| Abstract: | The main aim of the present study was to find out whether the dynamic characteristics of responses of limb extensor muscles to labyrinth stimulation were modified by the proprioceptive input elicited by appropriate displacements of the corresponding limb extremity. In cats decerebrated at precollicular or intercollicular level, the multiunit EMG activity of the medial head of the triceps brachii was recorded during roll tilt of the animal at the frequency of 0.15 Hz, +/- 10 degrees leading to selective stimulation of labyrinth receptors. This stimulation was then tested several times at regular intervals of 2 to 6 min for several hours while maintaining the ipsilateral forelimb in the horizontal extended position, i.e. with the plantar surface of the foot lying on the tilting table, or during passive flexion of the forepaw in plantar or dorsal direction. In all the experiments in which the forelimb was in the control position, the multiunit EMG responses of the triceps brachii were characterized by an increased activity during side-down tilt of the animal and a decreased activity during side up tilt. These responses were related to animal position and not to the velocity of animal displacement, thus being attributed to stimulation of macular, utricular receptors. Static displacement of limb extremities following plantar flexion of the forepaw greatly decreased the amplitude of the EMG modulation and thus the gain of the first harmonic component of the multiunit EMG responses of the ipsilateral triceps brachii to animal tilt. This reduced gain was due not only to a reduced number of motor units recruited during labyrinth stimulation, but also to a reduced modulation of firing rate of the active motor units, as shown by recording the activity of individual motor units. On the other hand, displacement of the same extremity in the opposite direction, i.e. following dorsiflexion of the forepaw, enhanced the amplitude of the EMG modulation and thus the gain of the multiunit EMG responses of the ipsilateral triceps brachii to animal tilt. This finding was mainly due to an increased recruitment of motor units during side-down tilt, although an increased modulation of the firing rate of individual motor units could not be excluded. In both instances, no changes in the phase angle to the responses were observed. The changes in response gain described above depended on the amount of passive displacement of the forepaw and persisted unmodified throughout the new maintained position.(ABSTRACT TRUNCATED AT 400 WORDS) |
