Excitatory and Inhibitory Effects of Midbrain Stimulation on γS- and γD-Motoneurones of a Cat Hindlimb Muscle
Areas in the midbrain where stimulation preferentially affects γs- and γd-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 γs-motoneurones, whereas active γd-motoneurones cause bag1 contractions. In confirmation of Fowle et al. (1992) chain fibres could regularly be recruited with bag2 (pure γs- activation), and these fibres were also recruited together with bag1 (mixed γs- and γd-activation) in 14 experiments, including several in which more than one spindle was isolated. It proved difficult to recruit bag1 fibres alone (pure γd- activation). Spontaneous γs-activity could also be inhibited. Here we document an example in which some γs-motoneurones were activated while others were inhibited.
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