Final answer:
The true statement is that the stimulation of stretch receptors in the urinary bladder initiates the micturition reflex. The micturition reflex involves the contraction of the detrusor muscle and relaxation of the internal sphincter, with involvement of both autonomic and somatic nervous systems, allowing urination to occur. The correct option is A).
Step-by-step explanation:
The statement "stimulation of stretch receptors in the urinary bladder initiates the micturition reflex" is true. When the bladder wall stretches due to filling with urine, the stretch receptors are activated. These receptors send nerve impulses to the sacral region of the spinal cord, triggering a spinal reflex that initiates the micturition process.
This spinal reflex leads to the contraction of the detrusor muscle and the relaxation of the involuntary internal urethral sphincter. Concurrently, the somatic motor neurons are inhibited, resulting in relaxation of the external urethral sphincter, making urination possible.
The micturition reflex can generate enough pressure to cause involuntary urination, especially when the bladder reaches certain volumes of 300 to 400 mL.
Also, it's important to note that the internal sphincter is involuntary, and it is the external urethral sphincter that is under voluntary control.
Relaxation of the internal sphincter is not voluntary but is triggered by the micturition reflex. When ready to void, the conscious signals lead to contraction of the detrusor muscle and relaxation of the internal urethral sphincter, forcefully expelling urine from the bladder.
Lastly, contrary to decreasing bladder pressure, the contraction of the detrusor muscle increases bladder pressure to facilitate the expulsion of urine.
The bladder's strength can diminish with age, but individuals can use voluntary contractions of abdominal skeletal muscles to increase intra-abdominal pressure, aiding in the emptying process when necessary, such as during forceful defecation and childbirth. Option A) is the correct one.