Final answer:
Micturition initiation is triggered by the stretching of the bladder wall which activates the stretch receptors, signaling the spinal cord and triggering a reflex that causes the detrusor muscle to contract and urethral sphincters to relax.
Step-by-step explanation:
Micturition Trigger
The initiation of micturition, or voiding, is a complex process controlled by the nervous system. The main trigger for micturition is the stretching of the bladder wall. As the bladder fills with urine, stretch receptors within the detrusor muscle of the bladder wall become activated. These receptors send signals to the sacral region of the spinal cord, initiating a spinal reflex. This reflex results in the parasympathetic neural outflow that causes contraction of the detrusor muscle and relaxation of the internal urethral sphincter, while somatic nerves relax the external urethral sphincter, allowing urine to flow through the urethra.
Voluntary control of urination involves the brain enabling the conscious decision to start micturition when it is appropriate. This control is mainly exerted by the external urethral sphincter, which is made up of skeletal muscle and innervated by somatic nerves. The interplay between involuntary (autonomic) and voluntary (somatic) nervous control is crucial for maintaining continence and allows an individual to decide the timing of bladder emptying.