Neural control of continence
At rest continence is assured by a competent sphincter mechanism, including not only the striated and smooth muscle sphincter but also the PFM and an adequate bladder storage function. Normal kinesiological sphincter EMG recordings show continuous activity of motor units at rest (as defined by continuous firing of motor unit potentials), which as a rule increase with increasing bladder fullness. Reflexes mediating excitatory outflow to the sphincters are organized at the spinal level (the guarding reflex).
The L region in the brainstem has also been called the ‘storage centre’. This area was active in PET studies of those volunteers, who could not void, but contracted their PFM. The L region is thought to exert a continuous exciting effect on the Onuf’s nucleus and thereby on the striated urinary sphincter during the storage phase; in humans it is probably part of a complex set of ‘nerve impulse pattern generators’ for different coordinated motor activities such as breathing, coughing, straining, etc. During physical stress (e.g. coughing, sneezing) the urethral and anal sphincters may not be sufficient to passively withhold the pressures arising in the abdominal cavity, and hence within the bladder and lower rectum. Activation of the PFM is mandatory, and may be perceived as occurring in two steps by two different activation processes.
Coughing and sneezing are thought to be generated by individual pattern generators within the brainstem, and thus activation of PFM is a preset coactivation – and not primarily a ‘refl ex’ reaction to increased intraabdominal pressure. But, in addition, there may be an additional refl ex PFM response to increased abdominal pressure due to distension of muscle spindles within PFM.
The PFM can of course also be voluntarily activated anticipating an increase in abdominal pressure. Such timed voluntary activity may be learned (the ‘knack procedure’).