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

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.