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Lab 9 (ƒ 10) - Cranial Nerve Nuclei and Brain Stem Circulation

Cranial Nerve III-Oculomotor Nerve

Components of the Oculomotor Nerve include

• The somatic motor component of the oculomotor nerve innervates the levator palpebrae, the superior, medial and inferior recti, and the inferior oblique muscle.
• The parasympathetic component of the oculomotor nerve consists of preganglionic parasympathetic efferents (Edinger-Westphal fibers) which terminate within the ciliary ganglion. It is the postganglionic fibers of the ciliary ganglion that form the short ciliary nerve and innervate the sphincter muscle of the iris and the ciliary muscles of the eye.

The somatic motor component of the oculomotor nucleus is associated with the elevation of the eyelid (levator palpebrae), vertical eye movements, converging eye movements, and also participates in conjugate horizontal eye movements. The parasympathetic Edinger-Westphal nucleus component controls the constriction of the pupil by contraction of the sphincter muscle of the iris and the lens curvature by contraction of the ciliary muscles during accommodation.

Damage to the lower motor neurons will result in paralysis of the muscles involved. The eyelid droops (ptosis) and the eye is deviated laterally (external strabismus) following damage of the oculomotor somatic component. Following damage of the parasympathetic (Edinger-Westphal or ciliary ganglion) neurons: the pupil is fully dilated (mydriasis), the pupillary light reflex is abolished and lens accommodation (near vision) is lost.

The nuclei of the oculomotor complex receive fibers from the vestibular nuclei, reticular formation, and from other extraocular motor nuclei. To coordinate muscle action in horizontal gaze, the interneurons of the abducens nucleus send axons via the medial longitudinal fasciculus to the oculomotor neurons controlling the medial rectus muscle. The vestibular input also travels via the medial longitudinal fasciculus to the extraocular motor nuclei for reflex adjustments of eye position to accommodate for changes in head position. Intercalated neurons of the reticular formation relay corticofugal inputs for control of reflex and volitional eye movements and for adjustments of eye muscles to focus on objects. The superior colliculi, which are involved in controlling eye movements, appear to influence extraocular motor nuclei indirectly via cell groups located in the periaqueductal gray and reticular formation. The superior colliculus controls vertical tracking eye movements to visual and acoustic stimuli. Damage to corticofugal fibers in the internal capsule may result in impairments of conjugate eye movements. The impairment, however, is often temporary.

Clinical Test of Cranial Nerve (III):

• See section on Abducens Nerve for extraocular muscle testing.
• Test pupillary (parasympathetic) response (direct): Ask patient to look in the distance while shining a bright light (usually from penlight) in patient's eye. The light should come in from the side, no more than six inches from the face, using the patient's nose as a barrier to light reaching the other eye. The examiner should observe the response of the pupil. Then repeat the test on the other eye.
• Test pupillary (parasympathetic) response (consensual): As above, but the examiner should observe the response of the pupil that does not have light shining on it.