INTRODUCTION   
The micturition cycle occurs in two phases. It consists of a filling phase and an emptying phase. Each phase requires a coordinated interaction between the bladder and the nervous system.

URINE STORAGE   
The sympathetic nervous system predominates during the storage phase. Alpha-adrenergic receptors in the base and neck of the bladder increase muscle tone. Simultaneously beta-adrenergic receptors in the body of the bladder depress detrusor tone. Sympathetic stimulation arises from the thoraco-lumbar portion of the CNS and is carried via the hypogastric nerve.   Afferent nerves from the bladder send signals to the brain, brainstem and sacral reflex center. The cerebrum and pons exercise control over bladder functions by switching to and from the storage mode. They accomplish this by facilitating or inhibiting parasympathetic receptors.   Parasympathetic (cholinergic) nerves release the neurotransmitter acetylcholine on or near the bladder smooth muscle which causes contraction of the detrusor muscle.   Cholinergic somatic fibers in the pudendal nerve stimulate the external sphincter during bladder filling. Stimulation by the pudendal nerve causes the external sphincter to contract, obstructing the outflow of urine.   Many drugs have been developed to take advantage of the adrenergic and cholinergic properties of lower urinary tract.

SACRAL REFLEX   
This diagram represents the sacral reflex detached from higher neuro center control (suprasacral injury). Sacral reflex contraction is initiated when:

  1. Stretch receptors in the bladder wall are activated.

  2. Stretch receptor output is transmitted via afferent fibers of the pelvic nerve to the sacral cord.

  3. Parasympathetic cell bodies within the cord are activated.

  4. Parasympathetic nerve endings release acetylcholine on or near detrusor cells.

  5. The bladder contracts.

 Without inhibitory input from the pons, motor neurons in the sacral cord continue to stimulate the external sphincter via the pudendal nerve. This diagram demonstrates Detrusor Sphincter Dyssynergia (DSD). Note the bladder straining against the external sphincter.   DSD often occurs in patients who have permanent SCI above the S-2. These patients usually require intermittent catheterization to avoid overdistention.

CEREBRAL CONTROL The ability to control urination is dependent upon intact communication between the cerebrum, pons, sacral cord and peripheral nerves.   We become aware of the need to void when the urine volume is about 150cc. Ascending afferent fibers relay sensation to the pons and then to the cerebrum. The cerebrum determines if the surroundings and social conditons are appropriate to void. If it is appropriate to void, the cerebrum switches the pons to emptying mode which inhibits sympathetic (not shown) output and pudendal nerve stimulation of the external sphincter.   When the sympathetic system is quieted, the parasympathetic system is free to have maximum effect. Detrusor contraction ensues.