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Distribution and properties of muscle spindles in the caudal segmental muscles of the rat together with some comparisons with hind limb muscle spindles.

The afferent activity in sensory nerve fibres was recorded when lateral segmental muscles of the rat tail were stimulated mechanically. The muscles were then examined histologically and the muscle spindle content established. The distribution of spindles along the tail length and their innervation pattern was obtained. The overall ratio of primary to secondary endings was approximately two to one. Most single spindles (67% of sample) were intermediate; parallel (dual) spindles (29% of sample) usually combined a simple with an intermediate spindle; complex spindles were rare. 
 
 
 
The maximum muscle extension during tail-bending movements was a submaximal stimulus to tail spindles; there was no resting discharge in the straight tail position in the absence of fusimotor stimulation. 
 
 
 
The spindle endings were compared with those in rat hind limb muscles in terms of dynamic index, vibration sensitivity, spindle complexity and conduction velocity of afferent fibres. In all four aspects the tail values were more restricted in range; all caudal spindle endings had lower dynamic indices, lower vibration sensitivity and lower afferent conduction velocities than those found in primary endings in the three hind limb muscles examined (peroneus longus, peroneus brevis and gastrocnemius lateralis). A positive correlation between spindle afferent conduction velocity and vibration sensitivity of caudal spindle endings was recognized, and the possible role of afferent fibre size in determining spindle properties discussed.