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

Abstract

SUMMARY1. Functionally isolated single fibres were prepared from the 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 fibres innervating muscle receptors.2. These afferent units were remarkably sensitive to muscle stretch even under anaesthesia deep enough to eliminate all stretch evoked e.m.g. activity in masseter. Moreover, these units responded to gentle pressing of the surface of either the masseter, temporalis or pterygoid muscles.3. 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.4. In reflexly induced jaw movements, the highest discharge frequency of these spindles was observed during the jaw opening phase. However, they were also activated during the jaw closing phase, indicating that the fusimotor innervation to the source spindle was still preserved.5. From these results, it is concluded that branching of the spindle afferent outside the capsule occurs in the muscle nerve innervating cat's jaw closing muscles.

Interpretation of fusimotor activity in cat masseter nerve during reflex jaw movements.

“…Of the other units, one was thought to be a masseter spindle afferent, and four temporalis spindle afferents. Similar units have been reported in masseter nerve filaments by Kato, Kawamura & Morimoto (1982), who suggested that their presence in the central ends of cut masseter nerve filaments could be explained on the basis of branching by a parent axon to innervate more than one muscle spindle. If this explanation is correct, the above results would suggest that an axon can branch to innervate spindles in different closely related muscles (i.e.…”

Noradrenaline triggers muscle tone by amplifying glutamate‐driven excitation of somatic motoneurones in anaesthetized rats

“…Not only did noradrenergic drive intensify the magnitude of the masseteric reflex, it also induced a potent increase in reflex duration. During baseline conditions, reflex activation increased masseter tone for 11 s even though reflex pathways were continuously activated for 20 s. This response occurs because Ia spindle afferents rapidly adapt to persistent activation by slowing their discharge activity; this reduces muscle tone by reducing glutamate release on to motoneurones (Kato et al 1982; Chandler, 1989). However, when noradrenergic drive on to motoneurones was increased, masseter tone was elevated throughout the entire reflex activation period.…”

“…6). The short‐lived nature of this response presumably results from the initial activation of Ia spindle afferents, which then rapidly adapt to continued muscle stretch (Hulliger, 1979; Kato et al 1982).…”