Distribution of muscle spindles in a simply structured muscle: Integrated total sensory representation

Abstract

Background:The distribution of muscle spindles (Sps) in a small muscle of simple architecture, the capsularis at the cat's hip joint, was quantified to reveal the patterns of proprioceptive representation in the transverse and sagittal planes as well as to model the effect a local disturbance in muscle length would have on total Sp discharge.Methods: Locations in serial cross-sections of the 32 and 38 Sps in 2 muscles, 1 perfused with the hip joint flexed and the other extended, were plotted, and their patterns of integrated sensitivity calculated assuming that (1) the discharge rate of a Sp afferent varies linearly with change in length along the Sp's axis, and (2) that within a local disturbance produced by contraction of a motor unit (MU), lengths decrease either linearly or as the square of the distance from its center.Results: The isomeric pattern of ''integrated, total Sp representation on cross-section'' showed two peaks of sensitivity in the half of the muscle that had been next to the joint capsule, offset by low representation in a small, central area and along the extensive zone bordering the laterally facing ''superficial surface.'' The equivalent radius of an idealized symmetrical MU territory was estimated from distributions of the few fast, oxidative-glycolytic fibers found in two muscles, and the effect of a MU's contraction on net Sp discharge predicted when the unit was positioned at distinctive sites within the pattern. As an index of Ia and II afferent representation in the sagittal plane, the distribution of the nucleated regions of Sps and the summed lengths of segments of Sp axial bundles and capsules, respectively, within successive 1-mm segments of the muscle were graphed.Conclusions: The longitudinal representation and structure of the muscle are not suited for reflex adjustment of differences in length along the muscle. The isomeric pattern of high relief in the transverse plane suggests that in this ϳ0.2 g muscle, the localization of myotatic reflexes might be accommodated but the need for adjustment in activation of MUs seems minimal. This is because the muscle is not compartmentalized, its fibers extend between the muscle's origin and insertion, their angle of pinnation is low, and greater than 90% are of slow type. The distribution of Sps is consistent with gauging length of the entire muscle and hence angulation at the hip joint. Anat.

A comparative analysis of the encapsulated end‐organs of mammalian skeletal muscles and of their sensory nerve endings

“…Other features of the sensory complements are apparently unconstrained, or subject to random variation and may therefore be supposed to be of little or no functional significance. For example, spindles tend to be located close to the divisions of the intramuscular nerve as a consequence of the developmental process, but they would probably function equally well as if arranged more evenly (Eldred et al. 1998), so there is no selective pressure for them to be arranged in a deterministic way.…”

“…But their abundance and intramuscular distribution seem to worry many authors, though few would go as far as Kokkorogiannis (2008), who is ready to deny the function of spindles as length sensors on those bases. The concerns about number and distribution appear to be largely intuitive: careful observation coupled with modelling is always liable to demonstrate the fallibility of intuition, as in the case of the functional significance of the uneven distribution of spindles within muscles (Eldred et al. 1998).…”

An allometric analysis of the number of muscle spindles in mammalian skeletal muscles

“…Most of the data used in this analysis have been obtained from various published sources as follows: human, Voss (1971); cat, Oshima (1938), Chin et al . (1962), Richmond & Abrahams (1975), Gonyea & Ericson (1977), Bakker & Richmond (1982), Richmond & Stuart (1985), Scott & Young (1987) and Eldred et al. (1998); rat, Arendt & Asmussen (1974), Maier (1979), Pfister & Zenker (1984) and van der Wal (1988); and guinea‐pig, Martini & Palmieri (1970, 1971).…”

The innervation of the muscle spindle: a personal history

“…Probably not (Eldred et al. ). As we shall see in the following section, some aspects of the organisation of the intrafusal motor supply are also subject to random variation.…”