Gesineke C. Bangma

The fasciculus longitudinalis medialis in the lizard Varanus exanthematicus

SummaryIn the present study the vestibular components of the fasciculus longitudinalis medialis (flm) were investigated in the lizard Varanus exanthematicus with various tracing techniques: anterograde transport of horseradish peroxidase to study vestibulo-oculomotor and vestibulospinal projections, the multiple retrograde fluorescent tracer technique for the cells of origin of such projections. Internuclear projections between the oculomotor and abducens nuclei could also be studied in this way.Rather extensive vestibulo-ocular projections passing via the flm were demonstrated. Mainly ipsilateral ascending projections arise in the dorsolateral vestibular nucleus, mainly contralateral ascending projections in the ventromedial vestibular nucleus and adjacent parts of the ventrolateral and descending vestibular nuclei. Furthermore, distinet bilateral ascending projections of the nucleus prepositus hypoglossi were demonstrated. Extensive vestibulospinal projections pass via the flm and form the medial vestibulospinal tract. This largely contralateral descending pathway arises predominantly in the ventromedial and descending vestibular nuclei. Terminal structures presumably arising in the ventromedial and descending vestibular nuclei were found on contralateral neurons, probably motoneurons innervating neck muscles.Vestibular neurons with both ascending (presumably to extra-ocular motoneurons) and descending projections to the spinal cord are present in all vestibular nuclei, although preferentially in the ventromedial vestibular nucleus and adjacent parts of the ventrolateral and descending vestibular nuclei. However, also in the dorsolateral vestibular nucleus a substantial number of double labeled neurons were found. These vestibular neurons with both vestibulomesencephalic and vestibulospinal projections are probably involved in combined movements of eyes and head.Evidence for reciprocal internuclear connections between the oculomotor and abducens nuclei was found. Neurons in the dorsal part of the oculomotor nucleus probably project to the ipsilateral abducens nucleus, while neurons in the abducens nucleus most likely project to the contralatcral oculomotor nucleus. These recpprocal internuclear connections between the oculomotor and abducens nuclei probably play an important role in conjugate horizontal eye movements.

Materials and Techniques

The present cytoarchitectonic analysis of the brain stem of the savanna monitor lizard, Varanus exanthematicus (Fig. 1), is based on Nissl (cresylecht violet), Kluver-Barrera (1953), and Haggqvist (1936) stained material. Four transversely sectioned (15 µm) Nissl stained, one Kluver-Barrera, and one Haggqvist stained series were available. In addition, a Nissl and a Kluver-Barrera stained sagittal series (15 µm) were used. For comparison, transversely and sagittally sectioned series of the central nervous system of the tegu lizard, Tupinambis nigropunctatus, were available.

Reticulospinal and vestibulospinal pathways in the snake Python regius

SummaryIn the present HRP study extensive reticulospinal projections and more modestly developed vestibulospinal pathways have been demonstrated in the snake Python regius. The funicular trajectories of the main reticulospinal pathways have been shown: via the lateral funiculus pass spinal projections of the nucleus reticularis superior pars lateralis, the nucleus reticularis inferior and nucleus raphes inferior; via the ventral funiculus fibers arising in the nucleus reticularis superior and nucleus reticularis medius. Spinal projections of the locus coeruleus and subcoeruleus area reach their targets via both the lateral and ventral funiculi. Two vestibulospinal pathways have been demonstrated: an ipsilateral tractus vestibulospinalis lateralis arising in the ventrolateral vestibular nucleus, and a contralateral tractus vestibulospinalis medialis from the descending and ventromedial vestibular nuclei. After HRP gel implants into the vestibular nuclear complex direct vestibulocollic projections to motoneurons in the rostral spinal cord were observed.Spinal projections from the ventral part of the nucleus reticularis inferior and the descending and ventromedial vestibular nuclei are mainly aimed at the thin “neck area” (approximately the first 50 spinal segments). This area is extensively used in such acts as orientation and prey-catching, requiring a rather delicate brain stem control.

A crossed rubrobulbar projection in the snakePython regius

In the present study a distinct crossed rubrobulbar projection has been demonstrated in the snake Python regius, a limbless vertebrate which lacks a rubrospinal tract. This rubrobulbar projection is presumably involved in the neural control of mastication. The red nucleus may relay cerebellar influence to the trigeminal and facial nuclei.

