The reptilian vestibular and cerebellar gray with fiber connections

Recent electrophysiological studies in the turtle Chrysemys picta have suggested that a neural correlate of the eye-blink reflex can be evoked in an in vitro brainstem-cerebellum preparation by electrical rather than natural stimulation of the cranial nerves. Discharge recorded in the abducens nerve, which is similar to EMG recordings from extraocular muscles during eye retraction, is triggered by a brief electrical stimulus applied to the ipsilateral trigeminal nerve. Evidence also suggests that pairing a one-second electrical stimulus applied to the posterior eighth nerve immediately prior to a single shock to the trigeminal nerve results in classically conditioned abducens nerve discharge in response to the previously neutral eighth nerve stimulus. In view of these physiological findings, the aim of the present study was to examine the central projections of trigeminal and posterior eighth nerve inputs to elucidate the anatomical substrates that may underlie the in vitro eye-blink reflex and possible pathways involved in reflex conditioning. Neurobiotin (NB) or fluorescein dextran (FD) was pressure injected into the cut end of either the trigeminal or posterior eighth nerve of the in vitro brainstem-cerebellum. Following trigeminal nerve injections, both tracers showed label in the ipsilateral trigeminal nuclear complex. Direct projections to the ipsilateral principal and accessory abducens motor nuclei were observed, suggesting that the eye-blink reflex is monosynaptic. Trigeminal nerve axons were also observed to terminate in the ipsilateral cerebellar cortex. The results of the posterior eighth nerve injections showed axonal projections and terminals in the cochlear, vestibular and principal sensory trigeminal nuclei. Terminal label was also observed in the ipsilateral cerebellar cortex, deep cerebellar nuclei, and in the principal and accessory abducens motor nuclei. Results from the NB cases suggested transneuronal transport of this tracer substance, whereas the FD cases did not. The present findings suggest that convergence of trigeminal and posterior eighth nerve inputs occurs in the ipsilateral cerebellar cortex, the principal sensory trigeminal nucleus, and the principal and accessory abducens motor nuclei. These regions of convergence may therefore be considered as potential sites of synaptic modification during in vitro studies of the conditioned abducens nerve reflex.

Section: Labeled Pathways Following Trigeminal Nerve Injectionsmentioning

confidence: 99%

Central Trigeminal and Posterior Eighth Nerve Projections in the Turtle Chrysemys picta Studied in vitro

Herrick

Keifer²

1998

“…The vestibular complex is divided into five nuclei [Weston, 1936] which show variable degrees of development in the different reptilian lines. The ventrolateral and dorsolateral nuclei are more highly differentiated in quadrupedal forms and the tangential nucleus more developed in serpentine forms [Stefanelli, 1944a, b].…”

Section: Discussionmentioning

confidence: 99%

Projections of the Vestibular and Cerebellar Nuclei in Rana pipiens

Montgomery

1988

The efferent projections and cytoarchitecture of the vestibulocerebellar region were examined to determine the nuclear boundaries and potential homologies. The anterior portion of the vestibular complex projects to the ipsilateral oculomotor and trochlear nuclei and is the major source of commissural fibers. Neurons in the rostromedial portions of the complex project to the contralateral trochlear nucleus. Large neurons in the ventrolateral portion of the complex give rise to a bilateral vestibulospinal pathway. Medium-sized neurons in the neuropil and small neurons in the central gray giving rise to bilateral projections to the spinal cord and oculomotor nuclei as well as commissural and ipsilateral cerebellar efferents. Projections from the nucleus of the cerebellum reach the contralateral spinal cord and cerebellar nucleus and there is also a bilateral projection to the ventral rhombencephalic and mesencephalic basal plates. The medial portion of the nucleus gives rise to commissural, ipsilateral mesencephalic and contralateral spinal projections. The lateral portion of the nucleus projects to the contralateral ventral mesencephalon. On the whole, the results of this investigation substantiate the division of the anuran vestibular complex in anurans into nuclei which may be homologous to the superior nucleus and nucleus of Deiters in mammals. The case for distinct descending and medial nuclei is less compelling. Further, it appears possible to divide the nucleus of the cerebellum into medial and lateral components whose connectivity is similar to that of reptiles and to a lesser extent mammals.

“…Ventrolateral vestibular n. N. vestibularis ventrolateralis [Weston, 1936] N. of Deiters [Weston, 1936] Ventral n. of Deiters [Sanders, 1929] Deiters' n. [Sanders, 1929] Lateral vestibular n. [Ebbesson, 1967] 2. Dorosolateral vestibular n.…”

Section: Vestibular Complexunclassified

“…Dorsal n. of Deiters [Sanders, 1929] N. vestibularis dorsolateralis [Weston, 1936] N. vestibularis anterior [Larsell, 1967] 3. Dorsomedial vestibular n.…”

Section: Vestibular Complexunclassified

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Projections of the Intraotic Ganglion to the Medullary Nuclei in the Tegu Lizard, Tupinambis nigropunctatus

DeFina¹,

Webster²

1974

The auditory and vestibular projectionsof the posterior ramus of the statoacoustic nerve were traced using silver degeneration techniques in the tegu lizard, Tupinambis nigropunctatus. Four tegus were perfused 7, 14, 21, and 28 days after labyrinthectomy, and three were perfused 21 days after cochlear ductectomy; 21 days was found to provide optimal degeneration patterns. Both surgical procedures destroyed the intraotic ganglion of the posterior ramus, which innervates the posterior crista, basilar papilla, macula lagena, macula sacculus, and possibly the macula neglecta. The vestibular components of the posterior ramus project to the medioventral part of the dorsolateral nucleus, the dorsal and dorsomedial parts of the ventrolateral nucleus, the dorsal part of the descending nucleus, and throughout the dorsomedial nucleus. No direct vestibulocerebellar tract is associated with the intraotic ganglion. The auditory components of the posterior ramus project to the entire nucleus angularis and nucleus magnocellularis. The nucleus laminaris receives either no primary statoacoustic fibers or very sparse projections on its lateral border.