Responses of forelimb extensors to sinusoidal stimulation of macular labyrinth and neck receptors.

In precollicular decerebrate cats the multiunit EMG activity was recorded from forelimb extensors (triceps brachii) during isolated or combined sinusoidal stimulation of labyrinth and neck receptors at frequencies of 0.026--0.15 Hz. As expected from previous work, the first harmonic component of the EMG responses to roll tilt of the animal leading to selective stimulation of labyrinth receptors was characterized by an increased activity during side-down tilt and a decreased activity during side-up tilt (labyrinth responses); on the other hand just the opposite changes were elicited for the same directions of neck rotation (neck responses). For the peak amplitude of displacement of 10 degrees, the responses were always related to position and not to velocity of displacement. In some experiments in which the sensitivity of the neck responses was usually comparable to that of the labyrinth responses and the corresponding peaks were almost 180 degrees out of phase, a cancellation of the responses occurred during head rotation, leading to costimulation of labyrinth and neck receptors. In other experiments, however, in which the sensitivity of the neck responses was higher than that of the labyrinth responses, the forelimb extensors responded to head rotation in a fashion predictable from the vectorial summation of the responses obtained from separate macular and neck stimulation.