Med 6573: Nervous System
University of Minnesota Medical School Duluth
Friday, 29 February 2008
Spring Semester
Dr. Donna J. Forbes
11:00 –12:00
Visual Pathway
References:
Nolte Chapter 17
Netter, plates 86, 114
Related handouts from other course faculty
At the end of this lecture, the student should be able to:

Describe the relationship of bipolar and ganglion cells in the retina to the visual pathway.

Describe the relationship of retinal quadrants to visual field quadrants.

Describe what is meant by visual fields, both monocular and binocular.

Describe the relationship of the macula vs. peripheral parts of the retina to the visual fields and the
visual pathway.

Describe the importance of "corresponding points" on the retinae of the two eyes. Explain what is
meant by the ‘blind spot’.

Describe the course of the visual pathway from the retina to the visual cortex, including which fibers
decussate, where they decussate, the tracts and nuclei involved.

Describe the representation of the visual fields throughout the visual pathway.

Describe the visual field deficits that would result from damage to the visual pathway at major points
along its course from retina to cerebral cortex.

Describe the components (nuclei, tracts, etc.) and basis of the:
 direct and consensual pupillary light reflex
 pupillary dilation
 accommodation or near reflex

Define or describe:
 Amblyopia
 Argyll Robertson pupil
 Diplopia
 Hemianopsia
 Homonymous vs. heteronymous visual deficits
 Horner’s syndrome
 Meyer’s loop
 Quadrantanopsia
 Retinotopic organization
 Scotoma
 Strabismus
Med 6573: Nervous System
I.
Visual System Pathways
The Visual Pathway
(Netter pl. 114; Nolte fig. 17-24 to 17-26)
Reti na
op tic nerve
op tic ch iasm
op tic tract
La teral gen icu late
nu cl of thala mus
ge niculocalcarin e tra ct
(op tic radi atio n)
Vi sual Cortex
(Are a 17 )
A. Retina (Netter pl. 86; Nolte figs. 17-2, 17-4 to 17-13; Dr. Downing’s materials on “Histology of the
Eye”)
1. Retinal cell types
a. Receptors: rods and cones
b. Bipolar cells
c. Ganglion cells
 Optic nerve is formed by the axons of these ganglion cells.
2.
Retinal quadrants:
a. The retina is divided into quadrants by horizontal and vertical lines or meridians, which
pass through the fovea:
Right eye
Left eye
UTQ UNQ
UNQ UTQ
nose
LTQ
LNQ
LNQ
Macula with fovea centralis
in the center
LTQ
Optic papilla or nerve head
(blind spot)
RETINA
(As you would see it through the patient’s pupil.)
Temporal Hemiretina
Nasal Hemiretina
UTQ = upper temporal quadrant
LTQ = lower temporal quadrant
UNQ = upper nasal quadrant
LNQ = lower nasal quadrant
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Med 6573: Nervous System
B.
Visual System Pathways
Visual Fields (Nolte fig. 17-31)
1. Visual field = the entire area which can be
"seen" by the patient without movement of
the head and with the eyes fixed on a single
spot.
Temporal Field
of Left Eye
Nasal Field of
Left Eye
2. For convenience, the same lines or
meridians that define retinal quadrants also
define quadrants of the visual field.
Upper Field
F
3. Mapping of Visual Fields: Visual fields are
recorded on charts so that the field is
represented as the subject sees it. Thus,
for the left eye, the upper field is plotted
above, the lower field is plotted below, the
temporal field is plotted to the left, and the
nasal field is plotted to the right.

Each eye is tested separately so “A” is
the plot for the left eye and “B” is the
plot for the right eye. Each of these
plots is a Monocular Visual Field.

Numbers along the meridians are
degrees of visual angle from the center
of the fovea (F).
Lower Field
Normal Monocular Visual
Field of Left Eye
4. Monocular Visual Field = total extent of
the external world perceived by one eye
without movement of the head or change of
ocular fixation.

Visual field is not round.

Blind spot is 15° to the temporal side of
the visual field and on the horizontal
meridian, corresponding to the optic
disk that is located on nasal side of
retina.
F
Normal Monocular Visual
Field of Right Eye
Right visual field
Left visual field
5. Binocular Visual Field includes the
monocular field of each eye.


