Cross-Correlation Assessment of Synaptic Strength of Single Ia Fiber Connections with Triceps Surae Motoneurones in Cats

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1. The relation between excitatory post-synaptic potentials (e.p.s.p.s) produced by single Ia 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 > 0.05).

3. A significant positive relationship (r = 0.76; P < 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%/µV. The k values were more weakly related to e.p.s.p. amplitude (r = 0.67; P < 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 > 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. Ramp-e.p.s.p. rise times were often substantially shorter or longer than rest-e.p.s.p. rise times, but did not differ in a consistent direction. Correlogram peak area and m.p.i. in firing probability were better related to ramp- than to rest-e.p.s.p. properties.

6. Properties of the correlogram peaks were not obviously related to (a) intrinsic motoneurone properties, including rheobase current and conduction velocity, (b) the type of Ia fibre-motoneurone connection, homonymous or heteronymous, or (c) the condition of the spinal cord, intact or acutely transected.

7. It is concluded that cross-correlograms provide a direct measure of synaptic strength at single Ia fibre-motoneurone connections. These correlograms confirm that single Ia fibres raise motoneurone firing probability in proportion to the amplitude of the underlying e.p.s.p.s.

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