Neuroanatomy, Pontine Micturition Center

Excerpt

The neural circuitry that controls urination, or micturition, is complex and highly distributed. It involves pathways at various levels of the brain, the spinal cord, and the peripheral nervous system and is under the mediation of multiple neurotransmitters. To fully understand the role that the Pontine Micturition Center (PMC) plays in regulating urination requires a basic understanding of the structures and innervations involved.

The mechanisms of storing and eliminating urine depend on the coordinated activity of the two functional units of the lower urinary tract: the urinary bladder and urinary outlet. The bladders' primary function is storing urine and contracting to void the stored urine. It is comprised mainly of smooth muscle, namely the detrusor muscle. The urinary outlet through which urine passes upon exiting the bladder consists of:

1. The bladder neck

2. The urethra

3. The urethral sphincters

If either of the sphincters contracts, the urethra is blocked, and urine will not flow out.

As a summary of innervation of these structures:

1. Sympathetic innervation originates in the thoracic spine and serves to inhibit urination. Sympathetic nerves inhibit the detrusor muscle and activate the internal sphincter to contract.

2. Parasympathetic innervation originates in the sacrum and serves to promote urination. Parasympathetic nerves activate the detrusor muscle causing it to contract and relax the internal sphincter.

3. Both of these innervations act on the internal sphincter, which is made of smooth muscle and is an extension of the detrusor muscle.

4. Both innervations are autonomic and involuntary.

In contrast to the previous autonomic pathways, a third innervation is that of the somatic motor nerves, which originate in the sacrum. These nerves act primarily on the external urethral sphincter, which is composed of striated muscle, and under voluntary control. When the somatic nerves stimulate the sphincter, it contracts and prevents urine from flowing.

The PMC, also known as Barrington's nucleus, is located in the brainstem and works with other regions of the brain to coordinate micturition. The primary role of the PMC when it is activated is to stimulate urination. It achieves this through activation of parasympathetic neurons, triggering the detrusor muscle to contract and indirectly inhibits the somatic nerves that maintain the external sphincter closed in contraction.