USMLE Step 1 Review 04 13 Anatomy Anus 3

USMLE Step 1 Review 04 13 Anatomy Anus 3


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Begin 04 13 Anatomy Anus 3 Transcription

Now, above all of these internal and external sphincters of the anal canal, we have one more muscle which is actually the most important muscle of the sphincters of the rectum.

What is it?

  • This is the puborectal muscle.

What is the overall shape of puborectalis?

  • It is a sling muscle.

What does the loop of the sling of puborectalis pass around?

  • It passes posteriorly to the perineal flexure of the rectum.

It then travels forward and up on both sides to attach where?

  • To the posterior aspect of the pubis bone on both sides.

As puborectalis normally pulls forward on the loop of the sling what does it do to the perineal flexure?

  • It collapses it and increases the angulation. As a result of this it closes off the bottom of the rectum and provides a very effective barrier to the passage of feces from the rectum into the anus.

What happens when puborectalis relaxes?

  • The perineal flexure straightens out, the rectum opens up, and there is passage of feces down against the internal and external sphincters of the anal canal.

Now puborectalis is actually part of what group of muscles?

  • The levator ani muscles, including another.

What is that?

  • The pubococcygeal muscle. All of the sphincter muscles of the perineal flexure, of the rectum, and the anal canal are normally under constant tone and constricted to hold feces in the rectum.

When the rectum becomes sufficiently full, they all relax and feces can be kept in the rectum only by voluntary action upon what two muscles?

  • The levator ani muscles including puborectalis and the external sphincter muscles. Okay, we’re going to look at the large intestines for a second.

In the colon, what is the term for segmental squeezing of the chyme, moving it back and forth in the colon, much like the segmentation contractions in the small intestine?

  • This is haustral shuttling in the colon.

And what is the term in the colon for the progressive contractile wave that is preceded by a wave of relaxation?

  • This is peristalsis.

Now peristalsis in the colon move the chyme at about what rate?

  • At about five centimeters per hour. Quite slowly.

Up to about how many hours can it take for chyme under normal conditions to move all the way through the colon?

  • Up to 48 hours.

What other type of movement of chyme happens in the colon, about three to four times daily?

  • This is a mass movement.

And what is a mass movement?

  • It is a long sustained peristaltic wave that sweeps fecal material all the way into the rectum.

During the long periods of haustral shuttling and peristalsis in the colon, what keeps the rectum free of fecal material?

  • The frequency of contractions is greater in the rectum than in the sigmoid colon, causing fecal material to constantly to be pushed out of the rectum back into the sigmoid colon.

Because of this, what often happens when a suppository is placed into the rectum?

  • It is pushed back up into the sigmoid colon.

Is the overall effect of the neural input to the colon stimulatory or inhibitory?

  • The overall effect is inhibitory.

What happens then to colonic tone when the enteric nervous system is eliminated such as in Hirschprung’s Disease?

  • There is a large increase in colonic tone.

Let’s look at the rectum for a second.

If the rectosphincteric reflex acts in a filled rectum to initiate defecation and this is blocked by conscious contraction of puborectalis and the external sphincters, what two things happen after the urge to defecate has been blocked?

  • First, the internal anal sphincter which has been relaxed closes back up and second, the rectum as a whole relaxes to accommodate the feces within it.

What will happen in terms of defecation in individuals lacking alpha-motoneuronal control over the external anal canal?

  • They will defecate whenever the rectum is filled with fecal material.



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