Jaw muscle proprioception and mesencephalic trigeminal cells in birds

Abstract

Extracellular record of the electrical activity of the mesencephalic trigeminal cells located within the posterior commissure was carried out in immobilized ducks by means of tungsten microelectrodes. The effects of jaw movements and stretches of some isolated jaw muscles on such unitary discharge were investigated. The great majority of the recorded units were fired by opening the jaw. The responses consisted of a short latency (2–5 msec) sustained increase in the discharge rate (up to 260 pulse/sec), which ceased immediately as soon as the opening was released. Such units responded also to the ipsilateral movements of the jaw without opening the jaw and to a moderate stretch of the ipsilateral adductor mandibulae externus superficialis and retractor anguli oris muscles. About 15% of the recorded units were inhibited by opening the jaw and showed a high discharge frequency when the jaw was closed. The activation of the unitary discharge of the trigeminal mesencephalic cells brought about by opening the jaw was inhibited by electrical stimulation of the muscles, identifying the units as muscle spindle afferents. The conclusion was reached that the mesencephalic trigeminal cells of the posterior commissure are concerned with the jaw muscle proprioception in ducks.