A comparison of the recurrent inhibition of alpha- and gamma-motoneurones in the cat.

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Abstract
The degree of recurrent inhibition of tonically firing .alpha.- and .gamma.-motoneurons to triceps surae muscles was assessed in decerebrated cats by measuring the change in probability of firing caused by an antidromic volley in other motoneuron axons. In 9 cats 91% (31 of 33) of .alpha.- and 54% (25 of 46) of .gamma.-motoneurons could be inhibited by antidramic volleys in .alpha.-motoneuron axons. The degree of recurrent inhibition, expressed as the average reduction in probability of firing during the response, was typically in the range 50-95% for .alpha.-motoneurons compared to 20-85% for .gamma.-motoneurons. The duration of recurrent inhibition was 20-50 ms for .alpha.-motoneurons and 5-40 ms for .gamma.-motoneurons. Duration was dependent on frequency of firing of a neuron, being shorter at high frequencies than at low frequencies. When .alpha.- and .gamma.-motoneurons had similar frequencies of discharge, durations of their recurrent inhibition were comparable. Raising the strength of electrical stimulation to elicit an antidromic volley in .gamma.- as well as .alpha.-motoneuron axons never produced or increased recurrent inhibition in either type of motoneuron. The quantitative differences in recurrent inhibition of .alpha.- and .gamma.-motoneurons are discussed in relation to control of firing frequency.