Gastrointestinal Physiology: Motility II Flashcards by Joel Glotfelty

In general, the duodenal cap is relaxed when the

Pyloric antrum contracts

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Contractions are irregular, affected by both the gastric and post-bulbar duodenal

Pacemakers

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A few gastric longitudinal muscle fibers enter the transitional zone of the duodenum, and the enteric system is

Continuous

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Caused by the entering chyme

-may initiate contraction

Lumenal distension

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The most common mechanical process seen in the small intestine

Rythmic segmentation

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Intracellular gastric and intestinal smooth muscle structure and tissue organization allows for changes in volume with little change in

Pressure

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Allow for efficient electrical communication between cells leading to waves of rings of contractions

Gap Junctions

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Contractile mechanism of gastric and intestinal smooth muscle allows for tonic contractions, a process called

Latching

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An inherent rhythmical fluctuation in the resting membrane potential of the muscle cells

Slow waves, Basal Electrical Rythm (BER), and Electrical Control Activity (ECA)

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The amplitude (5 to 15 mV), duration (1-5 sec), and frequency of ECAs vary along the

Gut

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Lie between the circular and longitudinal muscle layers and within the inner dense circular muscle layer and generate ECAs

Interstitial cells of Cajal

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ECAs are communicated to the muscle cells via

Gap Junctions

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Caused by fluctuations in ion conductance in the plasma membrane

ECA’s

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The rising phase of the ECA is due to influx of Ca2+ or

Ca2+ and Na+ voltage-gated channels

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Is the balance of Ca2+ and Na+ influx with K+ efflux via Ca2+ activated K+ channels

The ECA plateau phase

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Is closure of Ca2+ and Na+ channels and continued K+ efflux via Ca2+ activated and delayed K+ channels

Falling phase of ECA

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The frequency is regulated by pacemaker areas located along the track. What is the frequency of

) Gastric ECA
) Small intestinal ECA?

) 3/min

2. ) 8-12/min proximal to distal

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An ECA with spike potentials on the plateau. These occur when depolarization is sufficient to activate the appropriate voltage-dependent Ca2+ channels

Electrical Response Activity (ERA) or spike potential

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Elicit muscle contraction by allowing more Ca2+ to enter the smooth muscle cells.

ERAs

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Have a 1:1 relationship with contractions

ERAs

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May modulate, but not initiate ECAs

Neuroendocrine agents

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Such input may change (increase or decrease) the number of ERAs by altering

Ion channel activity

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CAN induce ERAs

-leads to longer, stronger contractions

Excitatory neurotransmitters such as ACh and gastrin

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The inhibitory neuroendocrines (e.g., norepinephrine, VIP, nitric oxide and epinephrine) can

Reduce ERAs

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In summary, ECA frequency determines the

Maximum rate of contraction

Whereas ERAs initiate

Contraction

Excitatory mediators increase the number and strength of contractions by increasing the number of

ECAs

In the stomach and intestine, is a wave of 1) contraction of the longitudinal muscle and relaxation of the circular muscle aboral to a site, and 2) relaxation of the longitudinal muscle and contraction of the circular muscle oral to, and at, the site

Peristalsis

These motor activities involve the enteric nervous system and GI peptides `setting the stage', and ECAs inducing

Contractions

Muscle contraction and relaxation are coordinated by

Interneurons

Dependent on where and how strongly muscles are contracting and relaxing

Rate of movement of luminal contents

There is an increase in the amount (but not the strength) of muscle contraction in the stomach and small intestine during the

Digestive period (compared to interdigestive period)

However, movement of chyme from the stomach through the small intestine by segmentation during the digestive period is

Slower

What is the average resident (or transit) time in the stomach?

2-4 hrs

What is the average transit time in the small intestines?

2-4 hour transit time

What is the minimum transit time in the large intestines?

Minumum 12-18hr. (average is 42-52 hrs)

Causes a reduction of enteric inhibitory neuron activity that results in a generalized increase in the tone of GI muscles, particularly sphincters -Increases transit time

Morphine

Can actually reduce diarrhea or abnormally rapid transit time

Morphine

Over distention of a segment of intestine results in a generalized inhibition of intestinal muscle activity. This is called the

Intestino-intestinal reflex

Over distention in the colon leads to a generalized inhibition of intestinal muscle activity. This is called the

Colonic-intestinal reflex

Gaseous distention, due to local bacterial fermentation results in

Increased transit time

This is consequent to peritoneo-intestinal reflex (handling of the intestine during abdominal surgery and peritoneal irrigation

Adynamic Ileus

Results in a generalized inhibition of intestinal muscle activity - accompanied by gaseous distension - increases transit time

Adynamic Ileus

Functions to delay transit of chyme into colon, thus allowing increased time for absorption

Ileocecal sphincter

The ileocecal sphincter also functions to prevent

Bacterial overgrowth in ileum

Distension of the lower ileum leads to

Relaxation of ileocecal sphincter

Distension of the upper cecum leafs to increased contraction of the

Ileocecal sphincter

Segmental, nonperistaltic contractions that slow fecal stream in the large intestine

