Recent Developments in the Physiology of the Mammalian Muscle Spindle

Abstract

Experiments have been carried out on passive muscle spindles of the soleus muscle of anaesthetised cats. Use is made of the property of spindles to show after-effects. It is proposed that after a muscle stretch the intrafusal fibres of spindles fall slack, due to their thixotropic property. It is possible to take up the slack selectively in individual intrafusal fibres by stimulating their fusimotor (γ) fibres. It was shown that vibration sensitivity of a slack spindle could be restored after a period of stimulation of γ static (γs) but not γ dynamic (γD) axons. Similarly the level of resting discharge of the slack spindle could be significantly raised, as could the response to a slow stretch, by stimulating γS but not γD axons. It is concluded that, at muscle lengths at which spindles show after-effects, responses of passive spindles come from the terminals of the afferent fibre which lie on intrafusal fibres innervated by γS axons, the bag2 fibre and perhaps also chain fibres but not the bag1 fibre, innervated by γD axons. This conclusion remains to be reconciled with recent observations which suggest that the sensory region of the bag1 fibre is more compliant than that of bag2 and chain fibres.