Mastication-induced modulation of orofacial sensory inputs as seen in the jaw reflex and single neuronal activities in the face primary somatosensory cortex of the rabbit.

The aim of the present study was to compare the effect of mastication on non-nociceptive orofacial sensory transmission of the somatosensory pathways and that on the jaw reflex arc. To achieve this, single-unit activities of the face primary somatosensory cortex (face-SI) neurons responding to low-intensity electrical stimulation of the inferior alveolar nerve (IAN) and the jaw-opening reflex (JOR) were recorded during mastication in awake rabbits. The entire masticatory sequence was divided into the preparatory period (PP), the rhythmic-chewing period (RC) and the preswallow period (PS). A total of 112 face-SI neurons were tested for the effects of mastication. The responsiveness of most neurons and the JOR were modulated during different periods of mastication. The IAN-evoked activity was attenuated in a majority of the neurons during at least one of the masticatory periods. The JOR was suppressed during the RC and PS, while the modulatory effect was variable during the PP. Regression analysis showed that the modulatory effect of mastication on the IAN-evoked neuronal activity and that on the JOR was not well correlated. It was notable that a considerable number of face-SI neurons showed mastication-related activity although their responsiveness to IAN stimulation was strongly suppressed (i.e. almost "gated out") during mastication. The findings indicate that (1) sensory transmissions of the somatosensory and reflex pathways may be modulated in a different manner during mastication, and that (2) activity of face-SI neurons during mastication may not necessarily be a simple reflection of movement-induced stimulation of orofacial mechanoreceptors.