Trochlear Nerve

TROCHLEARIS NERVUS, s. patheticus, trochlear nerve, the fourth pair of cranial nerves, innervates the superior oblique muscle of the eyeball. The paired nucleus (nucl. n. trochlearis) is located in the mesencephalon, at the level of the inferior colliculus, in the central gray matter surrounding the cerebral aqueduct, partly projecting into the medial longitudinal fasciculus and directly caudal to the nuclei of the oculomotor nerve [see separate table (p. 815-816), Fig. 5]. The trochlear nerve differs from all other cranial nerves by the dorsal exit of its rootlets from the brainstem. The rootlet of the trochlear nerve, upon emerging from the nucleus, passes through the central gray matter into the superior medullary velum, where the rootlets of both trochlear nerves decussate, and emerges externally near the frenulum of the medullary velum, wrapping around the lateral surface of the upper part of the pons and cerebral peduncles; lateral to the oculomotor nerve, the trochlear nerve enters the dura mater, passes through the lateral wall of the cavernous sinus, where it receives sympathetic fibers from the cavernous plexus and sensory fibers from the ophthalmic nerve (V), enters the orbital cavity through the superior orbital fissure, and is directed to the superior oblique muscle. The superior oblique muscle also receives a portion of its fibers from the ipsilateral nucleus of the trochlear nerve. The connection of both trochlear nerve nuclei with each other and with other centers for the eye muscles is carried out through the axons of intercalated cells, the medial longitudinal fasciculus, and other systems. The associative connections for conjugate movements are the same as for the oculomotor nerve (see Oculomotorius nervus). Pathology. The trochlear nerve may suffer in various pathological processes of the orbit, the superior orbital fissure, the cavernous sinus, the base of the brain, the region of the brainstem, specifically in the upper sections of the pons and in the midbrain, as well as from pressure on the corpora quadrigemina, e.g., on the part of a tumor in the anterior section of the cerebellum. The trochlear nerve is usually involved in pathology together with the oculomotor nerve. Isolated unilateral paralyses of the trochlear nerve are extremely rare. The pathology does not differ from the pathology of the oculomotor nerve (see Oculomotorius nervus). Clinically, a disorder of the trochlear nerve is manifested by paralysis of the superior oblique muscle; movements of the eyeball downward and inward are thereby difficult. When the patient looks downward, the eyeball deviates outward; strabismus increases if the patient looks downward and in the direction opposite to the affected trochlear nerve, and disappears when looking downward and in the direction of the affected trochlear nerve. Treatment - see Oculomotorius nervus.