Location and Structure of the Detrusor Muscle
The detrusor muscle (m. detrusor) is the main muscular component of the urinary bladder wall. It is a smooth muscle that works in coordination with the internal urethral sphincter (composed of smooth muscle) and the external urethral sphincter (composed of skeletal muscle) during the micturition reflex.
Functions of the Detrusor in the Micturition Cycle
The function of the detrusor correlates with the two main phases of the urinary bladder cycle:
- Filling phase: The detrusor remains relaxed while outlet resistance at the bladder neck increases.
- Emptying phase: Once a threshold volume is reached, the micturition reflex is triggered, leading to reflex contraction of the detrusor. Effective voiding requires simultaneous relaxation of both the internal and external urethral sphincters.
Innervation of the Detrusor Muscle
Muscle activity is regulated by the autonomic nervous system:
- Parasympathetic innervation: Originates from the sacral spinal cord segments (S2–S4); preganglionic fibers travel via the pelvic splanchnic nerves. The neurotransmitter is acetylcholine, which acts on muscarinic receptors on the detrusor muscle to cause contraction, resulting in bladder emptying.
- Sympathetic innervation: Sympathetic neurons originate in the thoracolumbar spinal cord segments (Th10–L2, specifically including lower thoracic and upper lumbar segments Th12, L1, and L2). Activation of the sympathetic nervous system causes relaxation of the detrusor and closure of the internal sphincter, promoting urine storage. Sympathetic nerves release norepinephrine, which acts on β-adrenergic receptors to relax the detrusor.
Pathologies and Clinical Disorders Involving the Detrusor
Neurological lesions at various levels lead to lower urinary tract dysfunction:
- Detrusor instability: Premature involuntary contractions of the detrusor during the filling phase, reflecting impaired inhibitory control over detrusor activity.
- Detrusor hyperreflexia: Occurs when a lesion is located above the sacral spinal cord segments, disrupting descending inhibitory pathways to the detrusor. Typical symptoms of isolated hyperreflexia include urgency, urge incontinence, and low post-void residual volume.
- Detrusor sphincter dyssynergia: Involuntary contraction of the detrusor muscle occurring simultaneously with failure of the external urethral sphincter to relax.
- Detrusor areflexia: A condition resulting from disruption of the afferent or efferent innervation of the detrusor. Conus medullaris syndrome, for example, features detrusor areflexia combined with overflow incontinence (paradoxical ischuria).