Final answer:
The micturition reflex is the neurological process that governs the act of urination, involving the contraction of the bladder and the coordination of internal and external sphincters, which is under both voluntary and involuntary control. It is triggered by stretch receptors in the bladder when the bladder volume reaches a certain threshold.
Step-by-step explanation:
The micturition reflex is involved in the process of urination or voiding. It involves a complex interplay of involuntary and voluntary actions controlling the internal and external urethral sphincters. When the bladder volume reaches about 150 mL, a need to urinate is first sensed, but this initial urge can be voluntarily overridden. As the bladder continues to fill, these urges intensify and become harder to resist. The reflex mechanism is based on stretch receptors in the bladder wall transmitting signals to the spinal cord, triggering parasympathetic outflow that results in the contraction of the detrusor muscle and relaxation of the involuntary internal urethral sphincter. Alongside, the spinal cord decreases activation to somatic motor neurons, allowing the external urethral sphincter to relax so that urination can occur.
This process is under conscious control, where one can maintain continence by preventing the relaxation of the external urethral sphincter. However, the ability to suppress the micturition reflex diminishes as the bladder reaches full capacity (approximately 300 to 400 mL), leading to the act of bladder contraction and the perceived need to void, or in other words, incontinence if not voided.
With age, the bladder's ability to contract forcefully decreases, but voluntary contractions of abdominal muscles can aid in bladder emptying, a mechanism also used in other physiological activities such as forceful defecation and childbirth.