Haustral shuttling

In the large intestine, there is occasional peristaltic activity to

Push stool forward

Functions to mix or knead the chyme to increase exposure to the epithelial surface, and to facilitate absorption of electrolytes and water

Large intestine

Cecal and colonic structural modifications account for the slow motility in the

Large intestine

Has much slower aboral movement and an increased oral movement of chyme/fecal material

Large intestine

In the large intestine, longitudinal muscles are gathered into 3 bundles called the

Taenia coli

This anatomical alteration results in a non-uniform shortening of the

Large intestine

We see less coordination in contractile activity in the large intestine due to

Reduced electrical coupling between muscle cells

The large intestine has which two sets of pacemaker cells?

1. ) ECA generating cells | 2. ) Myenteric potential oscilation cells

Located at the submucosal border of circular muscle that initiate normal changes of resting membrane potential

ECA generating cells

Located between the circular and longitudinal cells that initiate additional low irregular amplitude and high frequency changes in resting membrane potential

Myenteric potential oscilation cells

This results in changes in membrane potentials that are irregular in

Timing, amplitude. and shape

Can start at various foci and travel in both directions

Depolarization of Myenteric potential oscilation cells

Can be initiated by these depolarizations

Segmental depolarizations

Relative to the rest of the small intestine, we see an increase in the frequency of muscle contractions in the

Sigmoid colon

This frequency is further increased in the

Rectum

Initiate higher rates in the sigmoid colon and rectum

ECA pacemaker zones

Much of the time the short propulsive contractions are directed orally. This increased rate of contraction may aid in retarding movement of chyme in the distal large intestine and allow more time for

Reabsorption of salts and water

The increase in contractions in the rectum tends to keep the rectum empty of fecal matter and foreign objects, and facilitates the use of

Suppositories

There is local activity somewhere in the large intestine most of the time, but an individual area may be quiescent for long periods. Normally, we only see peristaltic activity

1 to 3 times per day

In fasting, the average net aboral movement of feces is

5 cm/hr

When eating, the aboral rate increases to

14 cm/hr

Most reflexes in the large intestine are expected to be mediated by the

ENS and PNS and gastrin and/or CCK

Increases the aboral rate to 20 cm/hr

The cholinomimetic drug carbachol

Increase the frequency of aboral propulsive activity leading to an increase in the aboral rate of movement

Gastrocolic movement and duodenocolic reflex

Increases ileal emptying causing distention of the colon and may lead to colonocolonic reflexes

Gastroileal reflex

The distention of one part of the colon leading to relaxation of other parts (sympathetic mediation)

Coloncolonic reflex

The coloncolonic reflex is under what type of mediation?

Sympathetic

Increases the length of propulsive movement by decreasing resistance to chyme movement

Coloncolonic reflex

Increases resistance to movement of chyme/feces by stimulating generalized contraction

Morphine

Decreases resistance to movement of chyme/feces by inhibiting generalized contractions -An old laxative

Castor Oil (Ricinoleic acid)

The effect of castor oil is through activating

Prostaglandin EP3 receptors

Movement of material from the cecum and transverse colon into the sigmoid colon and rectum

Mass movement (peristalsis)

Disappear during mass movement

Haustra

Mass movement is an important initiator of

Defecation

Can be induced by eating, ileal emptying, discussion of food or lectures on defecation

Mass movements

Mass movements often will not occur if the individual has prolonged periods of

Physical inactivity

Involves activation of enteric reflexes, reinforced by autonomic activity, and voluntary control

Defecation

The rectum is empty and relaxed, the internal anal sphincter is contracted to near maximum tone and the EAS is near minimum tone during

Resting conditions

Step one- Moving an adequate amount of fecal material into the rectum stimulates stretch receptors that initiate the

Rectosphincteric reflex (urge to defecate)

Enteric and parasympathetic mediated reflexes lead to relaxation of the

Internal anal sphincter (IAS)

Sympathetic mediated reflex then leads to contractions of the

External Anal Sphincter (EAS)

Step two - Movement of fecal material into the anal canal induces relaxation of the

EAS

During step two, descending and sigmoid colon and rectal muscles all

Contract

Increased intraluminal pressure in the rectum, increased intra-abdominal pressure and descent of the pelvic floor, and contraction of the distal colonic and rectal longitudinal muscles are the driving forces of

Fecal expulsion

Increased intra-abdominal pressure and descent of the pelvic floor pulls the

Anal canal walls open

Descent of pelvic floor and contraction of the distal colonic and rectal longitudinal muscles does what?

Straightens and shortens the rectum

What are 4 voluntary actions that facilitate expultion of feces?

1. ) Valsalva maneuver 2. ) Straighten posture 3. ) Abdominal contraction 4. ) Pelvic floor relaxation

What is the average number of shits per day in 1. ) US/England 2. ) Uganda

1. ) 1-3 times per day | 2. ) 16 times per day

Gastrointestinal Physiology: Motility II Flashcards

(96 cards)