Voiding Phase of Micturition

Volumes in Micturition

Whilst the capacity of the bladder varies from roughly 300-550ml, afferent nerves in the bladder wall signal the need to void the bladder at around 400ml of filling. Micturition itself is typically a short-lasting event.

Urinary flow rate in a full bladder is:

• 20-25ml/s in men
• 25-30ml/s in women

After a successful voluntary void, a normal healthy adult may retain about 50 mL of urine (post-void residual).

Neural Control of Micturition

The passing of urine is under unconscious parasympathetic control via a spinal reflex which undergoes remodelling during childhood, allowing it to be overridden at various control points by higher voluntary somatic control.

For micturition to occur the detrusor muscle must contract while the internal (only present in males) and external urethral sphincters relax concomitantly.

Primitive Micturition Reflex

As the bladder reaches capacity, stretch receptors in the detrusor muscle are intensely activated and bladder afferents send signals via the pelvic nerve (nerve roots S2-4) to the spinal cord. Efferent signals are sent back to the detrusor via parasympathetic fibres in the pelvic nerve, releasing acetylcholine (ACh) at the neuromuscular interface.

Ach acts on muscarinic ACh receptors (M₃ receptors) on the detrusor muscle, causing contraction and increased intra-vesicular pressure. This is the micturition reflex.

After 2 years of age, following extensive remodelling, voluntary control of this reflex arc is gained by higher centres. Instead of synapsing in the spinal cord, information from bladder afferents ascend via the spinal cord to the periaqueductal gray in the midbrain, to the pontine micturition centre (PMC) and cerebral cortex.

Though the PMC now stimulates the efferent limb of this reflex, the primitive spinal reflex is still relevant in pathological states.

Control by Higher Centres

Higher brain centres can voluntarily inhibit or facilitate the micturition reflex, primarily via the PMC. The PMC and micturition reflex is normally inhibited by higher centres until the decision to urinate is made.

Upon the voluntary decision to urinate, inhibition of the PMC is released. This allows the PMC to stimulate the sacral preganglionic parasympathetic neurons, causing detrusor contraction.

The PMC also suppresses Onuf’s nucleus in the sacral spinal cord, reducing the somatic activity of the pudendal nerve (S2–S4) and voluntary contraction of the external urethral sphincter. This causes relaxation of the external urethral sphincter.

Simultaneously, in response to bladder afferents from a full bladder, the PMC also reduces sympathetic stimulation of the hypogastric nerve (T10-L2), allowing relaxation of the internal urethral sphincter. This allows the urethra to distend, enabling the passage of urine.

Bladder Emptying

Once micturition has begun, bladder contractions lead to increased stimulation of afferents and increased contraction in a positive feedback and self-regenerating process. This ensures bladder emptying.

In the female, urination is assisted by gravity, while in the male, bulbospongiosus muscle contractions along the length of the penis also helps to expel all of the urine.

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Figure 1
Diagram summarising the neural pathways involved in voiding.