Sacral spinal interneurones and the control of urinary bladder and urethral striated sphincter muscle function

Normally, during bladder filling (continence) and expulsion (micturition) t here is a reciprocity between the pattern of activity ill the urinary bladd er sacral parasympathetic efferents and the somatic motoneurones innervatin g the striated external urethral sphincter muscle. The co-ordination of thi s pattern of reciprocal activity appears to be determined by excitatory and inhibitory actions of a variety of segmental afferents and descending syst ems with sacral spinal actions. These actions may in part be mediated throu gh lower lumbar and sacral excitatory anti inhibitory spinal interneurones. Over the past 30 years, both neuroanatomical and electrophysiological appr oaches have been used to reveal an ever-increasing richness in the neuronal network in the lower;er spinal cord related to the bladder and striated ex ternal urethral sphincter muscle. The purpose of this review is to present an overview of the identified excitatory and inhibitory spinal interneurone s hypothesized to be involved in the central networks controlling the sacra l bladder parasympathetic preganglionic neurones and striated urethral sphi ncter motoneurones during continence and micturition.