Final answer:
The voiding reflex of micturition primarily involves the contraction of the detrusor muscle, which works in conjunction with the relaxation of both the internal and external urethral sphincters to enable urination.
Step-by-step explanation:
The voiding reflex of micturition occurs as a result of contraction of the detrusor muscle. The sequence of the micturition reflex begins when stretch receptors in the bladder wall get activated as the bladder fills with urine. This sensation leads to a spinal reflex that promotes the relaxation of the involuntary internal urethral sphincter and the contraction of the detrusor muscle.
Shortly after, if the conditions allow, the somatic nervous system will inhibit the motor neurons responsible for the external urethral sphincter, enabling it to relax. This dual action — the contraction of the detrusor muscle and the relaxation of both urethral sphincters — allows urine to be expelled from the bladder.
While the detrusor muscle plays a critical role in this process, voluntary control over the external urethral sphincter is essential for timed voiding and continence. Hence, while the detrusor muscle is responsible for the forceful expulsion of urine, both sphincters need to be relaxed for micturition to occur smoothly.