Final answer:
The process of urination is managed by the micturition reflex which involves both involuntary and voluntary control mechanisms. The bladder's filling triggers nerve signals that coordinate the contraction and relaxation of muscles involved in urination, with the external urethral sphincter under voluntary control.
Step-by-step explanation:
The coordinates of the process of urination are primarily managed by the micturition reflex. This complex sequence is a combination of involuntary and voluntary controls that ensure the proper expulsion of urine from the bladder. The beginning of this reflex is triggered when the bladder volume reaches around 150 mL, sending a signal to the brain that generates the initial urge to void.
The micturition reflex involves both the autonomic and somatic nervous systems. During the filling of the bladder, sympathetic nerves maintain the contraction of the external urethral sphincter via cholinergic neurons, preventing urination. Once the bladder is sufficiently full, afferent signals via sacral pelvic nerves later activate parasympathetic neurons that lead to the detrusor muscle's contraction, causing the bladder to empty.
Voluntary control over urination is exercised through the somatic nervous system, which controls the external urethral sphincter. This muscle can be consciously relaxed to allow urination to take place. It is pertinent to note that voluntary control may diminish in certain populations such as infants, older individuals, or those with specific neural impairments.