Branching of muscle spindle afferents of jaw closing muscles in the cat.

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Abstract
Functionally isolated single fibers were prepared from te cut central ends of the masseter nerve in cats. Of those firing in response to muscle stretch, most were .alpha. motor units but some had the properties of afferent fibers innervating muscle receptors. These afferent units were remarkably sensitive to muscle stretch even under anesthesia deep enough to eliminate all stretch evoked EMG [electromyogram] activity in masseter. These units responded to gentle pressing of the surface of either the masseter, temporalis or pterygoid muscles. On the basis of their responses to ramp stretch, to high-frequency vibration, to suxamethonium, and to gentle pressing of the muscle surface, the majority were considered to arise from primary or secondary endings of muscle spindles. In reflexly induced jaw movements, the highest discharge frequency of these spindles was observed during the jaw opening phase. They were activated during the jaw closing phase, indicating that the fusimotor innervation to the source spindle was still preserved. Branching of the spindle afferent outside the capsule occurs in the muscle nerve innervating cat''s jaw closing muscles.