Distinctive modes of static and dynamic fusimotor drive in jaw muscles

Abstract

This paper describes the results of recordings from fusimotor and alphamotor axons of the cat masseter nerve and from muscle spindle afferents from the jaw closing muscles. Criteria are described for recognising fusimotor from alpha motor axons and their behaviour is examined during cyclic reflex jaw movements under light anaesthesia. Approximately one third of the fusimotor fibres showed an increased and ‘sustained’ discharge during a movement sequence. The rest showed strong modulation with maximum frequency during the muscle shortening phase and were referred to as ‘modulated’. By recording from primary and secondary spindle afferents during separate but very similar experiments it was deduced that the ‘sustained’ type of response was probably in dynamic fusimotor fibres and the ‘modulated’ type in static fusimotor fibres. New observations of jaw muscle spindle afferent discharge in freely moving alert cats indicated that the same sort of modulated static fusimotor drive may accompany masticatory movements but that this does not normally overcome the unloading effect of unobstructed shortening. Sometimes, when extra loading slows shortening, a considerable enhancement of spindle afferent firing is seen which could have a significant load compensating effect.