Final answer:
Micturition, or urination, depends on the contraction of the detrusor muscle and relaxation of the internal and external urethral sphincter muscles. The bladder's ability to expel urine involves both involuntary reflexes and voluntary control, critical for maintaining urinary continence.
Step-by-step explanation:
Yes, micturition is dependent on the contraction of the detrusor muscle and the relaxation of both the external and internal urethral sphincter muscles. The detrusor muscle, a smooth muscle in the wall of the bladder, is controlled by the autonomic and somatic nervous systems. As the bladder fills, stretch receptors in the bladder wall send signals to the sacral region of the spinal cord, generating a spinal reflex. This causes the detrusor muscle to contract and the involuntary internal urethral sphincter to relax. Concurrently, somatic motor neurons are inhibited, allowing the external urethral sphincter to relax, enabling urination.
During the micturition reflex, the internal sphincter opens involuntarily while the external sphincter, a striated muscle under voluntary control, requires conscious effort to relax. This voluntary control is developed with maturity and is necessary for urinary continence. The bladder can hold urine until approximately 300 to 400 mL is reached, at which point incontinence can occur if not voided.
Voluntary contractions of abdominal skeletal muscles can help expel urine more forcefully, especially as bladder strength can diminish with age. Nerve pathways, including the hypogastric, pelvic, and pudendal nerves, are integral in coordinating this complex process of micturition.