Reflex and reticular modulation of first-order proprioceptive neurons of the mesencephalic trigeminal nucleus

Several investigations have shown that the vagus nerve and the reticular formation can affect the reflex responses of the masticatory muscles. The present research has been devoted to analyze the mechanism of such modulations of the masseteric reflex in the lamb. Extracellular records of the electrical activity of the mesencephalic trigeminal nucleus (MTN) was carried out in immobilized lambs by means of tungsten microelectrodes. Units were found which responded to lowering the jaw and to stretching the masseter muscle: they were identified as the first-order neurons of the masticatory proprioception on the basis of their electrophysiological properties. Single-shock or repetitive electrical stimulations of the cervical vagus nerve and of the bulbo-pontine reticular formation could affect the unitary discharge of the MTN: different patterns of activation and inhibition of the MTN units were seen; however, the activation was the most prominent effect. The responses did not depend upon the circulatory effects of the vagal stimulation. Thus the conclusion can be reached that the vagus and the reticular substance can modulate the masseteric reflex at level of the perikarya of the afferent pathway. Such a statement is supported also by the presence of synaptic boutons on the soma of the MTN neurons.