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

Normally, during bladder filling (continence) and expulsion (micturition) there is a reciprocity between the pattern of activity in the urinary bladder sacral parasympathetic efferents and the somatic motoneurones innervating the striated external urethral sphincter muscle. The co-ordination of this pattern of reciprocal activity appears to be determined by excitatory and inhibitory actions of a variety of segmental afferents and descending systems with sacral spinal actions. These actions may in part be mediated through lower lumbar and sacral excitatory and inhibitory spinal interneurones. Over the past 30 years, both neuroanatomical and electrophysiological approaches have been used to reveal an ever-increasing richness in the neuronal network in the lower spinal cord related to the bladder and striated external urethral sphincter muscle. The purpose of this review is to present an overview of the identified excitatory and inhibitory spinal interneurones hypothesized to be involved in the central networks controlling the sacral bladder parasympathetic preganglionic neurones and striated urethral sphincter motoneurones during continence and micturition.