An investigation of muscle spindles in the pelvic floor muscles

Investigators : Ms Margaret Sherburn, Prof Mary Galea, A/Prof Chris Briggs

The aim of this study is to investigate the density, morphology and distribution of muscle spindles within the pelvic floor muscles.

While transabdominal displacement measures of pelvic floor muscle activation are reliable, there is still variation in individual muscle contractions. This variation is typical of all striated muscle activity, however in some women these variations are large. One reason for this is a poor perception of pelvic floor muscles. Symptoms of this poor perception are a difficulty of initiating a muscle contraction in the presence of an intact neural system, poor perception of muscle activation, of a sustained contraction and a poor ability to repeat a contraction of similar effort, or modify a contraction in the absence of myopathy. This poor perception creates difficulties in undertaking pelvic floor muscle training.

One of the anatomical structures which enables us to sense muscle activity and thereby modulate change in muscle activation patterns, is the muscle spindle. Muscle spindles (Fig 1), along with Golgi tendon organs are mechanoreceptors, or force transducers, which provide the sensory mechanism for muscle activation. They are unique sensory receptors in that they have both sensory and motor innervation. and respond to both rate of change of muscle length and to static activity, for maintenance of posture or precision in voluntary movement.

This study is a descriptive study investigating the density, morphology and distribution of muscle spindles within the pelvic floor muscles. Fresh pelvic floor muscle tissue will be gathered from 10 subjects both male and female of a variety of ages. Tissue will be gathered from an anterior and a posterior site on both the left and right sides of the both the pubococcygeus and one from each iliococcygeus muscles, six sites in total. The tissue will be transferred to Histology at The University of Melbourne and then carefully labelled before being embedded in paraffin wax for sectioning. The muscle samples will then be sectioned and those at 1 mm intervals will be transferred to glass slides for histological staining with Massons's trichrome and then examined under a light microscope for distribution of muscle spindles using stereological method called the disector. From these data, comparisons will be made between the two pelvic floor muscles pubococcygeus and iliococcygeus.

Figure 1 . An example of Massons's Trichrome stain in a section from Longus Colli. Muscle fibres are stained red, collagen is stained blue. The muscle spindles (centre) are easily distinguishable from the surrounding extrafusal muscle fibres.

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