Fusimotor Influence on Jaw Muscle Spindle Activity during Swallowing‐Related Movements in the Cat

Abstract

The activity patterns of muscle spindle afferents in jaw‐closer muscles were studied during reflex swallowing movements in anaesthetized cats. Simultaneous records were made of the electromyogram (EMG) in masseter and anterior digastric muscles and of the unloaded jaw movements. The underlying patterns of fusimotor activity were deduced by comparing afferent discharges occurring during active swallowing with those occurring when exactly the same movements were imposed passively. The interpretation of spindle behaviour was greatly facilitated by characterizing the afferents according to the evidence for their contact with the various intrafusal muscle fibres, derived from testing with succinylcholine. It was also valuable to have two different types of afferent recorded simultaneously. There was clear evidence of fusimotor activity occurring during active jaw closing so as to oppose the spindle silencing. This effect was most marked in b2c‐type afferents (probably secondaries) and was therefore attributed to a modulation of static fusimotor discharge approximately in parallel with α‐activity. Afferents with evidence of bag1 fibre contacts (primaries) showed much greater sensitivity to muscle lengthening during active movement than when the movement was imposed. This difference was exaggerated when anaesthesia was deepened for the passive movements. This was interpreted as evidence for a higher level of dynamic fusimotor activity maintained during active movements than at rest. The results support the view that for a variety of active jaw movements, static fusimotor neurone firing is modulated roughly in parallel with α‐activity but leading it so as to counteract spindle unloading. Dynamic fusimotor neurone firing appears to be set at a raised level during active movements. Anaesthesia appears to depress activity in the α‐motoneurones more than in γ‐motoneurones.

Static and dynamic γ‐motor output to ankle flexor muscles during locomotion in the decerebrate cat

“…If the additional spindle activity were sufficient it might alert the animal to the unexpected obstruction and cause a change in the strategy of the movement. Similar conclusions were derived from studies of rhythmic jaw movements in the lightly anaesthetized cat (Taylor et al 1997). The significance of the interrupted pattern of dynamic fusimotor firing in rhythmic movements has been suggested to be that it primes the primary spindle afferents to provide a very distinct signal to the central pattern generator indicating the onset of muscle lengthening.…”

Behavior of Jaw Muscle Spindle Afferents During Cortically Induced Rhythmic Jaw Movements in the Anesthetized Rabbit

“…Although the possibility remains that these different discharge patterns were caused by the dissimilar jaw movements induced, the firing patterns could also result from a difference in fusimotor activity. In support of this proposal, a very irregular discharge of the spindle afferents was noted during active movements, whereas spindle firing was more regular when the same movements were reproduced passively (Taylor et al 1997). It seems therefore that the fusimotor drive is more active in CRJMs than in reflexively evoked rhythmic jaw movements.…”

Patterns of fusimotor activity during locomotion in the decerebrate cat deduced from recordings from hindlimb muscle spindles

“…Studies in the jaw‐closing muscles, from both spindle afferent and γ‐motor recordings, have been consistently in favour of modulated patterns of firing in γ s neurones (e.g. Taylor & Appenteng, 1981; Taylor et al 1997). Although jaw muscles serve very different functions from hindlimb extensor muscles, it would be surprising if there were to be such a basic difference in the way in which the fusimotor system is used in the two cases.…”