MODULATION OF ACTIVITY OF SPINDLE AFFERENTS RECORDED IN TRIGEMINAL MESENCEPHALIC NUCLEUS OF RABBIT DURING FICTIVE MASTICATION

1. These experiments were based on the findings that antidromic firing is observed in first-order sensory afferents during fictive locomotion and were designed to test the hypothesis that a similar central modulation of afferent discharge occurs during mastication. To do this, spindle afferents were recorded in the trigeminal mesencephalic nucleus (Mes V) of anesthetized and paralyzed rabbits during fictive mastication. The cortical masticatory area was stimulated to induce mastication, and activity of the XIIth or the Vth nerves were recorded to monitor the masticatory motor rhythm. 2. Although we could find little evidence that antidromic discharges invade the somatic region of this class of sensory afferents, we did discover a previously unrecognized type of modulation of afferent firing. 3. Of 83 slowly adapting muscle spindle afferents, 33 were modulated during fictive mastication. In 28 cases, the modulation consisted of a phasic inhibition, whereas for the remaining units it could be either a phasic excitation (n = 2) or an excitation alternating with an inhibition (n = 3). 4. Rapidly adapting units were also tested when encountered. Tonic or phasic excitation was never observed. The presence of inhibition could not be verified for this population because tonic activity could not be maintained by passive stretch. 5. The main electroneurogram (ENG) burst of the XIIth and Vth cranial nerves occurred during the opening phase of the masticatory cycle, and in all cases where the records were clear (22 out of 33), phasic inhibition of the afferents coincided with the ENG burst. 6. There was no difference in the distributions of the modulated and of the unmodulated units along the length of the Mes V nucleus. 7. Approximately 40% of trigeminal spindle afferent cell bodies have dendrites, and we suggest that these are the ones rhythmically modulated during fictive mastication. The possible role of this modulation is discussed.