A functional analysis of the mesencephalic nucleus of the fifth nerve in the selachian brain.

Microelectrode recordings were taken from neurons of the mesencephalic nucleus of the fifth nerve (mes. V) and from the fifth motor nucleus of decerebrate dogfish ( Scyliorhnusand ) in response: to electrical stimulation of the peripheral branches of the trigeminal nerve; to electri­ cal excitation of the mes. V tract and in response to mechanical stimulation of the lower jaw. Responses were recorded from cells in the optic tectum at depths of 1400-2000 jj.m which were driven directly by peripheral nerve stimulation, a t latencies of 1.2-2.8 ms, even above 300 Hz. These cells had axons which entered only one of the three main peripheral branches of the trigeminal nerve. Other cells, located a t the caudal limit of the nucleus, were driven antidromically by electrical stimulation of the cerebellum. The axons in the peripheral branches supplied highthreshold, non-spontaneous, slowly adapting mechanoreceptors of the teeth and perioral skin; there was no evidence of a projection to any type of muscle receptor in the jaw musculature as is found in mammals. The mes. V neurons could also be excited synaptically, with a latency of about 12 ms, by single low-frequency shocks or by pairs of shocks, separated by 4-10 ms, applied to the same branches of the trigeminal nerve; the sensory nucleus of the fifth nerve was apparently involved in this path ­ way. When the mesencephalic pathway was stimulated monosynaptic e.p.s.ps were recorded from the trigeminal motoneurons. Because those mes. V neurons which have peripheral branches can be activated in two ways, each functions both as a primary sensory cell and as an interneuron.