Cross-correlation assessment of synaptic strength of single Ia fibre connections with triceps surae motoneurones in cats.

1. The relation between excitatory post-synaptic potentials (e.p.s.p.s) by single I a fibres and the resultant cross-correlograms in triceps surae motoneurones was investigated in barbiturate-anaesthetized cats. The e.p.s.p.s. were documented first, using the discharge of single Ia fibres evoked by muscle stretch to compile spike-triggered averages of motoneurone membrane potential. Subsequently, Ia fibre action potentials were cross-correlated with rhythmic discharge of the same motoneurones induced by intracellular injection of current. 2. Primary correlogram peaks were statistically significant for thirty-one of forty-nine single Ia fibre-motoneurone connections. Cumulative sums of correlograms were used to identify the onset and duration of peaks. For twenty cases involving more than 2000 trigger spikes, thirteen showed significant correlogram peaks. For these thirteen, the mean percentage increase (m.p.i.) in motoneurone firing probability, defined as the mean height of the correlogram peak above base line, ranged from 29 to 138%. The k values (maximum height divided by base line) ranged from 2.1 to 5.2. Peak duration varied from 1.8 to 3.2 ms. In the remaining seven cases the Ia e.p.s.p.s produced no significant correlogram peak (i.e. P greater than 0.05). 3. A significant positive relationship (r = 0.76; P less than 0.005) was found between m.p.i. in motoneurone firing probability and e.p.s.p. amplitude (n = 13), with a mean slope of 0.30%/microV. The k values were more weakly related to e.p.s.p. amplitude (r = 0.67; P less than 0.01). The correlogram parameter most strongly related to e.p.s.p. amplitude (r = 0.80) was correlogram peak area (number of spikes above base line per excitatory post-synaptic potential). E.p.s.p. rate of rise was not significantly related (P greater than 0.10) to either m.p.i. in firing probability (r = 0.28) or peak area (r = 0.36). 4. The shapes of the primary correlogram peaks could be accounted for largely by a function proportional to the e.p.s.p. derivative (after temporal alignment). Subtracting a function proportional to the e.p.s.p. derivative from the correlogram peak left either a negligible remainder or a remainder term whose duration was shorter than the e.p.s.p. 5. To investigate properties of single-fibre Ia e.p.s.p.s occurring near motoneurone threshold during repetitive firing, e.p.s.p.s were selectively averaged using Ia spikes occurring near the end of the depolarizing ramp in membrane potential. These 'ramp e.p.s.p.s' tended to be somewhat smaller (by ca.8%) than the 'rest e.p.s.p.s' produced at the same connections with the motoneurone at rest.(ABSTRACT TRUNCATED AT 400 WORDS)