The “size principle” and synaptic effectiveness of muscle afferent projections to human extensor carpi radialis motoneurones during wrist extension

A. Schmied, D. Morin, J-P. Vedel, S. Pagni
Exp Brain Res. 1997-02-01; 113(2): 214-229
DOI: 10.1007/bf02450320

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1. Exp Brain Res. 1997 Feb;113(2):214-29.

The “size principle” and synaptic effectiveness of muscle afferent projections to
human extensor carpi radialis motoneurones during wrist extension.

Schmied A(1), Morin D, Vedel JP, Pagni S.

Author information:
(1)Laboratory of Physiologie et Physiopathologie Neuromusculaire humaine UPR-CNRS
Neurobiologie et Mouvements, Marseille, France.

The question of whether muscle spindle afferents might control human motoneurone
activity on the basis of the “size principle” during voluntary contraction was
investigated by recording the discharge of single motor units (n = 196) in wrist
extensor muscles while stimulating the homonymous muscle spindles by means of
tendon taps. The mechanical stimuli were delivered with a constant post-spike
delay of 80 ms so that the resulting afferent volleys could be expected to reach
the motoneurones towards the end of the inter-spike interval (mean +/- SD
duration: 124.7 +/- 11.9 ms). In the six subjects tested, the response
probability was found to be significantly correlated with the motor units’
functional parameters. Differences in twitch rise times, twitch amplitudes,
recruitment thresholds and macro-potential areas were found to account for 18%,
9%, 6% and 2% of the differences in the response probability observed within the
whole population of motor units tested. These differences could not be due to
differences in firing rate for two reasons: first, the motor units were found to
discharge with a similar range of inter-spike intervals whatever their functional
characteristics; secondly, the weak positive correlation observed between the
response probability and the motor unit firing rate showed parallel regression
lines between the late-recruited fast-contracting motor units and the
first-recruited slowly contracting motor units, but the y-intercept was
significantly higher in the latter case. This confirmed that the responses of the
first-recruited slowly contracting motor units tended to be larger whatever the
firing rates. In most of the pairs tested in the same experiment, the motor units
which had the lowest recruitment thresholds, longest contraction times, smallest
contraction forces or smallest motor unit macro-potentials tended to produce the
largest responses, which also had the longest latencies. Taking the response
latency to be an index of a motoneurone’s conduction velocity and therefore of
its size, the data obtained with this index and with other functional indices
such as the twitch rise times and amplitudes, the macro-potential areas and the
recruitment thresholds-can be said to be fully consistent with the “size
principle”, as previously found in anaesthetized animals. It can be inferred that
the presynaptic inhibition which is liable to take action during voluntary
contraction does not seem to alter the graded distribution of the muscle afferent
projections to human wrist extensor motoneurones.

DOI: 10.1007/BF02450320
PMID: 9063708 [Indexed for MEDLINE]

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