Relationship between jaw movements and trigeminal motoneuron membrane-potential fluctuations during cortically induced rhythmical jaw movements in the guinea pig

Low-frequency stimulation (less than 10 Hz) of the masticatory area of the cortex of the anesthetized guinea pig evokes a jaw-opening twitch, which is produced by a short latency (≃ 12 ms) electromyographic (EMG) response in the jaw-opener digastric muscle, along with a short-latency (≃ 8 ms) hyperpolarizing potential in jaw-closer motoneurons. Stimulation of the same cortical area with high-frequency repetitive stimulation (15-80 Hz) results in rhythmic jaw movements (RJMs) with a cycle frequency of approximately 3 Hz. The RJM cycle is separated into 1) the burst phase, which is characterized by EMG activity in the digastric muscle; and 2) the interburst phase, during which no activity in this muscle is observed. The EMG activity in the burst phase is composed of subunits. These are short-duration (≃ 16 ms) EMG bursts, which follow each cortical stimulus at a short latency (≃ 12 ms). Cortical stimuli that occur during the interburst phase do not evoke digastric EMG activity. During the burst phase each cortical stimulus evokes a short latency (≃ 8 ms) hyperpolarizing potential in jaw-closer motoneurons and a short-latency (≃ 8 ms) depolarization in jaw-opener motoneurons. There is a characteristic cyclically occurring waxing and waning of these potentials during RJMs. In the interburst phase the membrane potential of jaw-closer motoneurons is depolarized to varying degrees with the occasional appearance of spike potentials. There is no activity in jaw-opener motoneurons in the interburst phase. Low-frequency cortical stimulation evokes a jaw-opening twitch with a time to peak of ≃ 30 ms and a peak amplitude of ≃ 0.1 mm. The RJMs that are evoked by high-frequency cortical stimulation are composed of the coalescence of these jaw-opening twitches. The jaw-opening movements that occur during RJMs have a time to peak of ≃ 100 ms and a peak amplitude of ≃ 1-2 mm. The jaw musculoskeletal system acts as a low-pass filter that transforms the individual twitches into smooth jaw movements.