Chapter 62: General Principles of Gastrointestinal Function— Motility, Nervous Control, and Blood Circulation

Question 1

The alimentary tract provides
the body with a continual
supply of water,
electrolytes, vitamins, and
nutrients. To achieve this
requires the followi:ng

Answer 1

• (1) movement of food through the alimentary tract;
• (2) secretion of digestive juices and digestion of the food;
• (3) absorption of water, various electrolytes, vitamins, and digestive products;
• (4) circulation of blood through the gastrointestinal organs to carry away the absorbed substances; and
• (5) control of all these functions by local, nervous, and hormonal systems.

Question 2

Figure 62-1 shows the entire alimentary tract

Each part is adapted to its specific functions:

Answer 2

• some to simple passage of food, such as the esophagus;
• others to temporary storage of food, such as the stomach; and
• others to digestion and absorption, such as the small intestine.

Question 3

Physiologic Anatomy of the Gastrointestinal
Wall

the following layers from outer surface
inward:

Answer 3

• (1) the serosa,
• (2) a longitudinal smooth muscle layer,
• (3) a circular smooth muscle layer,
• (4) the submucosa, and
• (5) the mucosa.

In addition, sparse bundles

of smooth muscle fibers, the mucosal muscle, lie in

the deeper layers of the mucosa.

The motor functions of

the gut are performed by the different layers of smooth

muscle

Question 4

The specific characteristics of smooth muscle
in
the gut are the following

Answer 4

• Gastrointestinal Smooth Muscle Functions as a Syncytium
• Electrical Activity of Gastrointestinal
  Smooth Muscle

Question 5

What is the arrangement of the smooth muscle fibers in the GIT?

Answer 5

The individual smooth muscle fibers in
the gastrointestinal tract are 200 to 500 micrometers in
length and 2 to 10 micrometers in diameter, and they are
arranged in bundles of as many as 1000 parallel fibers.

Question 6

What is the difference between the longitudinal muscle from the circular muscle in relation to its extended location?

Answer 6

In
the longitudinal muscle layer, the bundles extend longitudinally
down the intestinal tract; in the circular muscle
layer, they extend around the gut

Question 8

Within each bundle, how are the muscle fibers electrically connected?

Answer 8

gap junctions that allow low-resistance movement
of ions from one muscle cell to the next.

Therefore,
electrical
signals that initiate muscle contractions can
travel readily from one fiber to the next within each bundle but more rapidly along the length of the bundle than
sideways

Question 9

What is syncytium?

Answer 9

Each bundle of smooth muscle fibers is partly separated
from the next by loose connective tissue, but the
muscle bundles fuse with one another at many points,
so in reality each muscle layer represents a branching
latticework of smooth muscle bundles.

Therefore, each
muscle layer functions as a syncytium; that is, when an
action potential is elicited anywhere within the muscle
mass, it generally travels in all directions in the muscle

Question 10

The action potential that travels in the GIT depends on what?

Answer 10

The distance that it travels depends on the excitability
of the muscle; sometimes it stops after only a few millimeters and at other times it travels many centimeters
or even the entire length and breadth of the intestinal
tract.

Also, a few connections exist between the longitudinal
and circular muscle layers, so excitation of one of these
layers often excites the other as well.

Question 11

The smooth muscle of the gastrointestinal tract is excited by how?

Answer 11

almost continual slow, intrinsic electrical activity along the membranes of the muscle fibers.

Question 12

Smooth muscle of the gastrointestinal tract is excited
by almost continual slow, intrinsic electrical activity along
the membranes of the muscle fibers, this has
two basic types of electrical waves

Answer 12

(1) slow waves and
(2) spikes, both of which are shown in Figure 62-3.

In addition,
the voltage of the resting membrane potential of the
gastrointestinal smooth muscle can be made to change to different levels, and this, too, can have important effects in controlling motor activity of the gastrointestinal tract

Question 13

What is a slow wave?

Answer 13

Most gastrointestinal contractions occur
rhythmically, and this rhythm is determined mainly by
the frequency of so-called “slow waves” of smooth muscle
membrane potential.

