During locomotion γ-efferents are activated but the governing rules are not fully understood. An important issue concerns the γ drive to different muscles. Recently ‘tonic’ and ‘phasic’ (α-linked) γ discharge patterns were described in the nerves to FHL and FDL (Murphy, 2001). In the present experiments we sought evidence of the nature (i.e. static or dynamic) of the fusimotor drive to these muscles by recording spindle afferent responses to sinusoidal stretch.
Cats were anaesthetised with 2 % halothane in a 95 % oxygen and 5 % carbon dioxide mixture. The right hindlimb was denervated except for the FHL and FDL nerves. The cats were supported over a treadmill and decerebrated by a section at a 50 deg angle from the anterior edge of the superior colliculus. Ia (conduction velocities, 75-96 m s-1; n = 32) and group II (24-72 m s-1, n = 17) afferents were recorded in cut dorsal root filaments. Sinusoidal stretch (4-5 Hz, amplitude: Ia, 0.25 mm; group II, 1 mm) was applied to the parent muscle of the fixed right leg at rest and during locomotor activity. With these stretch parameters static γ activation decreases, but dynamic γ activation increases, Ia afferent stretch modulation (Taylor et al. 1985). In contrast, group II afferents are almost exclusively influenced by static γ-axons. Death was induced at the end of experiments by I.V. administered barbiturate overdose. Experiments were performed with approval from the University of Alberta Health Sciences Animal Welfare Committee.
Typically Ia and group II afferents from FDL and FHL had increased rates, but reduced modulation, throughout the step cycle compared with rest. In contrast, signs of phasic γ activity differed. Most FHL Ia afferents (13 of 18), but not group II, showed a clear increasing trend in modulation during the extension phase. The activity of about half the FDL units (seven Ia, three group II) varied rhythmically during locomotion, with increased firing related to extension.
We conclude that both muscles received enhanced static γ drive during walking. Fusimotor action during extension differed, with phasic dynamic γ activation in FHL and phasic static γ activation in FDL. Such differential control is consistent with muscle-specific γ drive.