The degree of short-term synchrony between alpha- and gamma-motoneurones coactivated during the flexion reflex in the cat.

Cross-correlation analysis of unitary neuronal discharges has been used to study the linkage between alpha- and gamma-motoneurones coactivated during the flexion reflex of the semitendinosus muscle in the decerebrated spinal cat. A flexion reflex was elicited by firm grip or squeeze of the ipsilateral heel, shank or foot. The stimulus excited the discharges of both alpha- and gamma-motoneurones and increased the frequency of discharge of those gamma-motoneurones that had shown a background discharge prior to intentional stimulation. Short-term synchrony was present between a high proportion of semitendinosus gamma-motoneurones both for background discharges (sixteen out of nineteen pairs) and during the flexion reflex (thirteen out of fifteen pairs). All nineteen pairs of alpha-motoneurones examined during the flexion reflex showed short-term synchrony of discharge. Few alpha-motoneurones displayed background discharges but synchrony was observed in the two instances studied. The degree of synchrony was measured as the ratio (kappa) of the total counts contributing to the peak of the correlogram over the number expected by chance alone. The ratio was higher when the average frequency of motoneurone discharge was low. Kappa was generally higher for alpha-motoneurone pairs than for gamma-motoneurone pairs. The higher degree of synchrony for alpha-motoneurones reflected their lower discharge rates. During the flexion reflex the degree of synchrony between gamma-motoneurones was greater than expected for that same discharge rate in the absence of intentional stimulation. Only twenty-seven out of forty pairings of an alpha- with a gamma-motoneurone showed a significant degree of synchrony of discharge. On average, the degree of synchrony for alpha/gamma pairs was lower than that for either alpha/alpha or gamma/gamma pairings at the equivalent discharge rate. The results support the conclusion that coactivation of alpha- and gamma-motoneurones during the flexion reflex occurs largely through independent sets of interneurones. The possibility is discussed that those alpha-motoneurones which showed short-term synchrony with gamma-motoneurones were skeleto-fusimotor (beta-motoneurones) in nature.