The ORIGINS AND CHARACTERISTICS OF CROSS‐CORRELATED ACTIVITY BETWEEN Γ‐MOTONEURONES IN THE CAT

Abstract

SUMMARYCross-correlation analysis of the naturally occurring discharges of y-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 ymotoneurones 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 y-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-l 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 y-motoneurones which could account for the dependency of k' on frequency of firing.

Cross‐correlation analysis of motor unit activity recorded from two separate thumb muscles during development in man.

“…It is possible that if more spikes had been included in the analysis (typically, approximately 5000 spikes from each spike train were used), small central peaks would have been seen; that is, there may have been a small degree of synchrony present but we were unable to detect it. Quantitative estimation of the amount of synchrony present is complicated by the fact that it is inversely affected by the discharge frequency of the contributing units (Ellaway & Murthy, 1985). In the present study, we found that the average firing rate of the units in the thumb abductor muscles, for a contraction of 10-20% MVC, was about 16 Hz over the age range 4-11 years and then decreased with increasing age until the adult value of about 13 Hz was reached.…”

Control of motor units in human flexor digitorum profundus under different proprioceptive conditions

“…The width of the peak (n bins) was measured from the cusum of the cross-correlogram. The strength of short-term synchrony for each pair was estimated with the k' value (k' = xÏnm, where x is the sum of the counts in the n bins of the peak, and m is the mean count expected to fall into any bin by chance, which was calculated from fifty bins in flat parts of the histogram on each side of the peak; Ellaway & Murthy, 1985). The strength of common drive of motor unit pairs was determined for 20-50 s of the 3 min contraction in 3-5 s epochs distributed throughout the trial, using the method of De Luca et al (1982): the algorithm is given in the Appendix.…”

How computational technique and spike train properties affect coherence detection

“…For the computational synchronization index (SI comp ), we used a cross-correlogram identical to those used for experimental synchrony indices (Bremner et al, 1991a;Datta and Stephens, 1990;Ellaway and Murthy, 1985;Logigian et al, 1988;Nordstrom et al, 1992;Wiegner and Wierzbicka, 1987), with lag times from −100 ms to 100 ms and 1-ms bins, with lag times centered at each bin. An event is counted each time a response spike from one train is within ±100 ms of a reference spike in the other train.…”

“…Instead of expending computational resources on finding peak regions by variance methods or visually identifying peak regions for thousands of motor unit pairs using CUSUM deflections (Ellaway and Murthy, 1985, Wiegner and Wierzbicka 1987, Nordstrom et al 1992, the peak region was fixed at −4.5 ms ≤ lag time ≤ +4.5 ms (the 9 central bins). The computational SI comp index was then used to express synchrony as the ratio of the mean bin population within (binpop pk ) and outside (binpop npk ) the peak region, as shown in equation (5).…”