ROLE OF PROPRIOCEPTIVE INFORMATION IN THE TEMPORAL COORDINATION BETWEEN JOINTS

Ten subjects made rapid, simultaneous movements of jaw (elevation or l owering) and right foot (ankle flexion or extension) in two experiment al situations: (1) in response to an external signal (reaction-time si tuation), and (2) in a self-paced situation. We calculated the mean ti me intervals between the onsets of electromyographic (EMG) activity of agonist muscles (tibialis anterior or gastrocnemius lateralis compare d with masseter or digastricus pars anterior) and those between the on sets of movement acceleration at each joint. Despite the fact that sub jects reported simultaneous jaw-foot movements, there was always a sho rt time interval between the two movements as between the agonist EMG activities. When the subjects were asked to perform a jaw elevation mo vement simultaneously with an ankle movement (flexion or extension), t he sign of the time interval was dependent on the situation of movemen t initiation. In the reaction-time situation, the jaw motor activity p receded that of the ankle, whereas the reverse temporal order was obse rved in the self-paced situation. This is consistent with a previous h ypothesis suggesting that the simultaneity of two motor actions is cen trally established through two separate central processes: reactive or predictive. When subjects tried to perform simultaneous jaw lowering and foot flexion or extension movements, the strict temporal order obs erved when considering jaw elevation and ankle movements disappeared. The jaw motor activity generally preceded that of ankle in the reactiv e situation, but, depending on the subjects, it preceded or followed t he ankle motor activity during self-paced movements. It is likely that the specific spindle supply of jaw muscles accounts for these results . Indeed, the jaw depressor muscles, in contrast to the elevators, lac k muscle spindles. Our results suggest that the kinesthetic inputs use d by the upper central nervous system to synchronize two rapid volunta ry movements are mainly those from spindles located in the muscles tha t accelerate the movement, suggesting a strong alpha-gamma linkage.