Monosynaptic and oligosynaptic contributions to human ankle jerk and H-reflex

Studies were undertaken in normal subjects to determine whether it is possible for oligosynaptic reflex pathways to affect motoneuron discharge in the ankle jerk and H-reflex of the soleus. It is argued that if the rising phase of the increase in excitability of the soleus motoneuron pool produced by tendon percussion or by electrical stimulation of the peripheral nerve lasts more than a few milliseconds and if the increase in excitability takes several milliseconds to reach the threshold for motoneuron discharge, these reflexes are unlikely to be exclusively monosynaptic. In relaxed subjects, changes in excitability of the soleus motoneuron pool produced by tendon percussion and by electrical stimulation of the tibial nerve were examined using conditioning stimuli just below threshold and a test H-reflex just above threshold for a reflex response. The increase in excitability due to tendon percussion had an average rise time of 10.8 ms and a total duration of approximately 25 ms. With electrical stimulation the rising phase appeared shorter, but it could not be measured accurately due to afferent refractoriness. In single motor units, the rise times of the composite excitatory postsynaptic potentials (EPSPs) set up by subthreshold tendon percussion and by subthreshold electrical stimulation of the tibial nerve were estimated from changes in the probability of discharge of voluntarily activated single motor units. Rise times were significantly longer with tendon percussion (mean +/- SD, 7.1 +/- 2.3 ms; n = 34) than with electrical stimulation (2.4 +/- 1.4 ms; n = 32). In four experiments in which a number of motor units were studied using identical mechanical and identical electrical stimuli, the poststimulus time histograms (PSTHs) for each stimulus were pooled to provide an estimate of the rise time of the excitability change in the motoneuron pool. The mean rise times of these four samples were 10.5 ms with mechanical stimulation and 4.5 ms with electrical stimulation. The spontaneous variability in latency of reflexly activated single motor units was 0.8-3.1 ms (average SD, 0.34 ms) in the tendon jerk, and 0.6-1.4 ms (average SD, 0.19 ms) in the H-reflex. Comparison of these figures with the measurements of rise time given above suggests that the composite EPSPs are larger than the background synaptic noise. With six motor units, the timing of reflex discharge in the tendon jerk when the subject was relaxed was compared with the timing of the change in probability of discharge due to apparently identical percussion when the units were activated voluntarily.(ABSTRACT TRUNCATED AT 400 WORDS)