The sensory projections and the motor complex of the trigeminal nerve of the reptile Varanus exanthematicus were studied with the methods of anterograde degeneration and anterograde and retrograde axonal transport. The primary afferent fibers diverge in the brainstem into a short ascending and a long descending tract. The former distributes its fibers to the principal sensory trigeminal nucleus, where nerves V1, V2, and V3 are represented along a lateromedial axis. The fibers of the descending tract enter the nucleus of this tract and the reticular formation. Both in the tract and its nucleus, nerves V1, V2 and V3 occupy successively more dorsal positions. A small contingent of nerve V1 fibers course to the accessory abducens nucleus. The descending tract extends caudally into the first and second cervical segments of the spinal cord. The trigeminal motor complex consists of dorsal, ventral, and dorsomedial nuclei. The m. adductor mandibulae externus (the main jaw closer) is represented in the dorsal nucleus, predominantly in its rostral part. The muscles innervated by nerve V3 are represented in the ventral nucleus, mainly in its caudal part. All three divisions of the trigeminal nerve contain peripheral branches of the mesencephalic trigeminal system. Collaterals of the central branches of this system were traced to the ventral motor and the principal sensory trigeminal nuclei.

The motor nuclei and sensory connections of the IXth, Xth, XIth, and XIIth cranial nerves of the reptile Varanus exanthematicus were studied with the methods of anterograde degeneration and anterograde and retrograde axonal transport. The motor nuclei of nerve IX are located ventrally in the rhombencephalon and are constituted medially by the large-celled glossopharyngeal part of the nucleus ambiguus and laterally by the small-celled nucleus salivatorius inferior. The motor nuclei of nerve X consist of the dorsomedially located dorsal motor nucleus of the vagus and the laterally located vagal part of the nucleus ambiguus. The rostral portion of the latter cell group contains smaller cells than its caudal portion and is rostrally continuous with the nucleus salivatorius inferior of nerve IX. The efferent axons of nerves IX and X arising from the ventrolateral medulla first course dorsomedially, form genua beneath the IVth ventricle, and then exit the brainstem. All primary afferent fibers of nerve IX and the majority of those of nerve X enter the solitary tract. Terminations of vagal fibers were observed in the postvagal portion of the nucleus of the solitary tract, the dorsal motor nucleus of the vagus, and the nucleus of the commissura infima. A small contingent of vagal fibers courses caudally just dorsolateral to the descending trigeminal tract. A separate spinal component of nerve XI could not be found. The bulbar component of this nerve forms part of nerve X and takes its main origin from a detached caudal element of the nucleus ambiguus. The motor nuclear complex of nerve XII consists of a large dorsal nucleus and a small ventral nucleus that extend from the medulla oblongata into the first segment of the cervical spinal cord.

In order to describe the central relations of both the afferent and efferent components of the VIIIth cranial nerve in one reptile, the methods of anterograde and retrograde axonal transport and anterograde degeneration were used to study the vestibular and cochlear projections and the efferent system of this nerve in Varanus exanthematicus. On the basis of cresyl violet and Klüver-Barrera staining, five vestibular nuclei, four cochlear nuclei, and two clusters of small cells which could not be designated as strictly auditory or vestibular are distinguished. The vestibular nuclei include the nucleus dorsolateralis, nucleus ventrolateralis, nucleus tangentialis, nucleus ventromedialis, and nucleus descendens. The well-developed cochlear nuclear complex includes the nucleus angularis, nuclei magnocellulares medialis and lateralis, and nucleus laminaris. The two cell clusters are located dorsolaterally in the brainstem just ventrolateral to the acoustic tubercle. The primary afferent vestibular fibers coursing in the anterior VIIIth nerve root distribute to the ventral portions of all vestibular nuclei except nucleus ventromedialis, whereas the fibers coursing in the posterior root project to the dorsal portions of these nuclei. In nucleus ventromedialis fibers of both roots do not segregate into ventral and dorsal portions. Other targets of the vestibular fibers are the two cell clusters, the granular layer of the ipsilateral cerebellum, the reticular formation, and the descending trigeminal tract and its nucleus. The primary cochlear fibers coursing in the posterior root terminate in nucleus angularis, nuclei magnocellulares medialis and lateralis, and the inner cell strand of nucleus laminaris. The efferent system is, ipsi- and contralaterally in the brainstem, composed of ventral and dorsal cell groups that extend from the level of the principal abducens nucleus caudally where they overlap with the facial motor nucleus. The fibers, which originate from the contralaterally located efferent cells, course beneath the IVth ventricle to exit the brainstem on the ipsilateral side.

