Skilled movement of distal limb muscles requires individual motor units to be activated in a highly focused manner by the primary motor cortex. By contrast, activation of axial muscles (necessary to maintain posture etc.) – while also under voluntary control – depends particularly on vestibular nuclei and reticular formation to create predetermined ‘motor patterns’. The PFM are not, strictly speaking, axial muscles, but several similarities to axial muscles can be proposed as regards their neural control. In any case, PFM are under voluntary control (i.e. it is possible to voluntarily activate or inhibit the fi ring of their motor units). EMG studies have shown that the activity of motor units in the urethral sphincter can be extinguished at both low and high bladder volumes even without initiating micturition.
To voluntarily activate a striated muscle we have to have the appropriate brain ‘conceptualization’ of that particular movement, which acts as a rule within a particular complex ‘movement pattern’. This evolves particularly through repeatedly executed commands and represents a certain ‘behaviour’. Proprioceptive information is crucial for striated muscle motor control both in the ‘learning’ phase of a certain movement and for later execution of overlearnt motor behaviours. It is passed to the spinal cord by fastconducting large-diameter myelinated afferent fi bres and is infl uenced not only by the current state of the muscle, but also by the efferent discharge the muscle spindles receive from the nervous system via gamma efferents. To work out the state of the muscle, the brain must take into account these efferent discharges and make comparisons between the signals it sends out to the muscle spindles along the gamma efferents and the afferent signals it receives from the primary afferents. Essentially, brain compares the signal from the muscle spindles with the copy of its motor command (the ‘corollary discharge’ or ‘efferents copy’) which was sent to the muscle spindle intrafusal muscle fibres by the CNS via gamma efferents. The differences between the two signals are used in deciding on the state of the muscle. The experiments were carried out in limb muscles, but it has been suggested that similar principles rule in bladder neurocontrol.