Classification of fusimotor fibers in the primate.

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.