The relative unimportance of the temporal pattern of the primary afferent input in determining the mean level of motor firing in the tonic vibration reflex

Abstract

1. A study has been made of the effect of varying the temporal arrangement of the mechanical stimuli used to elicit the tonic vibration reflex in the soleus muscle of the decerebrate cat. The reflex was elicited by brief mechanical pulses, applied repetitively, either as a regular series or, at the same mean frequency, in groups of 2, 3 or 4 pulses with a separation between the pulses of 3-5 msec. Mean frequencies of 140/sec and 100/sec were used. The amplitude of the pulses was such that it could be presumed that each pulse excited every Ia fibre from soleus to discharge a spike, irrespective of the patterning employed. 2. Alterations in the stimulus pattern produced only minimal alterations in the size of the resultant reflex recorded myographically. The grouped stimulation regularly tended to produce the larger effect, but even with groups of 4 at 100/sec the modal effect was only 10% of the pre-existing response; expressed another way this was equivalent to an increase of 11 Hz in the mean frequency of stimulation. Thus under these conditions grouping the stimuli cannot have had an appreciable effect either in increasing the firing frequency of those motoneurones which were already active, or in recruiting those which were initially quiescent. 3. Recording from individual motor units with fine electrodes placed on the surface of the muscle showed that they were not significantly changing their frequency of firing on altering the pattern of stimulation. 4. Gross electromyographic recording showed that the motor discharge was locked in time to the mechanical stimuli and of appropriate latency for it to be presumed that the actual discharge of impulses was triggered by Ia monosynaptic action. 5. Similar insensitivity to the temporal pattern of the afferent input was found when the motoneurones were excited via two separate channels, one being the pulsed mechanical stimulation of soleus, the other being the weak electrical stimulation of the nerve to the medial head of gastrocnemius; altering the relative timing of the two sets of stimuli had little effect on the myographic result. 6. Thus, during tonic firing the timing of the motor output reflects the timing of the afferent input while the mean motor output reflects the mean value of the afferent input, as seems physiologically appropriate. The latter finding is, however, paradoxical; as detailed in an appendix the amount of motor firing produced by synchronous Ia monosynaptic action might be expected to increase on grouping the stimuli and so apparently favouring e.p.s.p. summation.