What initiates the micturition reflex? a. stretch receptors in the wall of the urinary bladder signal the sacral region of the spinal cord. b. parasympathetic efferent fibers stimulate the contraction of the detrusor muscle. c. the internal urethral sphincter relaxes. d. the external urethral sphincter relaxes.

The correct answer is a. Stretch receptors in the wall of the urinary bladder signal the sacral region of the spinal cord.

Explanation:

The answers that take place automatically and quickly are called reflexes. When the smooth muscle in the bladder wall is stretched, the micturition reflex is triggered. Micturition (Urination) is the act of emptying the bladder (urinating). When the bladder is full of urine, the bladder wall stretch receptors trigger the micturition reflex. The detrusor muscle surrounding the bladder contracts. The internal urethral sphincter relaxes and allows urine to leave the bladder and pass into the urethra. Those two reactions are involuntary. The external urethral sphincter is voluntary. You must relax so that the urine passes through the urethra and leaves the body.