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Static Fusimotor Action During Locomotion in the Decerebrated Cat Revealed by Cross‐Correlation of Spindle Afferent Activity

Cross‐correlation of the discharges of muscle spindle afferents in ankle extensor and flexor muscles has been used to reveal the activity of static gamma (γS) motoneurones innervating chain intrafusal muscle fibres during locomotion. In the anaesthetised cat, the cross‐correlation of spindle afferents, jointly innervated by a γS‐efferent with chain fibre contacts, showed short duration synchrony (2‐8 ms) when the efferent was stimulated repetitively. In pre‐mammillary decerebrated cats, the cross‐correlograms of discharges of some pairs of spindle afferents showed similar short duration peaks of synchronisation and these were interpreted as being due to a common γS drive to chain intrafusal muscle fibres. The incidence of synchrony was low, and was similar at rest (5% of pairs) and during treadmill locomotion (7% of pairs). Phase dependence of synchrony was evident during locomotion in the flexor muscle. The synchrony of muscle spindle afferent discharge is discussed in relation to estimates of the numbers of spindles contacted by individual γS‐efferents.