These waves, shown in Figure 62-3,
are not action potentials.

Instead, they are slow, undulating
changes in the resting membrane potential.

Their
intensity usually varies between 5 and 15 millivolts, and
their frequency ranges in different parts of the human
gastrointestinal
tract from 3 to 12 per minute: about 3

Question 14

What is the intensity of the slow waves in the GIT?

Answer 14

Their
intensity usually varies between 5 and 15 millivolts, and their frequency ranges in different parts of the human gastrointestinal tract from 3 to 12 per minute: about 3 in the body of the stomach, as much as 12 in the duodenum, and about 8 or 9 in the terminal ileum.

Therefore,
the rhythm of contraction of the body of the stomach is
usually about 3 per minute, of the duodenum about 12 per minute, and of the ileum 8 to 9 per minute

Question 15

What is the rhytm of contraction of the body of the stomach?

Answer 15

about 3 per minute

Question 16

What is the rhytm of contraction of the body of the duodenum?

Answer 16

about 12 per minute

Question 17

What is the rhytm of contraction of the body of the ileum?

Answer 17

8 to 9 per minute

Question 18

What is the reason for the slow waves of the GIT?

Answer 18

The precise cause of the slow waves is not completely
understood, although they appear to be caused by complexinteractions among the smooth muscle cells and specializedcells, called the interstitial cells of Cajal, that are believed to act as electrical pacemakers for smooth muscle cells.

Question 19

What are interstitial cells of Cajal?

Answer 19

interstitial cells of Cajal, are specialized cells that are believed to act as electrical pacemakers for smooth muscle cells.

These interstitial cells form a network with each
other and are interposed between the smooth muscle layers,
with synaptic-like contacts to smooth muscle cells.
The interstitial cells of Cajal undergo cyclic changes in
membrane potential due to unique ion channels that periodically
open and produce inward (pacemaker) currents
that may generate slow wave activity.

Question 20

The slow waves usually do not by themselves cause
muscle contraction in most parts of the gastrointestinal
tract, except perhaps where?

Answer 20

stomach

Instead, they mainly
excite the appearance of intermittent spike potentials,
and the spike potentials in turn actually excite the muscle contraction.

Question 21

What are spike potentials?

Answer 21

The spike potentials are true action
potentials.

They occur automatically when the resting
membrane potential of the gastrointestinal smooth
muscle
becomes more positive than about −40 millivolts
(the normal resting membrane potential in the smooth
muscle fibers of the gut is between −50 and −60 millivolts).

Note in Figure 62-3 that each time the peaks of
the slow waves temporarily become more positive than
−40 millivolts, spike potentials appear on these peaks.
The higher the slow wave potential rises, the greater the
frequency of the spike potentials, usually ranging between
1 and 10 spikes per second. The spike potentials last 10 to
40 times as long in gastrointestinal muscle as the action
potentials in large nerve fibers, each gastrointestinal spike
lasting as long as 10 to 20 milliseconds.

Question 22

What is another difference between the action
potentials of the gastrointestinal smooth muscle and those of nerve fibers?

Answer 22

is the manner in which they are generated

In nerve fibers, the action potentials are caused
almost entirely by rapid entry of sodium ions through
sodium channels to the interior of the fibers.

In gastrointestinal
smooth muscle fibers, the channels responsible
for the action potentialsare somewhat different;they
allow especially large numbers of calcium ions to enter
along with smaller numbers of sodium ions and thereforeare calledcalcium-sodium channels.

These channels are
much slower to open and close than are the rapid sodium channels of large nerve fibers.

The slowness of opening
and closing of the calcium-sodium channels accounts for the long duration of the action potentials.

Also, the movement
of large amounts of calcium ions to the interior of
the muscle fiber during the action potential plays a special role in causing the intestinal muscle fibers to contract, as
we discuss shortly.

Question 23

What accounts for the long duration of the action potentials in GIT?

Answer 23

The slowness of opening
and closing of the calcium-sodium channels

Also, the movement
of large amounts of calcium ions to the interior of
the muscle fiber during the action potential plays a special role in causing the intestinal muscle fibers to contract, as
we discuss shortly.

