Comparison of volume-conducted far-field short-latency glossopharyngeal nerve evoked potentials recorded from the scalp with similarly obtained near-field potentials from the solitary nucleus in dogs.

In 11 dogs, potentials recorded from the scalp and from the solitary nucleus after stimulation of the glossopharyngeal nerve were compared. The far-field potentials recorded from the scalp consisted of negativity, with peak latency of 2.10 to 3.45 milliseconds (mean, 2.93 milliseconds), followed by positivity, with peak latency of 3.20 to 5.95 milliseconds (mean, 4.86 milliseconds) and duration of 4.65 to 6.95 milliseconds (mean, 5.70 milliseconds). The near-field potentials recorded from the solitary nucleus consisted of positivity, with peak latency of 2.15 to 2.70 milliseconds (mean, 2.45 milliseconds), followed by negativity, with peak latency of 4.05 to 5.05 milliseconds (mean, 4.39 milliseconds) and duration of 4.45 to 5.80 milliseconds (mean, 5.21 milliseconds). Comparison of the far-field potentials (n = 10) with the near-field potentials (n = 5) indicated that polarity of the waves was reversed and that the first peak's latency was slightly (approx 0.5 milliseconds) longer in the scalp-recorded far-field potentials. Neither the difference in latency of the second peak nor the difference in its duration, measured from the onset of the potentials to the return to the baseline of the activity, was significant. The results strongly suggest that the response in the solitary nucleus evoked by electrical stimulation of the glossopharyngeal nerve is the source of at least part of the scalp-recorded responses to stimulation of the same nerve. The scalp-recorded far-field potentials could, therefore, be characterized as volume conducted from the evoked response in the solitary nucleus.

CMAPs in pharyngeal and hyoid muscles evoked by nucleus solitarius stimulation in dogs.

Electrical stimulation of the solitary nucleus with a single pulse of 150 to 500 microA caused visible contractions of the muscles involved in swallowing but did not activate the central pattern generator for swallowing. Electromyographically there were short bursts of activity characterized by an onset of activity within 1 to 7 ms, a duration of 14 to 26 ms, and an amplitude of 10 to 4,800 microV. The CMAPs were recorded in the pharyngeal and hyoid muscles on both the ipsilateral and the contralateral sides of the stimulated solitary nucleus. Across all five pairs of muscles in which the recordings were made the latencies of the CMAPs in the contralateral muscles were significantly greater and the amplitudes significantly lower, but the durations were not significantly different. Bilateral activation of the muscles by unilateral stimulation may not result in symmetrical contraction of the pharyngeal muscles.

Primary projections and efferent cells of the VIIIth cranial nerve in the monitor lizard, Varanus exanthematicus.

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.

Mixed-junction axon terminals on identified principal abducens motoneurons in the monitor lizard.

Motoneurons in the principal abducens nucleus of the monitor lizard Varanus exanthematicus were identified by retrograde labeling following application of horseradish peroxidase to the abducens nerve. The ultrastructure and synaptology of thirty labeled neurons were studied. We observed a type of axon terminal which forms mixed junctions with the cell bodies and the initial axon segments of labeled motoneurons. The juxtaposed membranes of the terminals and the motoneurons display gap junctions and small asymmetric synaptic specializations. The mixed-junction terminals contain spherical synaptic vesicles which are located immediately adjacent to the synaptic junction. They may originate from local circuit neurons or from neurons extrinsic to the principal abducens nucleus.

The motor nuclei and sensory neurons of the IIIrd, IVth, and VIth cranial nerves in the monitor lizard, Varanus exanthematicus.

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.

Synaptic connections between primary trigeminal afferents and accessory abducens motoneurons in the monitor lizard, Varanus exanthematicus.

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 complex and primary projections of the trigeminal nerve in the monitor lizard, Varanus exanthematicus.

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 primary projections of the IXth, Xth, XIth and XIIth cranial nerves in the monitor lizard, Varanus exanthematicus.

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.

The motor nuclei and primary projections of the facial nerve in the monitor lizard Varanus exanthematicus.

The location of the motor nuclei and the projection of the primary afferent fibers of the facial nerve of the reptile Varanus exanthematicus were studied by means of the HRP method and the anterograde degeneration technique. The motor nuclei are located ventrolaterally in the rhombencephalon and are constituted by a medial cell group consisting of large, polygonal cells and a lateral cell group consisting of medium-sized, spindle-shaped, and multipolar cells. From HRP applications to the various branches of the facial nerve it could be concluded that the medial cell group represents the branchiomotor nucleus and the lateral cell group the superior salivatory nucleus. The efferent axons from the motor nuclei course dorsomedially toward the fourth ventricle, where they form a genu, and exit from the brainstem in the ventral fiber bundle of the facial nerve. The primary afferent fibers enter the brainstem in the dorsal bundle of the facial nerve. This bundle courses medially, enters the solitary tract, and diverges into rostrally and caudally running fibers. Part of the caudally directed fibers leave the solitary tract and course laterally toward the descending trigeminal tract. Some fibers enter the nucleus of this tract. There was no noticeable terminal degeneration in the solitary tract or in the descending trigeminal tract or its nucleus.