Muscle activity is thoroughly dependent on neural control. ‘Denervated’ muscle atrophies and turns into fibrotic tissue. Muscle – like every tissue – consists of cells (muscle fibres). But the functional unit within striated muscle is not a single muscle cell, but a motor unit. A motor unit consists of one alpha (or ‘lower’) motor neuron (from the motor nuclei in spinal cord), and all the muscle cells this motor neuron innervates.
The motor unit – in other words – is the basic functional unit of the somatic motor system; control of a muscle means control of its motor units. Thus, in discussing neural control of muscle, we really only need to consider the motor neurons in the spinal cord and all the influences they are exposed to. The function of pelvic floor and sphincter lower motor neurons is organized quite differently from other groups of motor Neurons. In contrast to the reciprocal innervations that is common in limb muscles, the neurons innervating each side of the PFM have to work in harmony and synchronously. Indeed, sphincters may be morphologically considered to constitute ‘one’ muscle which is innervated by two nerves (left and right)! By concomitant activity the PFM act as the ‘closure unit’ of the excretory tracts, the ‘support unit’ for pelvic viscera, and an ‘effector unit’ in the sexual response. In general, muscles involved in these functions from both sides of the body act in a strictly unifi ed fashion as ‘one muscle’: this has been demonstrated for the pubococcygei muscles, but has not really been documented for the whole group of PFM and sphincters.
However, as each muscle in the pelvis has its own unilateral peripheral innervation, dissociated activation patterns are possible and have been reported between the two pubococcygei (Deindl et al 1994) and between levator ani and the urethral sphincter.
The differences in evolutionary origin of the sphincter muscles and levator ani furthermore imply that unilateral activation may be less of an impossibility for the PFM than for sphincters. It can be postulated that The neural mechanisms controlling the different muscles involved in sphincter mechanisms and pelvic organ support may not be as uniform as has been assumed. How much variability there is in normal activation patterns of PFM is not yet clarifi ed. It is clear, however, that the coordination between individual PFM can efinitively be impaired by disease or trauma.