Excitatory and Inhibitory Effects of Midbrain Stimulation on γ_S- and γ_D-Motoneurones of a Cat Hindlimb Muscle

Abstract

Areas in the midbrain where stimulation preferentially affects γ_s- and γ_d-motoneurones have been reported (see Appelberg, 1981; Fowle, Taylor, Rodgers & Durbaba, 1992; Taylor, Durbaba, Rodgers & Fowle, 1993). In the tenuissimus muscle of anaesthetised animals we also found these effects in experiments in which the responses of Ia afferents to standard ramp and hold stretches were monitored during midbrain stimulation (Dickson & Gladden, 1990). Static effects tended to predominate. This was also the case in separate experiments in which activity of γ-motoneurones was detected by observing the contractions of the types of intrafusal fibre they control in spindles exteriorised under anaesthesia. Chain and/or bag₂ contractions indicate active γ_s-motoneurones, whereas active γ_d-motoneurones cause bag₁ contractions. In confirmation of Fowle et al. (1992) chain fibres could regularly be recruited with bag₂ (pure γ_s- activation), and these fibres were also recruited together with bag₁ (mixed γ_s- and γ_d-activation) in 14 experiments, including several in which more than one spindle was isolated. It proved difficult to recruit bag₁ fibres alone (pure γ_d- activation). Spontaneous γ_s-activity could also be inhibited. Here we document an example in which some γs-motoneurones were activated while others were inhibited.