The origins and characteristics of cross-correlated activity between gamma-motoneurones in the cat.

Cross-correlation analysis of the naturally occurring discharges of gamma-motoneurones to gastrocnemius and soleus muscles has revealed a tendency towards synchronized firing in the decerebrated, spinal cat. The primary feature of the cross-correlation between two gamma-motoneurones is a symmetrical peak centred at time zero with reference to discharges at the level of the cell bodies in the ventral horn of the spinal cord. The peak has a half-width in the range 3-10 ms. Secondary features of the correlograms are frequently present, especially when the primary peak is large. They consist of dips to either side of the peak and are mirrored in the respective autocorrelation functions of the two contributory neurones. This suggests that they arise as a result of periodicity in firing patterns of the neurones. The degree of synchrony has been measured as the ratio (k') of the total counts contributing to the peak over the number expected by chance. The value of k' was found to be dependent upon the firing frequencies of the neurones. Plots of k' against the geometric mean frequency of discharge showed a falling curvilinear relation as frequency increased. There was a weak inverse relation between the width of the cross-correlation peak and firing frequency. These relations had to be acknowledged before any assessment of change in the degree of synchrony could be made. The correlation between discharges is absent or weak in the decerebrated cat with an intact spinal cord and in intact cats anaesthetized with pentobarbitone or chloralose. Anaesthetics administered to a spinal animal did not suppress synchrony. The synchrony between gamma-motoneurones is characteristic of the short-term synchrony (Sears & Stagg, 1976) which arises from activity in shared presynaptic axons. In a few instances a stronger, more-tightly coupled synchrony was observed. The cross-correlogram peak had a half-width of 0.5-1.0 ms and was superimposed upon the usual wider-based peak. We presume that this intense form of synchrony has a different origin. The properties of the short-term synchrony are discussed with respect to (a) the mechanism of supraspinal control, and (b) a plausible model of the membrane properties of gamma-motoneurones which could account for the dependency of k' on frequency of firing. Experiments to determine the afferent source of synchronous firing and its distribution are presented in the following paper (Ellaway & Murthy, 1985).