Brainstem neurons projecting to different levels of the spinal cord of the dogfish Scyliorhinus canicula.

Neurons in the brain that project to different levels of the spinal cord in the dogfish Scyliorhinus canicula have been identified by retrograde labelling with horseradish peroxidase. Injections have been made at four different levels, namely, cervical (segments 3-6), pectoral (segments 16-18), pelvic (segments 34-36) and caudal (segments 60-80). Labelled neurons were located in the diencephalon, mesencephalon and rhombencephalon. Twenty nuclei contained labelled cells following cervical injections. Fourteen of these contained labelled cells following pectoral injections as did nine following pelvic injections and only seven after caudal injections. Of the three diencephalic nuclei projecting to the cord only nucleus thalamus ventralis pars medialis projected further than cervical regions. From the mesencephalon, the tectospinal projection, which arises from tectal and tectotegmental regions, reaches only as far as the cervical cord. The reticulospinal system, arising from cell groups in both the mesencephalon and rhombencephalon, provides the major descending pathway and reaches the most caudal levels. From the octaval region a vestibulospinal pathway arises from nucleus octavus magnocellularis, to reach all levels of the cord, and from nucleus octavus descendens, to project only as far as the pectoral cord. Other rhombencephalic cell groups--the trigeminal nuclei and nuclei B, F and G--project at least as far as the pectoral cord. We conclude that few brainstem nuclei can directly affect the whole spinal cord, whereas many nuclei may have an impact on its most rostral regions; here we have found labelled spinal interneurons with long descending axons that reach the most caudal cord levels.