Frequency-response analysis of vestibular-induced neck reflex in cat. I. Characteristics of neural transmission from horizontal semicircular canal to neck motoneurons.

1. Vestibular-induced neck muscle reflexes (vestibulocollic reflexes) were studied with frequency-response methods in unanesthetized, decerebrate cats. The horizontal semicircular canals were stimulated by oscillation of the turntable and EMG activity was recorded from neck extensor muscles. 2. The maximum firing rate of each motor unit was less than 40 spikes/s. The motor units were classified by their maximum firing rates into two groups: HF (high frequency) units which could fire above 20 spikes/s and LF (low frequency) units which could not exceed 20 spikes/s of firing. The HF units had larger gains than the LF units on the average. 3. Compound EMGs, which presumably represent activity of the whole muscle, were examined at different frequencies of stimulation. The gain of compound EMG responses depended on the spontaneous activity. When the spontaneous activity was low or too high, the gain was small. There was an intermediate spontaneous activity level at which the gain became maximal. 4. The phase lag and the gain of the frequency response were represented in a Bode diagram with respect to angular acceleration. The transfer function of the system from the vestibular nuclei to neck EMG activity was estimated by assuming a first-order lag-lead system. A phase lag of 70-80 degrees (0.05 Hz) was found between the averaged activity of vestibular nucleus neurons and cervical motor activity. A positive correlation was observed between the phase lag and the gain of each motor unit. These results cannot be attributed solely to the action of the vestibulospinal tract, but suggest the existence of a neural intergrator in the vestibulocollic reflex arc.