Dynamics of neck-to-forelimb reflexes in the decerebrate cat.

We have studied the neck-to-forelimb reflex evoked by head rotation around the longitudinal axis (roll) in the long and medial heads of triceps brachii of decerebrate, acutely labyrinthectomized cats. Reflexes were measured by recording mass electromyogram (EMG). As expected from the work of others, they were reciprocal in the two limbs, with excitation in the limb toward which the chin rotates. The reflex was sufficiently linear for a sinusoidal analysis. Although there was sometimes adaptation at stimulus frequencies of 0.1 Hz and below, response phase at these frequencies was usually in phase with position, and gain was flat. At higher frequencies there was some sensitivity to the velocity of the stimulus: gain increased with a slope of 10 dB/decade and phase advanced in some cats but not in others. Gain at low frequencies of head rotation, expressed as percent modulation of EMG, was typically 1%/deg or less. Reflexes evoked by head rotation in triceps and in the neck extensor splenius capitis have different dynamics. It remains to be determined whether this difference is due to activation of different receptors. We compared the dynamics of roll reflexes evoked by stimulation of neck receptors with those of vestibular reflexes evoked by tilt of the whole animal (23). Taking into account dynamics and gain, the two reflexes should cancel at low frequencies, as predicted by others. Above 0.2 Hz, cancellation becomes less effective.