Central mechanisms of selective fusimotor control

Experiments on anaesthetized cats have confirmed the existence of regions of the midbrain from which dynamic fusimotor effects can be produced by electrical stimulation. Simultaneous recording from hindlimb and jaw muscle spindles has shown that both of these widely different muscle groups are affected from identical or closely related regions. The anatomical structure common to many of the pathways, which are likely to be involved at the sites of effective stimulation is the fasciculus retroflexus and it is suggested that this indicates that the habenulo-interpeduncular system may be involved in generating "dynamic fusimotor set" at the onset of movements, as a possible function of its place as an output pathway for the limbic system. The results are discussed in relation to the other well known source of dynamic control of spindles, namely the MesADC.