Question 24

Explain the Changes in Voltage of the Resting Membrane
Potential of the GIT

Answer 24

In addition to the slow waves and spike potentials,
the baseline voltage level of the smooth muscle resting membrane potential can also change.

Under normal
conditions, the resting membrane potential averages
about −56 millivolts, but multiple factors can change thislevel.

When the potential becomes less negative, which is
called depolarization of the membrane, the muscle fibers become more excitable.

When the potential becomes
more negative, which is called hyperpolarization, the
fibers become less excitable.

Question 25

What are the factors that depolarize the membrane of GIT—that is, make it more excitable—?

Answer 25

* (1) **stretching of the muscle,** * (2) **stimulation by acetylcholine** released from the endings of parasympathetic nerves, and * (3) **stimulation by several specific gastrointestinal hormones**

Question 26

What are the important factors that make the membrane potentialmore negative in the GIT—that is, hyperpolarize the membrane and make the muscle fibers less excitable?

Answer 26

* 1) the effect of **norepinephrine or epinephrine** on the fiber membrane and * (2) **stimulation of the sympathetic nerves** that secrete mainly norepinephrine at their endings.

Question 27

What cause the smooth muscle contraction of the GIT ?

Answer 27

occurs in response to **entry of calcium** ions into the muscle fiber. As explained in Chapter 8, calcium ions, acting through a **calmodulin control mechanism,** **activate the myosin filaments in the fiber, causing attractive forces to develop between the myosin filaments and the actin filaments, thereby causing the muscle to contract**

Question 28

The slow waves of the GIT cause to enter what particular ion?

Answer 28

The slow waves **do not cause calcium ions to enter the smooth muscle fibe**r***(only sodium ions)***. Therefore, the slow waves by themselves usually cause no muscle contraction. **Instead, it is during the spike potentials,** generated at the **peaks of the slow waves**, tha**t significant quantities of calcium ions do enter the fibers and cause most of the contraction.**

Question 29

Some smooth muscle of the gastrointestinal tract exhibits tonic contraction, as well as, or instead of, rhythmical contractions. What is tonic contraction?

Answer 29

Tonic contraction is *c****ontinuous continuous*,** **not associated with the basic electrical rhythm** of the slow waves but **often lasting several minutes** or **even hours.** The tonic contraction o**ften increases or decreases in intensity but continues**

Question 30

Tonic contraction is sometimes caused by what?

Answer 30

* **continuous repetitive spike potentials**—the greater the frequency, the greater the degree of contraction. * At other times, tonic contraction is **caused by hormones or other factors that bring about continuous partial depolarization** of the smooth muscle membrane without causing action potentials. * A third cause of tonic contraction is **continuous entry of calcium ions into the interior of the** **cell brought about in ways not associated with changes****in membrane potential.** * The details of these mechanisms are still unclear.

Question 31

What is the Neural Control of Gastrointestinal Function?

Answer 31

The gastrointestinal tract has a nervous system all its own called the ***enteric nervous system***

Question 32

Where does the enteric nervous system of the GIT lies entirely?

Answer 32

It lies entirely in the ***wall of the gut, beginning in the esophagus and extending all the way to the anus***. The number of neurons in this enteric system is about **100 million,** almost **exactly equal to the number in the entire spinal cord**.

Question 33

What is the importance of the enteric nervous system?

Answer 33

This highly developed enteric nervous system is especially important in **controlling gastrointestinal movements and secretion.**

Question 34

The enteric nervous system is composed mainly of two plexuses, shown in Figure 62-4:

Answer 34

* (1) an **outer plexus** lying between the longitudinal and circular muscle layers, called the **myenteric plexus or Auerbach’s plexus,** and ( **O**h**MyAu)** * (2) an **inner plexus, called the submucosal plexus or** **Meissner’s plexus**, that lies in the **submucosa.** ( InSuMe) The nervous connections within and between these two plexuses are also shown in Figure 62-4.

Question 35

What does the myenteric plexus controls mainly?

Answer 35

gastrointestinal movements

Question 36

What does the submucosal plexusc ontrols mainly?

