DETECTION OF WEAK SYNAPTIC-INTERACTIONS BETWEEN SINGLE IA AFFERENT AND MOTOR-UNIT SPIKE TRAINS IN THE DECEREBRATE CAT

1. Spike trains from identified single Ia afferents from soleus and la teral gastrocnemius muscles were recorded (while 'in continuity' with the spinal cord) simultaneously with single-motor-unit EMG spike train s from the same muscles in decerebrate cats. 2. A total of 143 Ia affe rent-motor-unit pairs were examined for the presence of correlated act ivity between the Ia afferent and motor-unit and between the motor-uni t and la afferent. Four types of correlation were identified on the ba sis of the cross-intensity function estimated for individual la affere nt-motor-unit pairs. These correlations were attributed to the absence or presence of a central Ia afferent-motoneurone interaction or a per ipheral motor-unit-muscle spindle interaction. 3. In addition to the c ross-correlation-based second-order cross-intensity function, third-or der cumulants were defined and used further to investigate Ia afferent -motor-unit interactions. A third-order cumulant density-based approac h to signal processing offers improved signal-to-noise ratios, compare d with the traditional product density approach, for parameters charac terizing, certain kinds of linear processes as well as a description o f non-linear interactions. Two classes of third-order relations were d escribed. One class was associated with a strong central connection an d the other with a weak central connection. 4. Third-order cumulants e stimated for Ia afferent-motor-unit pairs with significant second-orde r central correlations were able to detect a period of decreased moton euronal excitability. In addition, temporal summation prior to spike i nitiation could be identified in cases where the afferent discharge wa s suitably high. 5. Third-order cumulants estimated for Ia afferent-mo tor-unit pairs in which no significant second-order central correlatio n existed identified the presence of weak synaptic interactions. It is argued that these interactions result from the summation from the rec orded Ia afferent discharge and other spontaneous synaptic inputs to t he motoneurone. 6. The results of the second-order cross-intensity ana lysis of Ia afferent-motor unit interactions, combined with those from the third-order cumulant density analysis. showed that 77 % of the re corded afferents had a detectable influence on motor-unit behaviour. 7 . The results of this study suggest that the third-order cumulant, bas ed on the analysis of spike trains, will provide a useful tool for det ecting synaptic interactions not found by the use of the second-order cross-correlation histogram alone, and may also be used to estimate th e time course of post-spike depression in motoneurones, as well as oth er non-linear regions of motoneurone membrane trajectory.