Sensory innervation of baboon muscle spindles

The number and distribution of primary and secondary sensory endings has been studied in 52 de-efferentated baboon muscle spindles and the axon diameters of the afferent fibres innervating these endings have been measured. Each spindle contained a single primary sensory ending; most of these endings were supplied by a single afferent nerve fibre. Each primary sensory ending consisted of a multi-branched network distributed on both nuclear bag and nuclear chain fibres. Beaded sensory terminals were prominent in the central part of the ending. Eighty one secondary endings were found in 45 spindles (87% of the number of spindles remained). Of these endings, 70% were found in the S1 position, 25% in the S2 position and 4% in the S3 location. The afferent axons supplying the most equatorial of these endings were of thicker mean diameter than those supplying the most polar endings. In addition, the juxta-equatorial secondary endings were similar in form, although less regularly organized than the primary endings. The more polar secondary endings rarely fromed terminal sensory enlargements and usually took the form of a fine spray of unmyelinated branches. A non-parametric statistical comparison of physiological and anatomical data in baboon spindles has suggested that the demarcation criteria for separation of primary and secondary spindle afferents, using conduction velocity, should be modified. It is suggested that afferent fibres of conduction velocity less than 60 m/sec should be classified as secondary afferents, and fibres of conduction velocity greater than 72 m/sec should be classified as primary afferents.