The locomotor discharge characteristics of ankle flexor gamma-motoneurones in the decerebrate cat.

1. The discharge patterns of ankle flexor, tibialis anterior (TA), gamma-motoneurones were recorded during locomotion in the decerebrate cat. 2. At rest gamma-efferents had no background discharge. During locomotion two patterns of gamma activity could be distinguished. Most units (16) were phasically recruited with homonymous electroneurogram (ENG) activity, while the remainder (5) were tonically active throughout the step cycle. 3. The modulation of phasic units was greater (P < 0.01) than tonic neurones. Phasic units had lower (P < 0.02) mean, but higher (P < 0.01) peak, rates during the step cycle. 4. The discharge rate of both types of efferent increased around the onset of ENG activity and peaked during ENG activity, or shortly after its cessation. The conduction velocities of phasic and tonic units overlapped widely. 5. It is proposed, on the basis of muscle spindle afferent recordings during locomotion, that TA phasic and tonic units correspond to static and dynamic gamma-motoneurones, respectively. This correspondence is functionally advantageous for the role of ankle flexor muscles during locomotion. Thus phasic static gamma discharge during flexion would aid muscle contraction via increased Ia afferent activity, while tonic dynamic gamma firing would enhance Ia afferent stretch sensitivity throughout the step cycle. Such enhancement during flexion would oppose unexpected muscle lengthening while, during extension, it would contribute to reciprocal inhibition of ankle extensor muscles. 6. The results are discussed in relation to strategies of gamma usage during rhythmic movements. It is postulated that, for such behaviour, muscle contraction is accompanied by coactivity in static and dynamic gamma-motoneurones. A functional rationale is suggested for this strategy.