Topological analysis of the brain stem of the sharks Squalus acanthias and Scyliorhinus canicula
Authors: Smeets Wilhelmus J. A. J. , Nieuwenhuys Rudolf 
Published: May 2005

The ventricular sulcal pattern and the cellular structure of the brain stem of the sharks Squalus acanthias and Scyliorhinus canicula have been studied in transversely cut Nissl, Klüver‐Barrera and Bodian stained serial sections. Five longitudinal sulci, the sulcus medianus inferior, the sulcus intermedius ventralis, the sulcus limitans, the sulcus intermedius dorsalis and the sulcus medianus superior could be distinguished in both species. In addition to these long, principal grooves, a number of shorter, accessory sulci appeared to be present. Although the neuronal perikarya in many places display a diffuse arrangement, with the aid of the usual cytoarchitectonic criteria 34 cell masses could be delineated in Squalus and 30 in Scyliorhinus. These cell masses can be distributed over the following categories )numbers added in brackets for Scyliorhinus, if different from those in Squalus(: primary efferent or motor, 10 )9(; primary afferent or sensory, 7: reticular formation, 5; “relay” centers, 9 )8(; nuclei of unknown relationships, 3 )1(. Seven of the cell masses found in Squalus and four of those found in Scyliorhinus have not been described before.

The morphological pattern of the cell masses and their relationship to the ventricular sulci were studied with the aid of a graphical reconstruction procedure termed topological analysis )cf. Nieuwenhuys, '74, and figs. 24, 25(. This analysis yielded for both sharks the following results: A sulcus limitans extends from the spinal level to the pretrigeminal region. This sulcus divides the greater part of the rhombencephalon into a basal plate and an alar plate. In the basal plate the sulcus intermedius ventralis marks the boundary between an area ventralis and an area intermedioventralis. The area ventralis contains two somatic motor centers )i.e., the rostral end of the spinal motor column and the medial part of the nucleus of VI( and by far the greater portion of the rhombencephalic medial reticular formation. The latter may be primarily considered as a somatic motor coordinating center. The area intermedioventralis contains the visceral motor nuclei of X, IX, VII and V. However, the basal plate also harbours a number of non‐motor centers, for example the inferior olive and the nucleus funiculi lateralis. In the caudal half of the rhombencephalon the sulcus intermedius dorsalis subdivides the alar plate into an area intermediodorsalis and an area dorsalis. The area intermediodorsalis is largely occupied by the common visceral sensory center of X, IX and VII; however, this area also contains the general somatic sensory nucleus descendens of V and the nucleus vestibularis magnocellularis. The area dorsalis is entirely occupied by two large lateral line nerve centers. The cell masses in the isthmus region and in the mesencephalon do not exhibit a clear‐cut morphological pattern. Functionally, however, a number of centers in the medial part of the tegmentum mesencephali )nuclei of IV and III, nucleus of the f.l.m., nucleus ruber( may be considered as a rostral continuation of the rhombencephalic somatic motor area. Similarly, the nucleus of Edinger‐Westphal represents the visceral motor zone at the midbrain level. The lateral part of the tegmentum mesencephali contains the nucleus of the torus semicircularis and the nucleus tegmentalis lateralis. It is suggested that these two nuclei together constitute a zone of somatic sensory relay centers.