Physiological properties of tandem muscle spindles in neck and hind-limb muscles

Although tandem muscle spindle complexes are found in small but significant numbers in most muscles, experimental investigation of their properties has been problematic because of the difficulty of distinguishing their afferents from those of "normal" single spindles. Of particular interest are the afferents from b2c capsules of tandem spindles, which unlike normal spindles contain only a static b2 nuclear bag fibre and some nuclear chain fibres. The absence of a dynamic b1 nuclear bag fibre from b2c spindles has engendered much speculation as to their response properties and their possible role in motor control. We have recently developed a method for the identification of afferents from b2c spindles in electrophysiological experiments, using infusion or topical application of succinylcholine (SCh). SCh causes the contraction of the dynamic b1 and static b2 nuclear bag intrafusal fibres, and paralyses the nuclear chain fibres. Afferents from b2c spindles are characterized by a strong "biasing" of their discharge rate to about 100 impulses per second (i.p.s.) when activated by SCh (reflecting the contraction of the static b2 fibre), while primary afferents from normal b1b2c spindles show a large increase in dynamic sensitivity as well as "biasing" (reflecting the contraction of both dynamic b1 and static b2 bag fibres). Histological examination of tenuissimus spindles activated by SCh has confirmed this relationship between the pattern of activation by SCh and the number of intrafusal nuclear bag fibres in the spindle. In this paper we review the value of SCh as a means of testing spindle afferents for functional inputs from sensory terminals on the nuclear bag fibres, and discuss the properties of b2c afferents from tandem spindles in the context of their possible function.