Detrusor

Where is the detrusor located and how is it structured?

The detrusor (m. detrusor) is the main muscular structure of the urinary bladder. It is a smooth muscle that participates in the micturition mechanism together with the internal sphincter, represented by smooth muscle, and the external sphincter, represented by striated muscle.

What are the functions of the detrusor in the micturition cycle?

The functioning of the detrusor is associated with the phases of bladder activity:

• Storage phase: the detrusor remains relaxed; meanwhile, resistance in the area of the internal urethral orifice increases.
• Voiding phase: upon reaching a threshold urine volume, the micturition reflex is triggered, leading to reflex contraction of the detrusor. Efficient emptying requires relaxation of both the internal and external urethral sphincters.

How is the detrusor innervated?

Regulation of muscular activity is provided by the autonomic nervous system:

• Parasympathetic innervation: the source is the sacral segments of the spinal cord (S2–S4); impulses travel via preganglionic fibers as part of the pelvic nerves. The neurotransmitter is acetylcholine; it acts on muscarinic receptors of the detrusor and causes its contraction, leading to bladder emptying.
• Sympathetic innervation: sympathetic neurons are located in the thoracolumbar segments of the spinal cord (Th10–L2); the lower thoracic and upper lumbar segments Th12, L1, and L2 are also indicated. Activation of the sympathetic nervous system causes relaxation of the detrusor and closure of the internal sphincter, facilitating urine storage. Sympathetic nerves release norepinephrine, and binding to β-adrenoceptors leads to detrusor relaxation.

What disorders and pathological conditions are associated with the detrusor?

Damage to the nervous system at various levels leads to urinary bladder dysfunctions:

• Detrusor instability: premature contraction of the detrusor during the bladder storage phase; the term indicates a disruption of the normal inhibitory control over detrusor contraction.
• Detrusor hyperreflexia: the lesion is located above the sacral segments of the spinal cord; fibers exerting an inhibitory effect on the detrusor are damaged. The main symptoms of isolated hyperreflexia are urgency, urge urinary incontinence, and a small post-void residual volume.
• Detrusor sphincter dyssynergia: involuntary contraction of the detrusor without relaxation of the external urethral sphincter.
• Detrusor areflexia: a condition resulting from impaired afferent or efferent innervation of the detrusor muscle; conus medullaris syndrome is characterized by detrusor areflexia combined with paradoxical ischuria.

Class and exam questions

What else to study on this topic

• Pontine micturition center
• Onuf's nucleus
• Pelvic nerves
• Overactive bladder syndrome
• Urodynamic study

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.