Answer 36

controls mainly **gastrointestinal secretion** and local blood flow. **S** for **S**secretion

Question 37

Although the enteric nervous system can function independently of these extrinsic nerves, what stimulation can greatly enhance or inhibit the gastrointestinal functions?

Answer 37

stimulation by the **parasympathetic** and **sympathetic systems** can greatly enhance or inhibit gastrointestinal functions, as we discuss later.

Question 38

Also shown in Figure 62-4 are sensory nerve endings that originate in the gastrointestinal epithelium or gut wall and send afferent fibers to both plexuses of the enteric system, as well as

Answer 38

* (1) to the **prevertebral ganglia** of the sympathetic nervous system, * (2) to the **spinal cord,** and * (3) in the **vagus nerves** all the way to the brain stem. These sensory nerves can elicit local reflexes within the gut wall itself and still other reflexes that are relayed to the gut from either the prevertebral ganglia or the basal regions of the brain

Question 39

What are the Differences Between the Myenteric and Submucosal Plexuses?

Answer 39

The submucosal plexus, in contrast to the myenteric plexus, is **mainly concerned with controlling function** **within the inner wall of each minute segment** of the intestine. For instance, **many sensory signals** originate from the **gastrointestinal epithelium** and are then **integrated in the submucosal plexus** to help control **local intestinal secretion**, **local absorption, and local contraction of the submucosal muscle that causes** various degrees of infolding f the gastrointestinal mucosa

Question 40

Describe the neurons of the myenteric plexus.

Answer 40

The myenteric plexus consists mostly of a linear chain of many interconnecting neurons that extends the entire length of the gastrointestinal tract

Question 41

Because the myenteric plexus extends all the way along the intestinal wall and because it lies between the longitudinal and circular layers of intestinal smooth muscle, it is **concerned mainly with controlling muscle activity along the length of the gut.** When this plexus is stimulated, its principal effects are what?

Answer 41

* (1**) increased tonic contraction, or “tone,” of the gut wall;** * (2) **increased intensity of the rhythmical contraction**s; * (3) **slightly increased rate of the rhythm of contraction;** and * (4) **increased velocity of conduction of excitatory** waves along the gut wall, causing more rapid movement of the gut peristaltic waves.

Question 42

Why does myenteric plexus should not be considered entirely excitatory?

Answer 42

**because some of its neurons are inhibitory**; their fiber endings secrete an inhibitory transmitter, possibly ***vasoactive intestinal polypeptide*** or some other inhibitory peptide.

Question 43

When are the inhibitory signals of myenteric plexus useful?

Answer 43

useful for **inhibiting some of the intestinal sphincter** muscles that **impede movement of food along successive segments of the gastrointestinal tract**, such as the ***pyloric sphincter,*** which _controls emptying of the stomach into the duodenum_, and the ***sphincter of the ileocecal valve,*** which _controls emptying from the small intestine into the cecum._

Question 44

What are the Types of Neurotransmitters Secreted by Enteric Neurons?

Answer 44

* (1) acetylcholine and * (2) norepinephrine. Others are * (3) adenosine triphosphate, * (4) serotonin, * (5) dopamine, * (6) cholecystokinin * , (7) substance P, * (8) vasoactive intestinal polypeptide, * (9) somatostatin, * (10) leu-enkephalin, * (11) met-enkephalin, and * (12) bombesin.

Question 45

What most often excites gastrointestinal activity?

Answer 45

**Acetylcholine**

Question 46

What almost always inhibits gastrointestinal activity?

Answer 46

**Norepinephrine** This is also true of epinephrine, which reaches the gastrointestinal tract mainly by way of the blood after it is secreted by the adrenal medullae into the circulation. The other aforementioned transmitter substances are a mixture of excitatory and inhibitory agents, some of which we discuss in the following chapter

Question 47

What does the Parasympathetic Stimulation ?

Answer 47

Increases Activity of the Enteric Nervous System.

Question 48

The parasympathetic supply to the gut is divided into, which were discussed in Chapter 60.

