Quantitative studies on motoneurons: Hypoglossal neurons in the boa constrictor, Constrictor constrictor

Snakes deliver odorants to the vomeronasal organ by means of tongue-flicks. The rate and pattern of tongue-flick behavior are altered depending on the chemical context. Accordingly, olfactory and vomeronasal information should reach motor centers that control the tongue musculature, namely, the hypoglossal nucleus (XIIN); however, virtually nothing is known about the circuits involved. In the present work, dextran amines were injected into the tongue of garter snakes (Thamnophis sirtalis) to identify the motoneurons of the XIIN. Tracers were then delivered into the XIIN to identify possible afferents of chemical information. Large injections into the XIIN yielded retrograde labeling in two chemosensory areas: the medial amygdala (MA) and the lateral posterior hypothalamic nucleus (LHN). Smaller injections only yielded labeled neurons in the LHN. In fact, the MA, which receives afferents from the accessory olfactory bulb, the rostroventral lateral cortex, and the nucleus sphericus, projects to the LHN. Injections into the MA did not show terminal labeling in the XIIN but in an area lateral to it. However, injections into the LHN gave rise not only to labeled fibers in the XIIN but also to retrograde labeling in the MA, thus confirming the chemosensory input to LHN. Injecting different fluorescent tracers into the tongue and into the LHN corroborated the projection from the LHN to the XIIN. The present report investigates further connections of the olfactory and vomeronasal systems and describes the afferent connections to XIIN in a nonmammalian vertebrate. The circuit for tongue-flicking behavior described herein should be evaluated using functional studies.

Neural substrates for tongue‐flicking behavior in snakes

Martı́nez-Marcos

Úbeda-Bañón

Halpern

2001

Cytoarchitecture of nucleus sphericus in the common boa, Constrictor constrictor

Ulinski

Kanarek

1973

The normal anatomy of nucleus sphericus in the common boa (Constrictor constrictor) is described here on the basis of routine light microscopic and Golgi material. Nucleus sphericus is a cup-shaped aggregation of cells lying in the forebrain posterior to the anterior commissure and consisting of three concentric layers. The marginal layer is subependymal, is continuous rostrally with the anterior commissure, and contains fusiform cells with discoidal dendritic fields tangent to the mural layer. Marginal cell axons entwine mural cell dendrites. The mural layer forms the walls of the cup and appears as a closed ring of cells in transverse sections. Mural cell dendrites extend radially into the marginal and hilar layers and horizontally within the mural layer. All of these dendrites are profusely covered by dendritic spines with conical heads and slender stalks. Mural cell axons ascend to the marginal layer. The hilar layer forms the cup's cavity containing scattered cells whose dendrites extend in all directions and whose axons approach the cup's mouth. Afferent fibers in the nucleus run predominately at right angles to mural cell dendrites bearing varicosities at irregular intervals.

Quantitative studies on motoneurons. II. Spatial and dimensional organization of hypoglossal motoneurons in the boa constrictor, Constrictor constrictor

Ulinski

1974

The spatial distribution and sizes of degenerating hypoglossal neurons were studied following the removal of the genioglossus or hyoglossus muscles or of unilateral glossectomy in common boas with survival times of 7, 14, 21 or 40 days. The spatial organization of this nucleus differs from that of most cranial nerve nuclei in lacking a subnuclear organization: genioglossus and hyoglossus motoneurons are spread evenly throughout the nucleus and intrinsic tongue neurons show only a slight tendency to accumulate in the caudal half of the nucleus. The genioglossus and tongue neurons probably serve both ipsilateral and contralateral muscles with the ipsilateral innervation being largest; the hyoglossus motoneurons probably innervate only ipsilateral muscles. Complementary analyses of the transverse sectional areas of both degenerating and non-degenerating neurons indicates that, although their size ranges overlap, genioglossus motoneurons are small and tongue and hyoglossus motoneurons are progressively larger.