The light microscopical structure of the mesencephalic nucleus of the fifth nerve in the selachian brain

Abstract

A description is provided of the mesencephalic nucleus of the fifth nerve (mes. V) and its axonal projections in the brains of dogfish ( Scylio - rhinus , Mustelus ) and rays ( Raja ), based upon light microscopic observations. The mes. V nucleus contains 800-1100 cells distributed about the midline at the ventral surface of the optic tectum. Each cell gives rise to a stout axon which courses laterally, then posteriorly on the ipsilateral side; the collected axons divide at the caudal extreme of the tectum into a medial bundle which enters the ventral granular eminence of the anterior cerebellar lobe, and a lateral bundle which enters the medulla and leaves the brain via the fifth nerve root. The medial bundle arises from a cluster of mes. V neurons at the posterior limit of the nucleus which comprise about 15% of the nuclear population. The cerebellar projection ascends the anterior lobe and appears to end in close proximity to Purkinje cell perikarya. A typical mes. V perikaryon is oval, flattened dorsoventrally with average dimensions of 41 µm x 56 µm. It emits a few stout dendrites from its dorsal surface which extend no more than 75 µm from the cell body. Processes from deep tectal cells and collaterals from commissural fibres appear to make synapses upon mes. V perikarya. In addition, neighbouring mes. V neurons often form clusters and also contact each other via short dendrites. A small subpopulation of mes V. neurons gives rise to stout, ventrally directed processes which enter the ependymal layer and ramify within it. These may have a neurosecretory function.

Mesencephalic Trigeminal Nucleus in Sharks

“…As reported in the ray and dogfish [Witkovsky and Roberts, 1975], in our preparations many fine fibers not of Mes V origin also appear to make contact with Mes V cells. These enter the nucleus from the periven tricular fiber system, from the commissural band of fibers dorsal to the nucleus and, especially toward the midline, from the lower portion of the optic tectum proper dorsal to the commissural fibers.…”

Oculomotor and Sensory Mesencephalic Trigeminal Neurons in Lungfishes: Phylogenetic Implications

“…1955;Gonzalez and Munoz, 1988;Roth et ai.. 1988] and one order of magnitude higher than in osteichthyes [40-50. Weinberg, 1928: Witkovsky andRoberts, 1975], Numbers of Mes V neurons do not simply reflect a phylogenetic trend in vertebrates [Weinberg, 1928], but rather correlate with the functional differentiation of the jaw muscles [DuBrul, 1960;Witkovsky and Roberts, 1975). The numbers of Mes V neurons in lungfishes confirm this notion: the jaw musculature of Protopterus and Neoceratodus is strong and uniquely adapted to a sophisticated feed ing mechanism [Luther.…”

“…This nucleus 260 von Burthcld O culom otor and Mesencephalic Trigeminal Neurons in Lungfishes is well developed in Protopterus and Neoceratodus. Num bers of Mes V cells (534-5%) are lower than in elasinobranchs [800-1100, Witkovsky and Roberts, 1975), but higher than in amphibians [60-200, Piatt, 1945;Kollros and McMurray. 1955;Gonzalez and Munoz, 1988;Roth et ai.. 1988] and one order of magnitude higher than in osteichthyes [40-50.…”

Circumventricular Mesencephalic Trigeminal Midline Ridge Formation in Cartilaginous Fishes: Species Variations

“…However, the cells of this posteri or group, while large, are smaller than those Mes V cells located more anteriorly (40-50 vs. 80-150 fim). It is possible that they comprise a population of cells of the Mes V complex unique to Mustelus and may correspond to cells reported by Witkovsky and Roberts [1975] to send pro cesses to the cerebellum. Witkovsky and Roberts [1975] used Mustelus in their work.…”

“…It is possible that they comprise a population of cells of the Mes V complex unique to Mustelus and may correspond to cells reported by Witkovsky and Roberts [1975] to send pro cesses to the cerebellum. Witkovsky and Roberts [1975] used Mustelus in their work. However, the observation of cerebellar projections requires experimental ana tomical confirmation.…”