Answer 48

* cranial and * sacral divisions

Question 49

Except for a few parasympathetic fibers to the __________ of the alimentary tract, the **cranial parasympathetic nerve fibers are almost entirely in the *vagus nerves.***

Answer 49

mouth and pharyngeal regions These fibers provide extensive innervation to the **esophagus, stomach, and pancreas** and somewhat **less to the intestines** down through the **first half of the large intestine**

Question 50

The sacral parasympathetics originate from where?

Answer 50

in the **second, third, and fourth sacral segments** of the spinal cord and pass through the pelvic nerves to the distal half of the large intestine and all the way to the anus.

Question 51

What are considerably better supplied with parasympathetic fibers than are the other intestinal areas.

Answer 51

**sigmoidal, rectal, and anal regions** These fibers function especially to execute the defecation reflexes, discussed in Chapter 63.

Question 52

Where does the postganglionic neurons of the gastrointestinal parasympathetic system are located mainly?

Answer 52

**myenteric and submucosal plexuses**. NOTE : Stimulation of these parasympathetic nerves causes general increase in activity of the entire enteric nervous system. This in turn enhances activity of most gastrointestinal functions.

Question 53

What is the usual action of Sympathetic Stimulation in GIT?

Answer 53

***Inhibits*** Gastrointestinal Tract Activity

Question 54

The sympathetic fibers to the gastrointestinal tract originate where?

Answer 54

in the spinal cord between segments **T5 and L2.**

Question 55

Where does most of the preganglionic fibers that innervate the gut enter?

Answer 55

Most of the **preganglionic fibers** that innervate the gut, after leaving the cord, **enter the sympathetic chains** that lie lateral to the spinal column, and many of these fibers then pass on through the chains to outlying ganglia such as to the celiac ganglion and various mesenteric ganglia. Most of the postganglionic sympathetic neuron bodies are in these ganglia, and postganglionic fibers then spread through postganglionic sympathetic nerves to all parts of the gut. The **sympathetics innervate essentially all of the gastrointestinal tract,**rather than being more extensive nearest the oral cavity and anus, as is true of the parasympathetics.

Question 56

The sympathetic nerve endings secrete mainly what?

Answer 56

norepinephrine but also small amounts of epinephrine.

Question 57

In general, stimulation of the sympathetic nervous system inhibits activity of the gastrointestinal tract, causing many effects opposite to those of the parasympathetic system. It exerts its effects in two ways:

Answer 57

* (1) to a slight extent by direct effect of secreted norepinephrine to inhibit intestinal tract smooth muscle (except the mucosal muscle, which it excites) and * (2) to a major extent by an inhibitory effect of norepinephrine on the neurons of the entire enteric nervous system.

Question 58

Strong stimulation of the sympathetic system can result to what?

Answer 58

* *inhibit motor movements of the gut** so greatly that this * *can literally block movement of food** through the gastrointestinal tract.

Question 59

Many afferent sensory nerve fibers innervate the gut. Some of them have their cell bodies in the enteric nervous system itself and some in the dorsal root ganglia of the spinal cord. These sensory nerves can be stimulated by

Answer 59

* (1) irritation of the gut mucosa, * (2) excessive distention of the gut, or * (3) presence of specific chemical substances in the gut. Signals transmitted through the fibers can then cause excitation or, under other conditions, inhibition of intestinal movements or intestinal secretion. In addition, other sensory signals from the gut go all the way to multiple areas of the spinal cord and even the brain stem. For example, 80 percent of the nerve fibers in the vagus nerves are afferent rather than efferent. These afferent fibers transmit sensory signals from the gastrointestinal tract into the brain medulla, which in turn initiates vagal reflex signals that return to the gastrointestinal tract to control many of its functions

Question 60

The anatomical arrangement of the enteric nervous system and its connections with the sympathetic and parasympathetic systems support three types of gastrointestinal reflexes that are essential to gastrointestinal control. They are the following:

Answer 60

1. Reflexes that are integrated entirely within the gut wall enteric nervous system 2. Reflexes from the gut to the prevertebral sympathetic ganglia and then back to the gastrointestinal tract. 3. Reflexes from the gut to the spinal cord or brain stem and then back to the gastrointestinal tract.

