The human urethral and anal striated sphincters seem to have no muscle spindles; their reflex reactivity is thus intrinsically different from the levator ani muscle complex, in which muscle spindles and Golgi tendon organs have been demonstrated. Thus, PFM have the intrinsic proprioceptive ‘servomechanism’ for adjusting muscle length and tension, whereas the sphincter muscles depend on afferents from skin and mucosa. Both muscle groups are integrated in reflex activity, which incorporates pelvic organ function.
The reflex activity of PFM is clinically and electrophysiologically evaluated by eliciting the bulbocavernosus and anal reflex. The bulbocavernosus reflex is evoked on nonpainful stimulation of the glands (or – electrically – the dorsal penile/clitoral nerve). As recorded electromyographically, it is a complex response: its first component thought to be an oligosynaptic and the later component a polysynaptic reflex. The polysynaptic anal reflex is elicited by painful (pinprick) stimulation in the perianal region. The constant tonic activity of sphincter muscles is thought to result from the characteristics of their ‘lowthreshold’ motor neurons and the constant ‘inputs’ (either of reflex segmental or suprasegmental origin). It is supported by cutaneous stimuli, by pelvic organ distension, and by intra-abdominal pressure changes.
Sudden increases in intra-abdominal pressure as a rule lead to brisk PFM (reflex) activity, which has been called the ‘guarding reflex’; it is organized at the spinal level. It needs to be considered that ‘sudden increases in intra-abdominal pressure’, if caused by an intrinsically driven maneuver (i.e. coughing) include feed forward activation of the PFM as part of the complex muscle activation pattern. The observed PFM activation in the normal subject (e.g. during coughing) is thus a compound ‘feed-forward’ and ‘reflex’ muscle activation.
Another common stimulus leading to an increase in PFM activity is pain. The typical phasic refl ex response to a nociceptive stimulus is the anal reflex. It is commonly assumed that prolonged pain in pelvic organs is accompanied by an increase in ‘reflex’ PFM activity, which would indeed be manifested as ‘an increased tonic motor unit activity’. This has so far not been much formally studied. Whether such chronic PFM overactivity might itself generate a chronic pain state and even other dysfunctions may be a tempting hypothesis, but has not been well demonstrated so far.
To correspond to their functional (effector) role as pelvic organ supporters (e.g. during coughing, sneezing), sphincters for the LUT and anorectum, and as an effector in the sexual arousal response, orgasm and ejaculation, PFM have also to be involved in very complex involuntary (‘refl ex’) activity, which coordinates the behaviour of pelvic organs (smooth muscle) and several different groups of striated muscles. This activity is to be understood as originating from so called ‘pattern generators’ within the central nervous system, particularly the brainstem. These pattern generators (‘reflex centres’) are genetically inbuilt.