layer of the
retina.
Figure 4 Page 6
Optic chiasm- a place where the left and right optic nerves cross, the fibers from the nasal
halves of the retina decussate (cross), but those from the lateral (temporal) halves do not.
Figure 4 Page 6
Optic tract- (after chiasm, t after n) when the optic nerves exit the optic chiasm, they are
now referred to as optic tracts, they carry axons to the lateral geniculate bodies of the
thalamus. The optic tracts end in the lateral geniculate bodies. Figure 4 Page 6
Lateral geniculate body- (also called the Metathalamus (part of the diencephalon) & the
lateral geniculate nucleus & superior colliculus) it is relay nuclei within the thalamus,
whose axons project ipsilaterally by way of the optic radiations.
Figure 4 Page 6
Superior colliculus- (another name for the lateral geniculate body) receives synapses from
the visual cortex, it projects to the spinal cord via the tectospinal tracts, which control
movements of the head, neck and eyes in response to visual stimuli.
Optic radiation- (also called geniculocalcarine fibers/tract) the optic tracts after ending in
the lateral geniculate body, form the optic radiations which lead to the calcarine cortex in
the occipital lobe. It carries impulses from the lateral geniculate bodies to the visual cortex.
Figure 4
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Meyer’s loop- sweep of geniculocalcarine fibers that curves around the lateral ventricle
reaching forward into the temporal lobe, before proceeding toward the calcarine cortex. It
carries optic radiation fibers representing the upper part of the contralateral visual field.
Primary visual cortex- also called striate cortex, Brodmann’s area #17, the visual
receptive area and the calcarine cortex is in the occipital lobe and is the final area to
receive impulses and is where the P100 is believed to be generated
Figure 4 Page 6
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Visual Defects
Impaired vision in one eye is usually due to a disorder involving the eye, retina or the optic
nerve. (before the chiasm)
Field defects can affect one or both visual fields. Lesions in the optic chiasm, optic tracts,
or visual cortex, show field defects (abnormal VEP) in both eyes. A lesion in the optic
tract could cause a normal VEP because some normal fibers will still get to each eye.
Fig 5 B-E P 7
Bitemporal hemianopia- a lesion of the middle part of the optic chiasm (pituitary tumors)
which injures decussating fibers of optic nerves will cause this defect in the temporal field
of each eye (blindness in the lateral or temporal half of the visual field for each eye)
Figure 5 B P 7
Homonymous hemianopia- a lesion in the optic tract (behind chiasm) that causes a field
defect in the temporal field of one eye, together with a field defect in the nasal (medial)
field of the other eye. This would show a normal PRVEP at MO but low amplitude at
either LO or RO. The visual field defect is on the side opposite to the lesion. A right
hemianaopia means the right side of each visual field is black (right temporal and left
nasal) and the actual lesion in the brain would be on the left. (& visa versa for a left
hemianaopia) Figure 5
C & E Page 7
Superior quadrantanopsia- temporal lobe lesions can produce a visual field deficit
involving the contralateral superior quadrant (pie in the sky) Figure 5 D
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A lesion prechiasmally (in front of the chiasm/optic nerve) is detected by abnormality of
VEP stimulation of the eye on that side.
Figure 5 A
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Postchiasmal lesions affecting only temporal or nasal portions of retina or optic nerve
may cause abnormalities only if you use half-field stimulation of that eye.
Ocular albinism is abnormal crossing of the optic fibers at the chiasm.
The most important factor affecting the rate of transmissions is myelination.
Leukodystophies are a group of hereditary disorders of myelin production and
maintenance.
The retrochiasmal (behind the chiasm) portion consists of 1) the optic tract, 2) the lateral
geniculate body, 3) the optic radiations and 4) the visual cortex.
Figure 4 Page 6
Nasal fibers transmit information perceived in the temporal ½ fields.
Figure 4 Page 6
Paradoxical projection-normal response to nasal fiber excitation (during ½ field stim) is
processed by opposite hemisphere due to crossing. It is seen over ipsilateral hemisphere.
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