The density of muscle spindles in the medial, intermediate and lateral columns of human intrinsic postvertebral muscles

Abstract

Studies of the mechanics of the trunk have reported that during the initial phase of lifting in the stooped position the body uses additional support of intratruncal pressure to limit the work done by the erector spinae muscle (Davis, 1956; Bartelink,1957). These workers showed that intrathoracic and intra-abdominal pressures varied proportionately with the work done. Kumar & Davis (1973) observed that variations in intra-abdominal pressures closely correlated with electrical activity in the postvertebral as well as the anterior abdominal wall muscles. They suggested that receptors in the erector spinae muscles might be implicated, but they did not specify which components of the muscle were involved. Work done by Etemadi (1963) and Bogduk (1980) have shown that the component slips of the muscle are multi-articular and therefore their pattern of recruitment during movement would depend critically on afferent inputs into the central nervous system. The quality of sensory innervation, and, particularly, the density of muscle spindles in these muscles is, therefore, of interest. Information on the sensory innervation of the back muscles is incomplete. The segmental distribution of muscle spindles in the intrinsic postvertebral muscles was reported by Gregor (1904) in some parts of the group, and by Amonoo-Kuofi (1982) in all members of the group, but the density of spindles in the individual columns was not given. In order to determine the relative importance of the different columns of back muscles in postural and dynamic functions, their respective spindle contents must be expressed in relation to the bulk of muscle. This paper reports on the densities of muscle spindles in postvertebral muscles, based on the areas occupied by individual columns at each segmental level.