Question 61

What are the Reflexes that are **integrated entirely within the gut wall enteric nervous system?** These include:

Answer 61

* reflexes that control much gastrointestinal secretion, peristalsis, * mixing contractions, * local inhibitory effects, and so forth.

Question 62

What are the Reflexes from the gut to the prevertebral sympathetic ganglia and then back to the gastrointestinal tract?

Answer 62

These reflexes **transmit signals long distances** to other areas of the gastrointestinal tract, such as: * signals from the stomach to cause evacuation of the colon **(the gastrocolic reflex),** * signals from the colon and small intestine to inhibit stomach motility and stomach secretion **(the enterogastric reflexes)**, and * reflexes from the colon to inhibit emptying of ileal contents into the colon **(the colonoileal reflex).**

Question 63

What are the Reflexes from the gut to the spinal cord or brain stem and then back to the gastrointestinal tract.? These include especially :

Answer 63

* (1) reflexes from the stomach and duodenum to the brain stem and back to the stomach—by way of the vagus nerves—to control gastric motor and ecretory activity; * (2) pain reflexes that cause general inhibition of the entire gastrointestinal tract; and * (3) defecation reflexes that travel from the colon and rectum to the spinal cord and back again to produce the powerful colonic, rectal, and abdominal contractions required for defecation (the defecation reflexes).

Question 64

The gastrointestinal hormones are released into where?

Answer 64

**portal circulation and exert physiological actions** on **target cells with specific receptors** for the hormone.

Question 65

The effects of the hormones persist **even after all nervous connections between the site of release and the site of action have been severed.** T or F

Answer 65

True The effects of the hormones persist **even after all nervous connections between the site of release and the site of action have been severed.**

Question 66

Table 62-1 Gastrointestinal Hormone Actions, Stimuli for Secretion, and Site of Secretion Gastrin

Answer 66

* Stimuli for Secretion: Protein Distention Nerve ( Acid inhibits release) Mneomonics: Baby Back RIBS!

Question 67

Table 62-1 Gastrointestinal Hormone Actions, Stimuli for Secretion, and Site of Secretion Gastrin

Answer 67

Site of Secretion **G cells** of the **antrum, duodenum, and jejunum** **G for G**

Question 68

Table 62-1 Gastrointestinal Hormone Actions, Stimuli for Secretion, and Site of Secretion Gastrin

Answer 68

Actions Stimulates 1. Gastric acid secretion 2. Mucosal growth

Question 69

Table 62-1 Gastrointestinal Hormone Actions, Stimuli for Secretion, and Site of Secretion **Cholecystokinin**

Answer 69

Stimuli for Secretion Protein Fat Acid

Question 70

Table 62-1 Gastrointestinal Hormone Actions, Stimuli for Secretion, and Site of Secretion **Cholecystokinin**

Answer 70

Site of Secretion **I cells of the duodenum, jejunum, and ileum** **C- I**

Question 71

Table 62-1 Gastrointestinal Hormone Actions, Stimuli for Secretion, and Site of Secretion **Cholecystokinin**

Answer 71

Actions Stimulates * Pancreatic enzyme secretion * Pancreatic bicarbonate secretion * Gallbladder contraction * Growth of exocrine pancreas Inhibits * Gastric emptying

Question 72

Table 62-1 Gastrointestinal Hormone Actions, Stimuli for Secretion, and Site of Secretion **Secretin**

Answer 72

Stimuli for Secretion * Acid * Fat

Question 73

Table 62-1 Gastrointestinal Hormone Actions, Stimuli for Secretion, and Site of Secretion **Secretin**

Answer 73

Site of Secretion S cells of the duodenum, jejunum, and ileum

Question 74

Table 62-1 Gastrointestinal Hormone Actions, Stimuli for Secretion, and Site of Secretion Secretin

Answer 74

**Stimulates** * Pepsin secretion * Pancreatic bicarbonate secretion * Biliary bicarbonate secretion * Growth of exocrine pancreas **Inhibits** * Gastric acid secretion

Question 75

Table 62-1 Gastrointestinal Hormone Actions, Stimuli for Secretion, and Site of Secretion **Gastric inhibitory peptide**

