Micturition: Anatomy and Physiology

Overview

• Parasympathetic nervous system ACTIVATES urination.
• Sympathetic nervous system INHIBITS it.
• Somatomotor system (volitional control) INHIBITS it.

Major Structures

The major functional structures of the lower urinary system:

• Urinary bladder
• Urethra
• Detrusor muscle (the bladder wall muscle).
• Internal urethral sphincter
• External urethral sphincter

Major Functions

Filling phase

During the filling phase:

• Detrusor relaxes (stretches).
• Internal sphincter contracts (closes).
• External sphincter contracts (closes).

Voiding phase

During the micturition (voiding) phase,

• Detrusor contracts.
• Internal sphincter relaxes (opens).
• External sphincter relaxes (opens).

Filling Circuitry

Sympathetic nervous system

• The sympathetic preganglionic origins are in the intermediolateral cell column from T10 – L2).
• The sympathetic fiber system acts on the detrusor muscle and internal urethral sphincter to promote bladder filling: this is an unconscious action.

Somatomotor system

• Onuf 's nucleus (aka nucleus of Onufrowicz) comprises the S2 – S4 motor neurons in the anterior horn of the spinal cord; these motor neurons provide volitional innervation to the pelvis.
• The somatomotor system acts on the external urethral sphincter to promote bladder filling: this is under volitional control – so you can "hold" your urine.

Voiding Circuitry

Stretch Receptors

• Stretch receptors, which are mechanoreceptors in the bladder walls excite the micturition response when there is sufficient bladder wall distention, typically at 400ml of urine.

Parasympathetic nervous system

• The parasympathetic preganglionic originates in the intermediolateral cell column of S2 – S4.
• It acts on the detrusor muscle and internal urethral sphincter to promote emptying: this is an unconscious action.

Review

• Sympathetic fibers inhibit bladder wall contraction and excite internal urethral sphincter constriction, which inhibits urination.
• Parasympathetic fibers excite bladder wall contraction and inhibit internal urethral sphincter constriction, which activates urination.
• Somatomotor efferents provide tonic activation of the external urethral sphincter, which inhibits urination.

pontine micturition center

• The pontine micturition center lies in the medial (M) region of the dorsolateral pontine tegmentum and the pontine continence center lies ventro-lateral to it in the lateral (L) region. Barrington first described the pontine micturition center, so it is often referred to as the Barrington nucleus.
• These regions receive innervation from the brain, including the periaqueductal gray area, frontal lobes, hypothalamus, limbic system, and others; their action on micturition is understood from their general roles in the nervous system.

Key Pathologic Conditions

• Urinary retenion (overdistension of the bladder) occurs from mechanical causes (enlarged prostate) or failure of bladder wall contraction (spinal shock).
• Stress incontinence involves sudden, unnanticipated micturition, which can occur from a weak external urinary sphincter or an overexcitable bladder (such as from chronic spinal cord injury).