Journal of Neurophysiology, Vol 39, Issue 1 9-19, Copyright © 1976 by APS
Classification of fusimotor fibers in the primate
P. D. Cheney and J. B. Preston
The classification and distinguishing characteristics of fusimotor fibres
of the baboon soleus muscle have been studied by determining the effects of
single fusimotor fiber stimulation on the response of isolated spindle
afferents to muscle stretch. As in the cat, fusimotor fibres in the baboon
were divisible into static and dynamic types on the basis of the effect of
their stimulation at 200/s on the dynamic index of the primary afferent.
Single fusimotor fibres had the same qualitative effect-static or
dynamic-on all the primary afferents they were found to influence. All
static fusimotor fibres produced at least 1 to 1 driving of primary
afferent discharge at 50/s if the muscle length was adjusted to optimize
conditions for driving. In contrast, 31 of 32 dynamic fusimotor fibres did
not produce driving of primary afferent discharge even though all were
studied at many different muscle lengths and frequencies of stimulation.
Therefore, fusimotor fibres in the baboon could be classified as static or
dynamic on the basis of their ability to produce driving of the primary
afferent. The ratio of isolated static to dynamic fusimotor fibers was 1.5
to 1. The mean conduction velocity of static fusimotor fibers was 24.1 m/s
and that of dynamics was 20.2 m/s. Although the difference between these
two means was statistically significant (P less than 0.001), the conduction
velocity histograms of static and dynamic fusimotors overlapped, precluding
classification by this means. Secondary afferents with one possible
exception were concluded to be exclusively activated by static fusimotor
fibers. Poststimulus effects of fusimotor stimulation on primary afferent
response to stretch were studied. With intervals between the end of
stimulation and the beginning of stretch of 0.1 s, dynamic fusimotor
stimulation produced facilitation of the primary afferent response to
stretch, whereas static fusimotor fibers produced depression.
