Frequency-response analysis of vestibular-induced neck reflex in cat. II. Functional significance of cervical afferents and polysynaptic descending pathways.

1. Three major inputs to cervical motoneurons were analyzed with frequency-response methods in decerebrate and unanesthetized cats: the afferent system including the gamma-loop, the vestibulospinal tract, and some polysynaptic tract that transmits vestibular influences in the vestibulocollic reflex. 2. The dorsal roots C1-C4 were cut in order to open the feedback loop through the gamma-fiber spindle-afferent system. No effects were observed on the gain and the phase lag of motor-unit response. However, the DC components of the response were consistently decreased by deafferentation both in motor-unit and compound EMG responses. Firing of muscle spindle afferents was modulated during oscillation of the turntable. The majority of responses of spindle afferents was in phase with the simultaneously recorded extrafusal motor activity. These results indicate the existence of alpha-gamma coactivation in the vestibulocollic reflex. 3. Intracellular recording from cervical motoneurons showed that monosynaptic EPSP's were induced by dorsal root stimulation. By far the larger EPSP's, however, were observed with latencies of 10-15 ms, showing the existence of more powerful polysynaptic routes. It is postulated that the cervical afferent system controls the gain of the reflex by changing the number of motor units participating in the vestibulocollic reflex through some polysynaptic pathways, and that the afferent system scarcely changes, the gain or phase lag of individual motor units through a short stretch reflex loop. 4. The medial vestibulospinal tract in the MLF was interrupted, but no effects were detected on the dynamic characteristics of the frequency response. This shows the existence of other effective descending pathways which function as an integrator. 5. Remarkable phase advances were induced by intravenous infusion of Nembutal. This suggests that the Nembutal decreased the activity of the polysynaptic pathways and that the neural integrator, composed of polysynaptic networks, was prevented from fulfilling its ordinary function.