Chapter 94 Rectum, Anus, and Perineum

Question 1

Which plexus innervates the rectum, internal anal sphincter, and rectococcygeus muscle

Answer 1

The pelvic plexus

Question 2

What is the pelvic plexus composed of

Answer 2

PSNS fibers in the pelvic nerves, S1, S2, S3 branches and sympathetic fibers in hypogastric nerve

Question 3

Describe PSNS fibers in the pelvic plexus in relation to the rectum and internal anal sphincter

Answer 3

PSNS fibers in the pelvic plexus are excitatory to the rectum and inhibitory to the internal anal sphincter**

Question 4

Describe SNS fibers in the pelvic plexus in relation to the rectum and internal anal sphincter

Answer 4

Sympathetic fibers in hypogastric nerve are inhibitory to the rectum and excitatory to the internal anal sphincter**

Question 5

Describe intrinsic systems role in defecation innervation

Answer 5

First step in defecation-Involuntary- Rectum distends then enteric nervous plexus (within rectal wall) inhibits rectal contraction and internal anal sphincter relaxation. Weaker than extrinsic system.

Question 6

Describe extrinsic systems role in innervation during defecation

Answer 6

After stimulating intrinsic system, distention of rectum eventually stimulates extrinsic nervous mechanism also via parasympathetic fibers in the pelvic nerves, that initiates the defecation reflex. Rectum contracts and tone in the internal anal sphincter is inhibited. Step 2

Question 7

Describe the voluntary control of defecation

Answer 7

During distention (after intrinsic and extrinsic systems have been stimulated)– anal mucosal receptors signal CNS via pudendal nerve that poo is coming.
Voluntary control takes over to facilitate defecation by relaxation of external anal sphincter.