The central area is the portion of the
visual field seen by both eyes
(binocular region); the areas at the
sides are the temporal monocular
crescents of the visual field seen only
by the nasal portions of each retina.
F
If eyes are aligned correctly by the
extraocular muscles, the image falls on
"corresponding points" of the two
retinas and the subject perceives a
Normal Binocular Visual Field
single image. Diplopia occurs when
the images are not aligned to fall on corresponding points of the retina.
Be sure to distinguish between the "Monocular visual field of the right eye" vs.
the "Binocular right visual field" and vice versa for the left counterparts.
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Med 6573: Nervous System
Visual System Pathways
6. The image on the retina of an object located in the visual field is both inverted and reversed from
right to left:
Nasal Retina of Left Eye
Left Visual Field
Right Retinal Halves
Temporal Retina of Right
Temporal Retina of Left Eye
Right Visual Field
Left Retinal Halves
Nasal Retina of Right Eye
Upper Visual Fields
Lower Retinal Halves of Both Eyes
Lower Visual Fields
Upper Retinal Halves of Both Eyes
NOTE: To avoid confusion and abide by convention, central representation, deficits, etc., will be
described in terms of visual fields and not retinal quadrants. (The literature is not consistent in its
terms of reference--so be sure that you know whether a given source is referring to visual fields or retinal
quadrants.)
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Med 6573: Nervous System
Visual System Pathways
C. Optic Nerve (SSA)
1. Extends from the retina to the optic chiasm.
2. Composed of the axons of ganglion cells in the
ipsilateral retina.
D. Optic Chiasm
1. Located in the anterior part of the sella turcica,
immediately in front of the pituitary gland.
2. Partial crossing of the axons from each optic
nerve occurs within the chiasm.
a. Axons from temporal fields cross.
b. Axons from nasal fields do not cross.
3. Partial crossing is essential for binocular vision.
E. Optic Tract
1. Continuation of the optic nerve axons from the
optic chiasm to the lateral geniculate nucleus of
thalamus.
2. Each tract contains axons that receive input from
the contralateral visual field.
a. L. optic tract receives from R. visual field.
b. R. optic tract receives from L. visual field.
Reminders about the optic nerve:
 Develops as an outgrowth of the
diencephalon.
 Essentially a CNS tract; therefore, not
a “true” cranial nerve.
 No associated cranial nerve nuclei in
the brain stem.
“Wilbrand’s Knee”: Immediately after
crossing in the optic chiasm, fibers from
upper temporal fields loop forward for a
short distance into the optic nerve before
they continue into the optic tract. See
Lesion #3 for clinical implications. Recent
evidence indicates this may be an artifact.
While most optic tract fibers terminate in
LGN, many also terminate in the superior
colliculus/pretectum (see Visual Reflexes).
Suprachiasmatic nucl. of the hypothalamus
(involved with synchronizing visual input
with circadian rhythm) also receives visual
input at the optic chiasm level.
From optic tract to visual cortex (post-chiasmatic portion of the pathway), each
side of the brain deals with the contralateral visual field.
F. Lateral Geniculate Nucleus (LGN)
1. Primary termination of optic tract axons.
2. Each LGN receives input from the contralateral visual field.
3. Precise retinotopic organization within the LGN.
G. Geniculocalcarine Tract (= Optic Radiation)
1. Axons of LGN neurons travel to Area 17 via the retrolenticular & sublenticular portions of the
internal capsule.
2. Axons representing upper and lower fields are separated by macular fibers and take different
routes to the cortex:
a. Fibers from upper visual fields travel forward into the temporal lobe as Meyer's loop before
turning toward the occipital cortex.
b. Fibers from lower visual fields travel more directly through the parietal lobe toward the
occipital cortex. (Dashed lines from LGN to cortex in diagram on page 4)
c. Macular fibers lie intermediate in the radiation. (Not really shown on the diagram.)
H. Visual Cortex
1. Primary visual cortex (Area 17 or striate cortex) is located on either side of and within the
calcarine fissure.
a. Upper visual fields project to the lingual gyrus.
b. Lower visual fields project to the cuneate gyrus.
c. Macular representation is most caudal in Area 17.
d. Peripheral field representation occupies the rostral two-thirds of Area 17 (monocular segment
is most rostral).
2. Association visual cortex (Areas 18 and 19)
a. Input from Area 17 and elsewhere.
b. Deals with complex aspects of vision - such as recognition of subjects and their significance.
c. Lesions of association visual cortex result in visual agnosia.
d. Projections to superior colliculus (via optic radiation and brachium of superior colliculus):
 automatic scanning movements (such as those involved in reading)
 accommodation/convergence reflex
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Med 6573: Nervous System
Visual System Pathways
II. Defects in the Visual Pathway
Left
Right
1. Normal visual fields
Not retinal
quadrants
2. Blindness of the right eye
3. Blindness of right eye + contralateral left
upper quadrantanopia
4. Bitemporal heteronymous hemianopsia
