Centres in the pons (brainstem) coordinate micturition as such, but areas rostral to the pons (the hypothalamus and other parts of the brain including the frontal cortex) are responsible for the timing of the start of micturition. The pontine micturition centre (PMC) coordinates the activity of motor neurons of the urinary bladder and the urethral sphincter (both nuclei located in the sacral spinal cord), receiving afferent input via the periaqueductal grey matter. The central control of LUT function is organized as an on–off switching circuit (or a set of circuits, rather) that maintains a reciprocal relationship between the urinary bladder and urethral outlet.
Without the PMC and its spinal connections coordinated bladder/sphincter activity is not possible, thus patients with lesions of the PMC and its spinal connections demonstrate bladder sphincter discoordination (dyssynergia). Patients with lesions above the pons do not show detrusor–sphincter dyssynergia, but have urge incontinence (due to bladder overactivity) and demonstrate noninhibited sphincter relaxation and an inability to delay voiding to an appropriate place and time.
Voluntary micturition is a behaviour pattern that starts with relaxation of the striated urethral sphincter and PFM. Voluntary PFM contraction during voiding can lead to a stop of micturition, probably because of collateral connections to detrusor control nuclei. Descending inhibitory pathways for the detrusor have been demonstrated (de Groat et al 2001). Bladder contractions are also inhibited by refl exes, activated by afferent input from the PFM, perineal skin, and anorectum.
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