The reflex activity of mammalian small‐nerve fibres

The efferent fibres to muscle, in the lumbosacral outflow of the cat, show a diameter spectrum with two distinct groups. The large fibres are gathered about a peak of 10-12,., while the 'small-nerve' group has a distribution peak near 5-6,u. (Eccles & Sherrington, 1930). Recent studies have shown that these efferent fibres can be functionally separated into two categories corresponding to the diameter groups (Kuffler, Hunt & Quilliam, 1951; Hunt & Kuffler, 1951 a). The larger fibres, conducting at rates above 50 m./sec., set up the characteristic motor-unit twitch response in ordinary muscle fibres. In contrast, the smaller fibres, conducting at 15-50 m./sec., innervate exclusively the muscle spindles and, by excitation of the intrafusal muscle fibres within the spindle, regulate the sensory discharge from these end-organs. The discharge from sensory endings in muscle spindles is principally governed by two factors which always interact, namely external stretch to muscles and a nervous control mediated via the small-nerve fibres. By means of the latter the afferent spindle discharge can be finely graded, through a process of facilitation, by the number and frequency of impulses reaching the spindle over individual small-nerve fibres. Further, since each spindle receives up to five or more small-nerve fibres, variation in the number of active small-nerve fibres provides additional regulation. Another factor to be considered is the distribution of individual small-nerve fibres within a muscle. Each small-nerve fibre, by branching, innervates several muscle spindles so that activity in such a fibre can influence the sensory discharge in a number of afferent fibres from different spindles. In the lumbosacral ventral roots of cat, approximately one-third of the fibres are concerned with this exclusive function of spindle regulation (for details see Kuffler et al. 1951; Hunt & Kuffler, 1951a). The participation of the small-nerve fibres in reflex activity has not previously been investigated. This is the scope of the present paper.