EMG properties of the motor units of the striated urethral sphincter

96 EMG properties of the motor units of the striated urethral sphincter Clare J.Fowler and Roger S.Kirby. Clinical Neurophysioiogy and Urology, Middlesex Hospital, London. Whereas the bladder is known to be richly innervated by parasympathetic fibres, and under the control of the autonomic nervous system, the innervation of the urethral sphincter remains the subject of some controversy. It has been claimed

that the striated muscle of the urethra of the cat is innervated by

sympathetic and parasympathetic fibres, in addition to somatomotor innervation (E]badawi 1982).

The striated muscle of the urethra and the

arranged smooth muscle fibres,

concentrically

together comprise the "voluntary" sphincter.

Motor units can be electrophysiologlcally recorded from the urethral sphincter, which in their configuration and in response to reinnervation are similar to skeletal striated muscle motor units (Fowler et al 1983.) However, these motor units fire tonically with an interference pattern that is related to the degree of bladder filling, and evidence of voluntary control is principally demonstrated by voluntary inhibition of their activity at the initiation of micturition and voluntary activation when arresting midstream micturition. Motor units

recorded from the striated muscle of the urethral sphincter

exhibit other features which distinguish them from skeletal muscle motor units: i) Studies of the configuration of individual motor units from the urethral sphincter in 10 patients with the condition of Progressive Autonomic Failure plus Multiple System Atrophy (PAF+MSA) have shown striking changes of reinnervation. Denervation and reinnervation of skeletal muscle motor units is not a prominent feature of PAF+MSA. ii) We have observed that a particular type of EMG activity, which has become known as 'pseudomyotonia' (PMT), is not infrequently recorded from the striated muscle of the urethral sphincter. Such activity is extremely unusual in ske]?tal striated muscle. PMT is comprised of complex repetitive discharges (CRDs) and Decelerating Burst discharges (DBs). The latter have auditory qualities highly reminiscent of myotonic discharges. Detailed analysis of tiqese dischar0es has shown that they are due to ephaptic transmission between muscle fihr,-~, as described in skeletal muscle.(Trontelj and St~]berg 1983). The potential to develop this activity so freely indicates unusual properties of the striated muscle fibres of the ui :thral sphincter. Elbadawi A. (1982) I'qeurourol Urodynam 1, 3-50. Fowler C.J., et al (1984) J Neurol Neurosurg Psych /47, 637-6/41. Trontelj J.V. and Stalberg E. (1t~83) .] Neurol Neurosurg Psych /46, 3]U-316