The normal striated sphincter muscles demonstrate some continuous motor unit activity at rest as revealed by kinesiological EMG. This differs between individuals and continues also after subjects fall asleep during the examination. This physiological spontaneous activity may be called tonic, and depends on prolonged activation of certain tonic motor units.
The ‘amount’ of tonic motor unit activity can in principle be assessed counting the number of active motor unit potentials or analysing the interference pattern by EMG; this has so far not been much studied. Thus, little is known about the variability and the normal range of tonic activity in normal subjects, and the reproducibility of findings; this makes it difficult to assess the validity of results from the few studies reporting activity changes accompanying LUT, anorectal or sexual dysfunction.
As a rule tonic motor unit activity increases with bladder filling, at the same time depending on the rate of filling. Any reflex or voluntary activation is mirrored first in an increase of the fi ring frequency of these motor units. On the contrary, inhibition of firing is apparent on initiation of voiding. With any stronger activation manoeuvre (e.g. contraction, coughing), and only for a limited length of time, new motor units are recruited. These may be called ‘phasic’ motor units. As a rule, they have potentials of higher amplitudes and their discharge rates are higher and irregular. A small percentage of motor units with an ‘intermediate’ activation pattern can also be encountered. It has to be stressed that this typing of motor units is electrophysiological, and no direct correlation to histochemical typing of muscle fibres has so far been achieved. With regard to tonic activity, sphincters differ from some perineal muscles; tonic activity is encountered in many but not all detection sites for the levator ani muscle and is practically never seen in the bulbocavernosus muscle. In the pubococcygeus of the normal female there is some increase of activity during bladder filling, and reflex increases in activity during any activation manoeuvre performed by the subject (e.g. talking, deep breathing, coughing).
On voiding, inhibition of the tonic activity of the external urethral sphincter – and also the PFM – leads to relaxation. This can be detected as a disappearance of all EMG activity, which precedes detrusor contraction. Similarly, the striated anal sphincter relaxes with defecation and also micturition.