We studied the anatomical pathway underlying the nictitating reflex in the monitor lizard Varanus exanthematicus by the anterograde degeneration technique combined with retrograde transport of horseradish peroxidase (HRP) and electron microscopy. After application of HRP to the abducens nerve, retrogradely labeled neurons were observed in the ipsilateral principal and accessory abducens motor nuclei. The transection, in the same experiments, of the root of the trigeminal nerve resulted in massive degeneration of myelinated fibers in the descending trigeminal tract. In the ipsilateral accessory abducens nucleus, we observed electron-dense degenerating axon terminals that formed asymmetric synaptic contacts with the primary and secondary dendrites of large neurons retrogradely labeled with HRP. A few of the degenerating terminals could be traced in serial sections to myelinated axons. No terminal degeneration was found in the contralateral accessory abducens nucleus or in the ipsilateral and contralateral principal abducens nuclei. The present results are complementary with the findings of previous light microscopic experimental tracing studies (Barbas-Henry, H.A., and A.H.M. Lohman, J. Comp. Neurol. 1986, 254:314-329; see also J. Comp. Neurol. 1988, 267:370-386), and strongly suggest the existence in Varanus of a monosynaptic, unilateral reflex pathway in which trigeminal fibers, presumably originating from the cornea, synapse with motoneurons of the bursalis and retractor bulbi muscles, which are located in the accessory abducens nucleus. This monosynaptic pathway may mediate a rapid unilateral eyeball retraction and nictitating membrane extension.

The motor nuclei of the oculomotor, trochlear, and abducens nerves of the reptile Varanus exanthematicus and the neurons that subserve the sensory innervation of the extraocular muscles were identified and localized by retrograde and anterograde transport of horseradish peroxidase (HRP). The highly differentiated oculomotor nuclear complex, located dorsomedially in the tegmentum of the midbrain, consists of the accessory oculomotor nucleus and the dorsomedial, dorsolateral, intermediate, and ventral subnuclei. The accessory oculomotor nucleus projects ipsilaterally to the ciliary ganglion. The dorsomedial, dorsolateral, and intermediate subnuclei distribute their axons to the ipsilateral orbit, whereas the ventral subnucleus, which innervates the superior rectus muscle, has a bilateral, though predominantly contralateral projection. The trochlear nucleus, which rostrally overlaps the oculomotor nuclear complex, is for the greater part a comma-shaped cell group situated lateral, dorsal, and medial to the medial longitudinal fasciculus. Following HRP application to the trochlear nerve, almost all retrogradely labeled cells were found in the contralateral nucleus. The nuclear complex of the abducens nerve consists of the principal and accessory abducens nuclei, both of which project ipsilaterally. The principal abducens nucleus is located just beneath the fourth ventricle laterally adjacent to the medial longitudinal fasciculus and innervates the posterior rectus muscle. The accessory abducens nucleus has a ventrolateral position in the brainstem in close approximation to the ophthalmic fibers of the descending trigeminal tract. It innervates the retractor bulbi and bursalis muscles. The fibers arising in the accessory abducens muscles form a loop in or just beneath the principal abducens nucleus before they join the abducens nerve root. The afferent fibers conveying sensory information from the extraocular muscles course in the oculomotor nerve and have their perikarya in the ipsilateral trigeminal ganglion, almost exclusively in its ophthalmic portion.