Physiological identification and property of afferents from the masseter muscle to the caudal part of the trigeminal nucleus in rat.

The present study was to identify the physiological properties of the afferent fibers from the masseter muscle to the caudal part of the spinal trigeminal nucleus of rat using a microelectrode technique. When electrical stimulation was applied to the masseter muscle, the evoked potentials were recorded with a shorter latency (the S-response) or a longer latency (the L-response). The threshold current intensity of the S-response was lower than that of the L-response. There were statistical significant differences between the S-response and the L-response in latency and in threshold current intensity. Both the S- and L-responses could follow by stimulation of low frequencies (10 to 30 Hz), indicating that the evoked responses were the component of the secondary neuron activities. On the anatomical and physiological assumption, the conduction velocities of the S- and the L-responses were calculated and they were in the range of that of A-delta fibers. We also observed that both the S- and the L-responses could not follow high frequency stimulation of the masseter muscle. This effect may reflect the phenomenon of fatigue in the finely myelinated fibers or polymodal nociceptors. Thus, the present study suggested the involvement of the afferents from the masseter muscle to the spinal trigeminal nucleus in the transmission and the relay of the masticatory muscle pain.