Journal of Neurophysiology, Vol 75, Issue 4 1703-1713, Copyright © 1996 by APS
Impulse initiation in the mammalian muscle spindle during combined fusimotor stimulation and succinyl choline infusion
R. W. Carr, D. L. Morgan and U. Proske
Department of Physiology, Monash University, Victoria, Australia.
1. This is a report of observations on the responses of the primary and
secondary endings of soleus muscle spindles of the anesthetized cat to the
combined effects of the depolarizing 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 fiber interacting with activity coming from bag2 and
chain fibers. 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, whereas 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 SCh was given, whereas 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 behavior.
None of the responses to stimulation of gamma D axons (9 gamma D axons with
8 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 fiber (innervated
by gamma D axons) and a second close to terminals on the bag2 and chain
fibers (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 fibers, adds to the SCh response. The degree of summation varies
depending on whether the gamma S activates bag2 fibers, chain fibers, or
both. The bag1 contracture, together with the effects of gamma D
stimulation, acts through a separate pacemaker and therefore does 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 fibers, and the contractions of
bag2 and chain fibers generate activity through a common impulse generator,
then this bears on the question of the functional independence of the bag2
and chain fiber systems.
