Relation between intrafusal and extrafusal activity in triceps surae muscles of the decerebrate cat: evidence for beta action.

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Abstract
To assess the relative influence of .gamma. and .beta. innervation on muscle spindles, the discharge patterns of 105 normally innervated medial gastrocnemius and soleus spindle receptor afferents were compared in resting and reflexively activated muscles of the decerebrate cat. Increases in afferent discharge rate arising above extrafusal threshold could result from .beta. (skeletofusimotor) activity superimposed on a background of .gamma. efferent discharge. In isometric muscles, primary and secondary spindle receptors responded to low-intensity mechanical or electrical stimulation of the contralateral limb with acceleration of discharge rate. This acceleration occurred at stimulus intensities insufficient to activate extrafusal fibers, and was therefore of .gamma. origin. Approximately half of the endings generated further rate increases when stimulation of the contralateral limb was sufficient to induce substantial increases in isometric force and EMG activity. Of 105 receptors, 8 were activated only at high force levels; 3 of these showed abrupt discontinuities consistent with recruitment of a new efferent (.GAMMA. and .beta.) fiber. For many spindles, a continuing increase in efferent excitation, extending well beyond extrafusal threshold, must have occurred. Primary and secondary spindle afferents showed significant alterations in their response to constant velocity stretch during contralateral limb stimulation; the responses were classified as static type, dynamic type and mixed, depending on their resemblance to the known patterns observed during stimulation of single static and/or dynamic .gamma.-fibers. Of 65 units, 11 showed unexpected additions to the dynamic response at intermediate or high forces. A 4th pattern, unloading, was identified when receptor discharge decreased with increasing force. Static and dynamic .gamma.-fibers were apparently active in the preparations, although .beta.-fibers could have influenced afferent discharge above estrafusal threshold. Since .gamma.-motoneurons are neither recruited nor strongly excited by muscle stretch, the unexpected additions to the dynamic response probably resulted from recruitment of a .beta.-fiber. Muscle spindle receptors that showed continuing rate increases with contralateral limb stimulation usually increased their average discharge during isotonic muscle shortening; other spindle afferents showed substantial rate reductions. The former rate increases imply that motor excitation of the muscle spindle must have continued to increase during crossed-extensor stimulation. Recordings from 61 .gamma.-fibers isolated from teased filaments of the medial gastrocnemius nerve in similar preparations showed activation thresholds lower than those of extrafusal fibers from the same muscle. There was no evidence of .gamma. recruitment of continuing rate modulation at intermediate or high force levels, and the .gamma.-fibers were largely unresponsive to stretch. Modifications of spindle receptor discharge arising at substantial levels of excitation of the motoneuron pool could result from recruitment or rate modulation of either .gamma.- or .beta.-motoneurons. The finding of consistently lower thresholds for .gamma.-fiber activation, the apparent saturation of .gamma.-fiber discharge near extrafusal threshold and the failure to demonstrate significant additional .gamma.-fiber recruitment or rate modulation above extrafusal threshold support the hypothesis of .beta.-motoneuronal influence on muscle spindles.