Answer 75

Stimuli for Secretion **Protein** **Fat** **Carbohydrates**

Question 76

Table 62-1 Gastrointestinal Hormone Actions, Stimuli for Secretion, and Site of Secretion **Gastric inhibitory peptide**

Answer 76

Site of Secretion **K cells** of the **duodenum and jejunum**

Question 77

Table 62-1 Gastrointestinal Hormone Actions, Stimuli for Secretion, and Site of Secretion **Gastric inhibitory peptide**

Answer 77

Stimulates * Insulin release Inhibits * Gastric acid secretion

Question 78

Table 62-1 Gastrointestinal Hormone Actions, Stimuli for Secretion, and Site of Secretion Motilin

Answer 78

Stimuli for Secretion **Fat** **Acid** **Nerve**

Question 79

Table 62-1 Gastrointestinal Hormone Actions, Stimuli for Secretion, and Site of Secretion **Motilin**

Answer 79

Site of Secretion **M cell**s of the duodenum and jejunum **MM**

Question 80

Table 62-1 Gastrointestinal Hormone Actions, Stimuli for Secretion, and Site of Secretion Motilin

Answer 80

Stimulates Gastric motility Intestinal motility

Question 81

Gastrin is secreted by the _____ in response to stimuli associated with i**ngestion of a meal**, such as **distention of the stomach**, the **products of proteins, and *gastrin releasing peptide****,* which is released by the**nerves of the gastric mucosa during vagal stimulation.**

Answer 81

"G” cells of the antrum of the stomach

Question 82

The primary actions of gastrin are

Answer 82

* (1) stimulation of gastric acid secretion and * (2) stimulation of growth of the gastric mucosa.

Question 83

Cholecystokinin (CCK) is secreted by what?

Answer 83

“I” cells in the mucosa of the duodenum and jejunum

Question 84

Cholecystokinin (CCK) is secreted mainly in response to what?

Answer 84

* digestive products of fat, * fatty acids, and * monoglycerides in the intestinal contents.

Question 85

Whatis the action of Cholecystokinin (CCK)?

Answer 85

This hormone **strongly contracts the gallbladde**r, expelling bile into the small intestine, where the **bile in turn plays important roles in emulsifying fatty substances**, and **allowing them to be digested and absorbed.** **CCK also inhibits stomach contraction moderately**. Therefore, at the same time that **this hormone causes emptying of the gallbladder**, it also **slows the emptying of food from the stomach to give adequate time for digestion of the fats** in the upper intestinal tract . CCK also **inhibits appetite to prevent overeating during meals by stimulating sensory afferent nerve** fibers in the duodenum; these fibers, in turn, send signals by way of the vagus nerve to inhibit feeding centers in the brain as discussed in Chapter 71.

Question 86

What is the first gastrointestinal hormone discovered is secreted by the “S” cells in the mucosa of the duodenum?

Answer 86

Secretin

Question 87

Secretin is released in response to what?

Answer 87

in **response to acidic gastric juice emptying** **into the duodenum from the pylorus of the stomach.** Secretin has a **mild effect on motility of the gastrointestinal** tract and **acts to promote pancreatic secretion of bicarbonate**, which in turn helps to neutralize the acid in the small intestine

Question 88

Gastric inhibitory peptide (GIP) is secreted by what and is mainly in response to what?

Answer 88

***mucosa of the upper small intestine***, mainly in response to **fatty acids and amino acids** but to a **lesser extent in response to carbohydrate.** It has a **mild effect in decreasing motor activity of the stomach**and therefore**slows emptying of gastric contents into the duodenum**when the**upper small intestine is already overloaded with food**products. GIP, at blood levels even lower than those needed to inhibit gastric motility,***also stimulates insulin secretion and for this reason is also known as glucosedependent insulinotropic peptide.***

Question 89

**Motilin** is secreted by the **stomach and upper duodenum** during when?

Answer 89

**fasting,** and the **only known function** of this hormone is to increase g**astrointestinal motility.**

Question 90

Motilin is released cyclically and stimulates waves of gastrointestinal motility called ______________ that move through the stomach and small intestine every 90 minutes in a fasted person. Motilin secretion is inhibited after ingestion by mechanisms that are not fully understood.

