Intracellular analysis of synaptic mechanisms controlling spontaneous and cortically induced rhythmical jaw movements in the guinea pig.

Abstract

Intracellular recordings were obtained from guiena pig jaw-closer motoneurons during cortically induced and spontaneously occurring rhythmical jaw movements (RJM). Short pulse-train stimulation of the masticatory area of the cortex produced an inhibitory postsynaptic potential (IPSP) in jaw-closer motoneurons, followed by a longer latency excitatory postsynaptic potential (EPSP). The cortically induced IPSP was compared to the IPSP induced by inhibitory inputs activated by intraoral stimulation with respect to their sensitivity to Cl- diffusion from the recording micropipette. It was suggested, based on electrophysiological techniques, that the cortical stimulus activates inhibitory premotor cells that have their inputs located predominantly on the soma region of the cell. The long-duration hyperpolarizing potentials evoked by repetitive cortical stimulation were shown to be IPSP. These IPSP were coincident with the digastric muscle electromyographic (EMG) burst phase of the RJM cycle. The long-duration IPSP were composed of short-latency subunit IPSP that were time-locked to each cortical stimulus. During spontaneous RJM the rhythmically occurring hyperpolarizing potentials in jaw-closer motoneurons were produced by an IPSP-generating mechanism. The IPSP produced in jaw-closer motoneurons by short pulse-train stimulation of the cortex, and the long-duration rhythmically occurring IPSP observed during RJM produced by repetitive cortical stimulation are produced by the same cortico-brain stem-motoneuronal pathway. The synaptic mechanisms producing the rhythmic membrane potential fluctuations in jaw-opener and -closer motoneurons during both spontaneous and cortically induced RJM are similar. A model for the proposed effects of cortical activation on jaw-opener and -closer motoneurons and a brain stem masticatory pattern generator is discussed.