Recent Developments in the Physiology of the Mammalian Muscle Spindle
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.
KeywordsMuscle Spindle Muscle Length Sensory Ending Intrafusal Fibre Afferent Response
Unable to display preview. Download preview PDF.
- Celichowski, J., Emonet-Dénand, F., Laporte, Y. & Petit, J. (1994). Distribution of static γ axons in cat peroneus tertius spindles determined by exclusively physiological criteria. Journal of Neurophysiology 72, 722–732.Google Scholar
- Chua, M. & Hunt, C.C. (1993). Relative compliance of bag1 and bag2 sensory regions of the cat muscle spindle. 32nd Internalional Congress of Physiological Sciences, Glasgow, 322, 1P.Google Scholar
- Gioux, M., Petit, J. & Proske, U. (1991). Responses of cat muscle spindles which lack a dynamic fusimotor supply. Journal of Physiology 432, 557–571.Google Scholar
- Gregory, J.E., Morgan, D.L. & Proske, U. (1986). After-effects in the responses of cat muscle spindles. Journal of Neurophysiology 56, 451–461.Google Scholar
- Price, R.F. & Dutia, M.B. (1987). Properties of cat neck muscle spindles and their excitation by succinylcholine. Experimental Brain Research 68, 619–630.Google Scholar
- Vallbo, Å, B. (1974). Human muscle spindle discharge during isometric voluntary contraction. Amplitude reactions between spindle frequency and torque. Acta Physiologica Scandinavica 90, 319–336.Google Scholar