Answer 90

**interdigestive myoelectric complexes**

Question 91

Functional Types of Movements in the Gastrointestinal Tract Two types of movements occur in the gastrointestinal tract:

Answer 91

* 1) propulsive movements * (2) mixing movements

Question 92

What is the function of propulsive movements?

Answer 92

which **cause food to move forward along the tract at an appropriate rat**e to **accommodate digestion and absorption**

Question 93

What is the funciton of the mixing movements?

Answer 93

which keep the intestinal contents thoroughly mixed at all times.

Question 94

The basic propulsive movement of the gastrointestinal tract is what?

Answer 94

peristalsis

Question 95

Describe peristalsis?

Answer 95

A contractile ring appears around the gut and then moves forward; this is analogous to putting one’s fingers around a thin distended tube, then constricting the fingers andsliding them forward along the tube. Any material in front of the contractile ring is moved forward. Peristalsis is an **inherent property of many syncytial** **smooth muscle tubes; stimulation at any point in the gut can cause a contractile ring to appear in the circular**muscle, and this ring then spreads along the gut tube. (Peristalsis also occurs in the bile ducts, glandular ducts, ureters, and many other smooth muscle tubes of the body.)

Question 96

Peristalsis also occurs on which part of the body?

Answer 96

* bile ducts, * glandular ducts, * ureters, and * many other smooth muscle tubes of the body.)

Question 97

What is the usual stimulus for intestinal peristalsis?

Answer 97

* distention of the gut. * That is **, if a large amount of food collects at** **any point in the gut,** the **stretching of the gut wall stimulates the enteric nervous system** to contract the gut wall 2 to 3 centimeters behind this point, and a contractile ring appears that initiates a peristaltic movement. * Other stimuli that can initiate peristalsis include **chemical or physical** irritation of the epithelial lining in the gut. * Also, **strong** **parasympathetic nervous signals** to the gut will elicit strong peristalsis

Question 98

What is the Function of the Myenteric Plexus in Peristalsis?

Answer 98

Peristalsis occurs only weakly or not at all in any portion of the gastrointestinal tract that has congenital absence of the myenteric plexus. Also, it is greatly depressed or completely blocked in the entire gut when a person is treated with atropine to paralyze the cholinergic nerve endings of the myenteric plexus. Therefore, **effectual peristalsis requires an active myenteric plexus**

Question 99

What is the directional movemet of Peristalsis?

Answer 99

Directional Movement of Peristaltic Waves Toward the Anus. Peristalsis, theoretically, ***can occur in either direction from a stimulated point,***but it**normally dies out rapidly in the orad (toward the mouth) direction** while continuing for a considerable distance toward the anus. The exact cause of this directional transmission of peristalsis has never been ascertained, although it probably results mainly from the fact that the myenteric plexus itself is “polarized” in the anal direction, which can be explained as follows.

Question 100

What is the law of the Gut?

Answer 100

***The peristaltic reflex plus the anal direction of movement of the peristalsis***is called the “law of the gut.”

Question 101

Explain the " Law of the Gut".

Answer 101

When a segment of the intestinal tract is **excited by distention and thereby initiates peristalsis**, the contractile ring causing the peristalsis normally **begins on the orad side** of the distended segment and **moves toward the distended segment**, pushing the intestinal contents in the anal direction for **5 to 10 centimeters before dying out.** At the same time, the gut sometimes relaxes several centimeters downstream toward the anus, which is called **“receptive relaxation,**” thus allowing the food to be propelled more easily toward the anus than toward the mouth. This **complex pattern does not occur in the absence of the myenteric plexus**. Therefore, the complex is called the **myenteric reflex or the peristaltic reflex.**

Question 102

What is the ***myenteric reflex or the peristaltic reflex?***

Answer 102

This complex pattern of Peristalsis reflex **does not occur in the absence of the myenteric plexus.** Therefore, the complex is called the myenteric reflex or the peristaltic reflex.