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)

SUMMARY1. To define the neural volleys responsible for the Achilles tendon jerk and the H reflex, muscle afferent activity was recorded using micro-electrodes inserted percutaneously into appropriate fascicles of the tibial nerve in the popliteal fossa.2. The response of soleus muscle afferents to tendon percussion consisted of a dispersed volley, starting 3-5-7-0 ms after percussion, increasing to a peak over 6 5-1 1 0 ms, and lasting 25-30 ms, depending on the strength of percussion. Electrical stimuli to the sciatic nerve at a level adequate to evoke an H reflex but subthreshold for the M wave produced a more synchronized volley, the fastest fibres of which had conduction velocities of 62-67 m/s, and the slowest 36-45 m/s.3. The wave of acceleration produced by percussion subthreshold for the ankle jerk spread along the skin at over 150 m/s. Midway between the bellies of the gastrocnemii it consisted of a damped oscillation with four to five separate phases and maximum amplitude approximately one-twentieth of that recorded on the Achilles tendon.4. With ten primary spindle endings, tendon percussion subthreshold for the ankle jerk elicited two to five spike discharges per tap, the shortest interspike intervals being 4-7 ms. Tendon percussion elicited single discharges from two Golgi tendon organs, and altered the discharge pattern of a single secondary spindle ending. The degree of dispersion of the multi-unit muscle afferent volley can be explained by the pattern of discharge of primary spindle endings.5. Percussion on the Achilles tendon evoked crisp afferent volleys in recordings from nerve fascicles innervating flexor hallucis longus, tibialis posterior, the intrinsic muscles of the foot and the skin of the foot. Electrical stimuli delivered to the tibial nerve in the popliteal fossa at a level sufficient for the H reflex of soleus produced either a volley in muscle afferents from the intrinsic muscles of the foot or a volley in cutaneous afferents from the foot.6. For comparable stimuli in the two positions, the H reflex was inhibited but the Achilles tendon jerk enhanced when the ankle was dorsiflexed from 1050 to 900.7. The duration of the rise times of the excitatory post-synaptic potentials (e.p.s.p.s) produced in soleus motoneurones by electrical stimulation, and by tendon percussion subthreshold for the H reflex and the ankle jerk respectively, was estimated from post-stimulus time histograms of the discharge of voluntarily

Average short-latency cerebral potentials were recorded from the parietal scalp to mechanical stimulation of the index and middle fingers and to electrical stimulation of the digital nerves in normal subjects. The early components of the cerebral potential, representing the arrival of the afferent volley at the sensorimotor cortex, were studied during stimulation of the fingers separately and together. When strong or moderate stimuli were used there was a suppressive interaction between the afferent input from the two fingers with either electrical or mechanical stimulation. During simultaneous stimulation of both fingers the size of the early component of the cerebral potential was less than predicted by simple addition of the potentials produced by stimulation of the fingers individually. When very weak stimuli, close to the level necessary for detection by the subject, the input from the two fingers produced additive or facilitatory interactions in the early components of the cerebral potential. These results suggest that there is convergence between the afferent inputs from the index and middle fingers along the somatosensory pathway. At levels of stimulation comparable to those which produced facilitation in the electrophysiological studies, simultaneous stimulation to both fingers was detected significantly more frequently than would be expected from the detection of stimulation to individual fingers.