THE SIZE PRINCIPLE AND SYNAPTIC EFFECTIVENESS OF MUSCLE AFFERENT-PROJECTIONS TO HUMAN EXTENSOR CARPI RADIALIS MOTONEURONS DURING WRIST EXTENSION

The question of whether muscle spindle afferents might control human m otoneurone activity on the basis of the ''size principle'' during volu ntary contraction was investigated by recording the discharge of singl e motor units (n=196) in wrist extensor muscles while stimulating the homonymous muscle spindles by means of tendon taps. The mechanical sti muli were delivered with a constant post-spike delay of 80 ms so that the resulting afferent volleys could be expected to reach the motoneur ones 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' functi onal 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 dif ferences could not be due to differences in firing rate for two reason s: first, the motor units were found to discharge with a similar range of inter-spike intervals whatever their functional characteristics; s econdly, the weak positive correlation observed between the response p robability and the motor unit firing rate showed parallel regression l ines between the late-recruited fast-contracting motor units and the f irst-recruited slowly contracting motor units, but the y-intercept was significantly higher in the latter case. This confirmed that the resp onses of the first-recruited slowly contracting motor units tended to be larger whatever the firing rates. In most of the pairs tested in th e same experiment, the motor units which had the lowest recruitment th resholds, longest contraction times, smallest contraction forces or sm allest motor unit macro-potentials tended to produce the largest respo nses, which also had the longest latencies. Taking the response latenc y to be an index of a motoneurone's conduction velocity and therefore of its size, the data obtained with this index - and with ether functi onal indices such as the twitch rise times and amplitudes, the macro-p otential areas and the recruitment thresholds - can be said to be full y consistent with the ''size principle'', as previously found in anaes thetized animals. It can be inferred that the presynaptic inhibition w hich is liable to take action during voluntary contraction does not se em to alter the graded distribution of the muscle afferent projections to human wrist extensor motoneurones.