Movement convergence and motor end plate location in a human muscle spindle

Abstract

In cat spindles when γ- or β-axons are stimulated the direction of movement of sarcomeres is towards motor end plates. This is true of both bag and chain fibres, with the exception of some extra-capsular contraction sites in bag₁ fibres which have no associated motor endings (Banks et al., 1978; Arbuthnott et al., 1982). The method required to obtain this information for human spindles has so far been applied to one spindle only, a spindle from a child. All movement was in fact directed towards motor end plates in dynamis bag₁ (Db₁), static bag₂ (Sb₂) and chain fibres. The identification of the Db₁ and the Sb₂ fibres was made by comparing their speed of contraction (see Gladden, 1985), but morphological differences were also sought. A difference in the conformation of bag fibres close to the equator in spindles of adults (Gladden, 1975) occurred in this and other children’s spindles, and may provide a simple way of recognising human Db₁ and Sb₂ fibres.