5. Left homonymous hemianopsia
6. Left upper homonymous quadrantanopsia
7. Left homonymous hemianopsia with
sparing of macular vision
Definitions
1. Strabismus (squint): Failure of coordination of extraocular eye muscles, resulting in deviation of
the affected eye and diplopia.
2. Diplopia: Double vision due to failure of the image to be aligned on corresponding points of the
retinae of the left and right eyes.
3. Amblyopia (lazy eye): Decreased visual acuity in the absence of anatomical defects in the
visual pathway. Secondary to strabismus; to avoid diplopia, the vision in one eye is suppressed
at the level of the visual cortex. Occurs in children with the critical period variously reported as
ending by age 3/4 years and up to 8 years.
4. Scotoma: Area or 'island' of visual loss within the visual field.
5. Quadrantanopsia: Defective vision or blindness in approx. one-fourth of the visual field.
6. Hemianopsia: Defective vision or blindness in approx. one-half of the visual field.
7. Homonymous defects: Visual defects restricted to either the right or the left visual field (postchiasmatic defects).
8. Heteronymous defects: Visual defects involving parts of both the left and the right visual fields
so that visual field defects are non-overlapping. (Almost always occur at the chiasm).
9. Congruous defects: Visual defects are equivalent in each monocular visual field. (#5 above)
10. Incongruous defects: Visual defects are not equivalent in each monocular visual field. (#3
above)
11. Altitudinal defects: Visual defects are in the upper or lower aspect of the visual fields. (#6
above)
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Med 6573: Nervous System
Visual System Pathways
III. Visual Reflexes
A. Pupillary Light Reflex (i.e. constriction in response to light; = Miosis) (Nolte fig. 17-38)
Pretectal Nuclei
Optic nerve + Tract + + Superior
Colliculus
Brachium of the
Superior Colliculus
Ipsilateral Nucleus
of Edinger Westphal
(pre-gang)
Ipsilateral
Ciliary
Ganglion
DIRECT
REFLEX
Constriction of
Pupil
(post-gang)
Posterior
Commissure
(pre-gang)
Pretectal
Nuclei +
Superior
Colliculus
AFFERENT LIMB (II)
Contralateral
Nucleus of
Edinger Westphal
(post-gang)
Constriction of
Pupil
CONSENSUAL
REFLEX
Contralateral
Ciliary
Ganglion
EFFERENT LIMB (III)
CENTRAL CONNEX
1. Afferents bypass the lateral geniculate nucleus and enter the brachium of the superior
colliculus (AKA superior brachium).
2. The reflex is abolished if either the afferent (II) or efferent (III) components are
damaged. (Lesion of II  Marcus Gunn pupil)
3. Blindness due to a lesion in the optic radiation or visual cortex would not result in loss
of the pupillary light reflex since this is a subcortical pathway.
4. In a patient with total blindness in one eye due to an optic nerve lesion, a consensual
light reflex can be elicited in the 'blind' eye upon stimulation of the 'good' eye.
B. Pupillary Dilation (= Mydriasis)
Decreased light to pupil
Severe pain
Strong emotional stimulus
?
Cortex,
Thalamus &
Hippocampus
?
Hypothalamus
(CNS control
center for AN S)
Reticular
Formation
Reticulospinal
fibers
Dilation
of pupil
Superior
Cervical
Ganglion
(post-ganglionic
sympathetic)
7
(pre-ganglionic
sympathetic)
Preganglionic
Sympathetic
Neurons in
Thoracic Cord
(T1-T2)
Med 6573: Nervous System
Visual System Pathways
Horner's Syndrome:
1. Pupillary constriction due to paralysis of the dilator pupillae muscle.
2. Ptosis due to paralysis of smooth muscle in the eyelid.
3. Flushed and dry skin due to vasodilation and absence of sweating.
Deficits result from interruption of the sympathetic innervation to the head. The lesion
producing the syndrome could be located in either the CNS or the PNS areas related to
this sympathetic innervation. Symptoms are ipsilateral to the lesion.
C. Accommodation Reflex (AKA “Near Reflex” or “Accommodation-Convergence Reaction”)
(Nolte fig. 17-38)
1. Initiated when gaze is shifted from a distant object to a near one and the image on the
retina becomes blurred.
2. Reaction involves:
a. Ocular convergence - both medial recti contract to align image. (GSE)
b. Pupillary constriction - constrictor pupillae muscles contract to sharpen the image.
(GVE)
c. Lens thickening - ciliary muscle contracts, increasing refractive power of the lens for
focusing on a near object. (GVE)
3. Efferent limb of the reflex is carried by the GSE and GVE components of the oculomotor
nerve.
4. Afferent limb and central connections of the reflex are unclear but probably some version of
the following:
optic tract  lateral geniculate nucleus of thalamus  optic radiation primary visual
cortex association visual cortex  optic radiation brachium of superior colliculus
superior colliculus the oculomotor nuclei oculomotor nerve (GVE and GSE
components)
5. Be aware of the term "Argyll Robertson pupil” where the pupil accommodates to near
objects but does not react to light.
a. Seen in patients with tabes dorsalis/tertiary syphilis, systemic lupus erythematosus and
diabetes mellitus.
b. Indicates that the pathways for pupillary constriction associated with these two reflexes
are somewhat different. There is some evidence that the lesion associated with an
Argyll Robertson pupil may be located in the pretectal area.
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