Responses of Ia and spindle group II afferents to single motor-unit contractions

1. The responses of deefferented Ia and spindle group II afferents to electrically activated twitch contractions of randomly selected motor units of the cat tibialis posterior muscle have been studied. Each afferent was paired with from 8 to 20 of the muscle's 60 motor units, and each afferent-motor unit interaction was recorded to two muscle lengths. 2. Cross-correlation histograms were compiled for each afferent-motor unit interaction studied as well as the average twitch tension produced by the motor unit. A numerical "coupling index" was computed for the histogram distributions to quantitate the extent of mechanical coupling between the receptor and the single motor units. 3. Qualitatively, no consistent differences were noted in the responses of Ia and spindle group II afferents to single motor-unit contractions. However, Ia afferents were responsive to a higher percentage of motor units with which they were tested (89%) and, on the average, displayed a significantly larger magnitude of response (mean coupling index, 0.72 +/- 0.04 SE) than the spindle group II afferents (66% of motor units; mean coupling index, 0.51 +/- 0.03). 4. The extent to which a motor-unit contraction altered the discharge pattern of a spindle afferent was not strictly related to the amount of force generated by the unit, nor to its contraction time. 5. Muscle length exerted a strong influence on both the qualitative and quantitative features of many of the motor unit-muscle receptor interactions. 6. These results suggest that the degree of "mechanical coupling" between a receptor and a motor unit is largely dependent on anatomical arrangements and reinforce the possibility that muscle receptors generate a "sensory partitioning" of the motor-unit population within a muscle.