Organization and Connections of the Sensory Trigeminal Nuclei

The sensory part of the trigeminal nuclear complex comprises three nuclei: the descending (or spinal) nucleus, the principal nucleus, and the mesencephalic nucleus. The descending and principal nuclei constitute a column of gray which extends from the entrance of the trigeminal nerve into the rostral part of the spinal cord, where it is continuous with the peripheral part of the dorsal horn.

The Reticular Formation

The reticular formation of the reptilian brain stem is composed of various types of cells, ranging from small to very large, reticulospinal neurons (Sect. 13.1). It contains monoaminergic and peptidergic components (Sect. 13.2). The rhombencephalic reticular formation is the main site of termination of afferents from the spinal cord (see Sect. 4.2.1.1 and Fig. 22), is extensively afferented by the tectum mesencephali (see Sect. 10.1.3.2) and also receives a direct projection from the striatum (see Sect. 11.2 and Fig. 45). Efferent fibers pass to the thalamus and even to the telencephalon (Sect. 13.3), but chiefly to the spinal cord (Sect. 13.4). The bulk of descending pathways to the spinal cord is formed by reticulospinal fibers (ten Donkelaar 1982). Prosencephalic structures exert their influence on brain stem and spinal motor mechanisms via the brain stem reticular formation. It seems likely that, in reptiles and in other lower vertebrates (e.g., Nieuwenhuys 1977), the reticular formation represents a “final common supraspinal” pathway by way of which higher nervous centers can influence spinal motor mechanisms (ten Donkelaar 1982).

The Vestibular Nuclear Complex and Related Structures

The reptilian vestibular complex is usually divided into at least five nuclei (Weston 1936), i.e., the nucleus vestibularis dorsolateralis (also described as the superior vestibular nucleus: Beccari 1911 ; Larsell 1926; Stefanelli 1944a), the nucleus vestibularis ventrolateralis, the nucleus vestibularis tangentialis, the nucleus vestibularis ventromedialis, and the nucleus vestibularis descendens (or inferior: Beccari 1911; Stefanelli 1944a). In turtles, in addition a nucleus vestibularis superior can be distinguished as a subdivision of the dorsolateral vestibular nucleus (Weston 1936; Cruce and Nieuwenhuys 1974; Miller and Kasahara 1979; ten Donkelaar and Nieuwenhuys 1979; Bangma et al. 1983). Stefanelli (1944 a, b; see also Schwab 1979; ten Donkelaar and Nieuwenhuys 1979) noted the considerable differences in the development of the vestibular nuclei and their connections. The ventrolateral and dorsolateral vestibular nuclei are strongly developed in those reptiles with wholly or partly quadrupedal locomotion, whereas the tangential nucleus is particularly large in species with serpentine movements.

The Solitary Tract and Related Nuclei

The reptilian solitary tract is a well-developed, thin-fibered bundle that receives fibers from cranial nerves VII, IX, and X (see Sect. 6.2). Its fibers terminate in the nucleus of the solitary tract and in its mediocaudal extension, the nucleus commissurae infimae (Jacobs 1979; Barbas-Henry 1982; Barbas-Henry and Lohman 1984). In the older literature, based on normal material, an ascending pathway to a sensory visceral nucleus located in the rostral part of the rhomben-cephalon just ventromedial to the nucleus isthmi, has been suggested (Shanklin 1930; Barnard 1936; see also ten Donkelaar and Nieuwenhuys 1979). In a more recent study in Varanus exanthematicus, ten Donkelaar and de Boer-van Huizen (1981b) introduced the term “parabrachial region” for this secondary visceral center.

Spinal Projections to the Brain Stem

Spinal projections reach the brain stem via the dorsal and the lateral funiculi. The dorsal funiculus contains ascending collateral branches of spinal primary afferent (dorsal root) fibers, the lateral funiculus the spinal lemniscus and spino-cerebellar projections. The course and site of termination of spinal projections to the brain stem are shown in Fig. 22 in which the distribution of degenerating fibers and preterminal structures following a hemicordotomy at the third spinal segment is presented.

Forebrain Projections to the Brain Stem

Input from higher brain centers to the brain stem arises particularly in the basal forebrain (see Sect. 11.2) and reaches the mesencephalic tegmentum via the lateral and medial forebrain bundles. In addition, several thalamic and hypothalamic nuclei project to the brain stem (Sect. 11.3), whereas the habenula sends the fasciculus retroflexus or habenulointerpeduncular tract to the nucleus interpeduncularis (Sect. 11.4) and adjacent reticular formation.