Tonic and phasic discharge patterns in toe flexor gamma-motoneurons during locomotion in the decerebrate cat.

To investigate the specificity of fusimotor (gamma) drive during locomotion, gamma-efferents were recorded from the flexor digitorum longus (FDL) and flexor hallucis longus (FHL) nerves in a decerebrate cat preparation. These nerves innervate hindlimb muscles that differ in some aspects of their mechanical action. For both FHL and FDL two stereotyped patterns of gamma activity were distinguished. Tonic units fired throughout the step cycle and had less modulation, but higher minimum rates, than phasic units, which were mainly recruited with ankle extensor [soleus (SOL)] electromyogram (EMG) activity. Differences in the relative timing of these patterns were apparent. In FHL the activity of phasic and most tonic neurons peaked after EMG onset. With FDL, tonic units generally reached maximum rate before, while phasic units peaked after, the beginning of EMG activity. During locomotion FHL and FDL alpha activity were rhythmically recruited with SOL. However, consistent with previous reports, FHL and FDL differed in their patterns of alpha activity. FHL was stereotyped while FDL was variable. Both FHL and FDL had activity related to ankle extensor EMG, but only FDL exhibited a peak around the end of this phase. No corresponding gamma activity was observed in FDL. In conclusion, 1) FHL and FDL received tonic and phasic fusimotor drive; 2) there was no alpha/gamma linkage for the late FDL alpha burst; 3) phasic gamma-efferents in both muscles received similar inputs, linked to plantar flexor alpha activity; and 4) tonic gamma-efferents differed, to the extent that they were modulated at all. The FHL units peaked with the plantar flexor alphas. The FDL neurons generally peaked before alpha activity even began.