1) Involves 20% of the optic nerve/tract that bypass the lateral geniculate and terminate instead in the midbrain
2) Mediate pupillary light and accommodation reflexes.
3) Clinically important because these reflexes provide a general indicator of damage to the midbrain as well as the functional state of underlying afferent and efferent pathways (even in an unconscious patient).
Pupillary light reflex
constriction of the pupil in response to light
A. When one eye is exposed to light, the pupils in both eyes constrict
1. Direct response - pupillary constriction in the exposed eye
2. Consensual response - pupillary constriction in the unexposed eye (Aniscoria=pupils of unequal size)
B. Light reflex pathway
1. Afferent limb - Retinal ganglion cells project through CN II, optic tract and brachium of superior colliculus to terminate in the pretectal area (region rostral to superior colliculus)
2. Neurons from pretectal area project bilaterally to terminate in the Edinger-Westphal nuclei (crossing axons project through the posterior commissure)
3. Efferent limb
a. Edinger-Westphal neurons project preganglionic parasympathetic fibers through CN III to terminate in the ciliary ganglia. Postganglionic parasympathetic fibers project from the ciliary ganglia to terminate on the sphincter pupillae muscles, causing bilateral pupillary constriction.
C. Collaterals of light reflex pathway
1. Collaterals from the pretectal neurons terminate in the superior colliculus.
2. Axons from the superior colliculus cross and form the tectobulbar (blink in response to flash of light) and tectospinal tracts (turn head and upper body toward visual stimulus)
Lesion 1: R optic n. Afferent limb--no direct or consensual response. Left eye: both L&R response.
Lesion 2: R preganglionic parasympathetic of CN III (=postganglionics or ciliary ganglion)
Light on right--only consensual response. Light on left--only direct response
adjustments in the eyes as an object moves from distant vision to close vision
A. Three components
1. Convergence - contraction of medial rectus muscles causes eyes to rotate medially
a. Allows image of object to fall on fovea of both retinas.
2. Rounding (accommodation) of lens - the ciliary muscles normally suspend the lens tautly, keeping the lens somewhat flattened. When the ciliary muscles contract, the tension on the lens is released and the lens rounds. (presbyopia=loss of elasticity)
a. Assures that the object is in focus on the retina
3. Pupillary constriction - constriction of the sphincter pupillae muscles
a. Decreases the amount of light, thus increasing the depth of focus
B. Accommodation pathways (sight must be intact)
1. Eye to visual cortex
Visual information projects via retinal ganglion cells through the optic nerve and optic tract to terminate in the lateral geniculate body. Axons from lateral geniculate project to the primary visual cortex (Area 17). The visual data projects to the association areas of the visual cortex (Area 18 and 19), which interpret the visual data.
Accomodation Reflex, cont'd
2. Cortex (Area 17, 18 and 19) to midbrain region
a. Fibers from the visual association areas and other regions of cortex project toward the
midbrain and terminate in the pretectal area and superior colliculus (via the brachium of
the superior colliculus).
b. Dorsal midbrain - control of vertical eye movement; ventral area - control of horizontal
3. Pretectal area to Edinger-Westphal and oculomotor nucleus
a. Cell bodies in the pretectal area project their fibers bilaterally (crossed fibers project
through the posterior commissure) to the Edinger-Westphal and oculomotor nuclear complex.
4. Motor and parasympathetic projections through CN III
a. Motor: Cell bodies in the oculomotor nucleus project via CN III and stimulate the
contraction of the medial rectus muscles, which results in convergence.
b. Autonomic: Cell bodies of the Edinger-Westphal nucleus project preganglionic parasympathetic fibers through CN III to terminate in the ciliary ganglion and the ciliary ganglion projects postganglionic fibers that innervate: 1) the ciliary muscles causing the lens to round and 2) the sphincter pupillae muscles, causing the pupils to constrict.
c. Within CN III, motor fibers are centrally located within the nerve and parasympathetic fibers are peripheral
Visually Evoked Movement of the eyes
tracking the movement of an object
A. From the visual association cortex to nuclei of CN III, IV and VI via MLF
CN III: motor fibers in the center and peripheral fibers are parasympathetic.
Tumor: affects parasympathetics first
Vascular disease/diabetes: affects motor fibers first
A. Relative afferent pupillary defect (RAPD) - Marcus-Gunn pupil--partial lesion of optic n.
1. Pupil in affected eye constricts in response to light only during the consensual response
2. Lesion is rostral to the optic chiasm in CN II
3. Best tested by swinging flashlight test (use filter. 0.9=90% of light not perceived)
a. Flashlight is moved between eyes at 2-3 second intervals
b. As flashlight shifts from normal eye to affected eye, the affected eye will dilate. Comparator--notice decrease in light and cause dilation
B. Parinaud's Syndrome -
1. One symptom is conjugate paralysis of upward gaze above the horizontal plane
2. Caused by compression of the superior collicular and pretectal areas
a. Lesion is usually a pineal tumor
C. Adies pupil - dilated pupil which fails to constrict in response to light (but will in accomodation)
1. Usually occurs in patients less than 30 years of age 2. Pupil will respond normally to drugs for miosis or mydriasis
3. Caused by lesion of ciliary ganglion or its postganglionic parasympathetic neurons (viral infection or vascular disease)
D. Argyll Roberts on pupil-chronically small and irregular pupil
1. Constricts more to accommodation than to light
2. Caused by bilateral damage to pretectal nuclei or their projections to the Edinger-Westphal nucleus
3. Associated with neurosyphilis, multiple sclerosis and diabetes