Which of the following best describes the two positive-feedback loops involved in the defecation reflex?

• A. Release of bile from the gallbladder and increased gastric motility.
• B. Inhibition of the internal anal sphincter and relaxation of the external anal sphincter.
• C. Stretching of the rectal wall increases peristalsis, and voluntary contraction of abdominal muscles increases rectal pressure.
• D. Activation of the sympathetic nervous system and decreased intestinal motility.

Answer: (C)

Verified step by step guidance

1. Understand the concept of positive-feedback loops: Positive-feedback loops amplify a physiological response rather than stabilizing it. In the context of the defecation reflex, these loops enhance the process of expelling feces from the body.
2. Identify the key components of the defecation reflex: The defecation reflex involves the rectum, anal sphincters, and abdominal muscles. Stretching of the rectal wall triggers peristalsis, which is a series of wave-like muscle contractions that move feces toward the anus.
3. Analyze the first positive-feedback loop: Stretching of the rectal wall by feces stimulates sensory neurons, which send signals to the spinal cord. This leads to increased peristalsis in the rectum, further pushing feces toward the anus and amplifying the reflex.
4. Examine the second positive-feedback loop: Voluntary contraction of abdominal muscles increases intra-abdominal pressure, which compresses the rectum and enhances rectal pressure. This action further facilitates the expulsion of feces.
5. Connect the loops to the correct answer: The two positive-feedback loops involved in the defecation reflex are stretching of the rectal wall increasing peristalsis and voluntary contraction of abdominal muscles increasing rectal pressure. These processes work together to ensure effective defecation.