URINE STORAGE MICTURITION PHYSIOLOGY

The physiology of urine storage and micturition
Urine storage
During bladder filling, bladder pressure remains low despite a substantial increase in volume. The bladder is thus highly compliant. Its high compliance is partly due to the elastic properties (viscoelasticity) of the connective tissues of the bladder and partly due to the ability of detrusor smooth muscle cells to increase their length without any change in tension. The detrusor is able to do this as a consequence of prevention of transmission of activity from preganglionic parasympathetic neurons to postganglionic efferent neurons a so-called  gating mechanism within the parasympathetic ganglia. In addition, inhibitory interneuron activity in the spinal cord prevents transmission of afferent activity from sensors of bladder filling.

Micturition
A spino-bulbar-spinal reflex, co-ordinated in the pontine micturition centre in the brainstem (also known as Barrington's nucleus or the M region), results in simultaneous detrusor contraction, urethral relaxation, and subsequent micturition. Receptors located in the bladder wall sense increasing tension as the bladder fills (rather than stretch). This information is relayed, by afferent neurons, to the dorsal horn of the sacral cord. Neurons project from here to the periaqueductal gray matter (PAG) in the pons. The PAG is thus informed about the state of bladder filling. The PAG and other areas of the brain (limbic system, orbitofrontal cortex) input into the PMC and determine whether it is appropriate to start micturition.

At times when it is appropriate to void, micturition is initiated by relaxation of the external urethral sphincter and pelvic floor. Urine enters the posterior urethra and this, combined with pelvic floor relaxation, activates afferent neurons which results in stimulation of the pontine micturition centre (located in the brainstem). Activation of the PMC switches on a detrusor contraction via a direct communication between neurons of the PMC and the cell bodies of parasympathetic, preganglionic motoneurons located in the sacral intermediolateral cell column of S2 - 4. At the same time that the detrusor contracts, the urethra (the external sphincter) relaxes. The PMC inhibits the somatic motoneurons located in Onuf's nucleus (the activation of which causes external sphincter contraction) by exciting GABA and glycine-containing, inhibitory neurons in the intermediolateral cell column of the sacral cord, which in turn project to the motoneurons in Onuf  s nucleus. In this way, the PMC relaxes the external sphincter.

Micturition is an example of a positive feedback loop, the aim being to maintain bladder contraction until the bladder is empty. As the detrusor contracts, tension in the bladder wall rises. The bladder wall tension receptors are stimulated and the detrusor contraction is driven harder. One of the problems of positive feedback loops is their instability. Several inhibitory pathways exist to stabilize the storage - micturition  loop.

Tension receptors activate bladder afferents, which via the pudendal and hygastric nerves inhibit S2 - 4 parasympathetic motor nerve output. An ongoing detrusor contraction cannot be overriden.
Afferents in the anal and genital regions and in the distribution of the posterior tibial nerve stimulate inhibitory neurons in the sacral cord, and these neurons inhibit S2 - 4 parasympathetic motor nerve output. This pathway can override an ongoing detrusor contraction. It is hypothesized that this system prevents involuntary detrusor contraction during sexual activity, defaecation, and while walking, running, and jumping.

Excitatory neurotransmission in the normal detrusor is exclusively cholinergic, and reciprocal relaxation of the urethral sphincter and bladder neck is mediated by NO, released from postganglionic parasympathetic neurons.