USMLE Step 1 Review 03 13 Anatomy Small Intestine 4

USMLE Step 1 Review 03 13 Anatomy Small Intestine 4


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Let’s continue our USMLE Review with Anatomy from the Gold Standard USMLE Step 1 Audio Review program.

Play USMLE Audio MP3 03 13 Anatomy Small Intestine 4 Below


Begin 03 13 Anatomy Small Intestine 4 Transcription

All right, let’s look at small intestine motility.

What is the nervous complex between the outer longitudinal muscle layer and the inner circular muscle layer?

  • The myenteric plexus of Auerbach.

In what way do these two layers of muscle work with each other?

  • They are antagonists.

What is the result of contraction of a section of longitudinal muscle?

  • It both shortens and dilates the relevant portion.

What is the result of contraction of circular muscle in the same area?

  • It both narrows and elongates that part of the intestine.

What two effects does the contraction of the smooth  muscles lining the small intestines have upon the chyme either mixing it or propelling it. What two motions of the small intestine cause mixing of the chyme?

  • Pendulum and segmentation movements.

What is the first step in segmentation?

  • About two centimeters of the intestinal wall contracts.

What happens then?

  • The chyme is split part of it moves back toward the stomach and part of it moves distally toward the colon.

What happens next in segmentation?

  • That portion of the intestinal wall relaxes and the chyme returns.

How long do segmentation contractions last?

  • For about 5-6 seconds.

How often do segmentation contractions occur in the duodenum?

  • About 12 times per minute.

And how often in the ileum?

  • About 8 times a minute.

During segmentation in the digestive phase, does the chyme make slow overall progress down the small intestine?

  • Yes it does.

And why?

  • Because segmentation occurs more frequently in the duodenum than in the ileum, and the overall effect is distal movement a fraction of a segmentation at a time.

How long does it usually take chyme to move from the duodenum all the way to the end of the ileum?

  • About 2-4 hrs.

In the small intestines, is peristalsis considered to be the primary mode of moving chyme all the way through the small intestines?

  • No, it’s a very minor roll.

What is peristalsis?

  • It is rolling contractions.

Sometimes along the entire length of the small intestines but usually just a short segment.

While chyme makes slow progress during the digestive period, during what period is it swept rapidly out of the small intestines?

  • During the interdigestive period.

And what is the term for this sweeping contraction? The migrating motor complex, MMC. How long do MMC’s last?

  • About 10 minutes.

And how often do they occur?

  • About every hour to hour and a half.

What is the term for the type of nervous impulses that control segmentation contractions?

  • Slow waves.

What cells within the wall of the small intestines control slow wave frequency?

  • Pacemaker cells.

Is the slow wave frequency influenced by neural activity or hormones?

  • No, it is not.

What influence do hormones have upon the slow waves?

  • They control the amplitude of the slow wave.

And because they control the amplitude of the slow wave, what else about the slow wave is also controlled?

  • The number of spikes generated by the slow wave the greater the amplitude the more spikes.

And how do the spikes relate to the strength of the segmentation contraction?

  • If there are not sufficient spikes the segmentation contraction will not occur. The more spikes there are, the stronger the segmentation contraction.

What two hormones reduce slow wave amplitude and thereby lessen segmentation contraction?

  • Secretin and glucagon.

And what four hormones increase slow wave amplitude thereby increasing strength of segmentation contractions?

  • Gastrin, Cholecystokinin (CCK), motilin and insulin.



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