Impulse initiation in the mammalian muscle spindle during combined
fusimotor stimulation and succinyl choline infusion.
Carr, R.W., D.L. Morgan and U. Proske.
Department of Physiology, Monash University, Victoria, Australia 3168,
Department of Electrical & Computer Systems Engineering.
APStracts 2:0328N, 1995.
SUMMARY AND CONCLUSIONS
1. This is a report of observations on the responses of the primary and
secondary endings of soleus muscle spindles of the anaesthetised cat to the
combined effects of the depolarising neuromuscular blocker, succinyl choline
(SCh), given intravenously, and fusimotor stimulation. The findings were
interpreted in terms of a dual pacemaker model for activity generated in the
bag1 intrafusal fibre interacting with activity coming from bag2 and chain
fibres. 2. In preliminary experiments it was found, using whole ventral root
stimulation at fusimotor strength, that spindle responses to fusimotor
stimulation were not blocked by SCh while extrafusal junctions blocked
rapidly. In the presence of SCh fusimotor responses of spindle secondary
endings were, on average, slightly larger than their control values before
giving SCh, while fusimotor responses of primary endings were slightly
smaller. 3. A study of the responses of spindle primary endings to stimulation
of single dynamic ( [gamma]D ) and static ( [gamma]S ) axons in the presence
of SCh revealed a fundamental difference in behaviour. None of the responses
to stimulation of [gamma]D axons (nine [gamma]D axons with eight primary
endings) showed significant summation with the responses to SCh. By contrast
the 20 [gamma]S axons studied showed varying degrees of summation with the
responses to SCh. The responses of secondary endings to [gamma]S stimulation
in the presence of SCh resembled those of primary endings and [gamma]S
stimulation. 4. To explain these differences it is proposed that the primary
ending has two separate sites of impulse initiation, one close to terminals on
the bag1 intrafusal fibre (innervated by [gamma]D axons) and a second close to
terminals on the bag2 and chain fibres (innervated by [gamma]S axons). It is
proposed that the maintained increase in spindle firing observed during SCh
infusion is the result of a bag2 contracture. The response to [gamma]S
stimulation, contracting bag2 and chain fibres, adds to the SCh response. The
degree of summation varies depending on whether the [gamma]S activates bag2
fibres, chain fibres or both. The bag1 contracture, together with the effects
of [gamma]D stimulation act through a separate pacemaker and therefore do not
sum with the steady increase in spindle firing in the presence of SCh. There
may be pacemaker switching between the bag1 generator and the bag2 and chain
generator. 5. If the model is representative of most spindles containing the
three kinds of intrafusal fibres, and the contractions of bag2 and chain
fibres generate activity through a common impulse generator, then this bears
on the question of the functional independence of the bag2 and chain fibre
systems.
Received 20 December 1995; accepted in final form 23 October 1995.
APS Manuscript Number J793-4.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 30 November 95