Ch. 63 - Propulsion and Mixing of Food in the Alimentary Tract

Describe the chewing process.

-chewing reflex -bolus of food in mouth initiates reflex inhibition of muscles of mastication, which allows lower jaw to drop -initiates stretch reflex of jaw muscles that leads to rebound contraction -automatically raises the jaw to cause closure of teeth -also compresses bolus against the linings of mouth, allowing jaw to drop and rebound again -repeats over and over again

What are the effects of gastrin on stomach emptying?

-secretion of highly acidic gastric juice by stomach glands -mild to moderate stimulatory effects on motor functions in the body of the stomach -enhance activity of pyloric pump -promotes stomach emptying

How many liters can the stomach hold?

0.8 to 1.5 liters

What are the types of factors that are continually monitored in the duodenum and can initiate enterogastric inhibitory reflexes?

1. degree of distension of duodenum 2. presence of any degree of irritation of the duodenal mucosa 3. degree of acidity of duodenal chyme 4. degree of osmolality of the chyme 5. presence of certain breakdown products in the chyme, esp breakdown products of proteins and fats

What routes mediate the enterogastric nervous reflexes from the duodenum?

1. directly from the duodenum to the stomach through the enteric nervous system in the gut wall 2. through extrinsic nerves that go to prevertebral symp. ganglia and back through inhibitory symp nerve fibers to stomach 3. probably slightly through vagus nerves to brainstem whre they inhibit normal excitatory signals transmitted to stomach through vagi.

What regulates stomach emptying?

1. signals from stomach and duodenum 2. increase gastric food volume increases rate of stomach emptying

What are the motor functions of the stomach?

1. storage of large quantities of food until it can be processed in stomach, duodenum, and lower GI tract 2. mixing of the food w gastric secretions to create chyme 3. slow emptying of the chyme into the small intestine at a rate suitable for digestion/absorption

What two effects do the parallel enterogastric nervous reflexes from the duodenum have on stomach emptying?

1. strongly inhibit "pyloric pump" propulsive contractions 2. increase the tone of the pyloric sphincter

What are the two major physiological parts of the stomach?

1. the "orad" portion - about the first 2/3 of the body 2. the "caudad" portion - the remainder of the body + the antrum

What are the two major anatomical parts of the stomach?

1. the body 2. the antrum

What are the three stages of swallowing?

1. voluntary stage - initiates swallowing process 2. pharyngeal stage - involuntary and constitutes passage of food through pharynx into esophagus 3. esophageal stage - involuntary, transports food from pharynx to stomach

How does the thickness of the pylorus compare to the stomach antrum?

50-100% greater

If all the jaw muscles are working together, what is the force by which teeth can be closed?

55 pounds on the incisors and 200 pounds on the molars

What nerves transmit the motor impulses from the swallowing center to the pharynx and upper esophagus?

5th, 9th, 10th, and 12th cranial nerves and a few of the superior cervical nerves

What is primary peristalsis?

A continuation of the peristaltic wave that begins in the pharynx and spreads into the esophagus during the pharyngeal state of swallowing.

What is achalasia?

A rare condition where the lower esophageal sphincter does not relax satisfactorily.

What happens to the stomach when the esophageal peristaltic wave approaches?

A wave of relaxation, transmitted through myenteric inhibitory neurons, precedes the peristalsis. The entire stomach and the duodenum become relaxed and are prepared ahead of time to receive the food.

What is swallowing also called?

Deglutition

How does the rate of digestion relate to total surface area?

Digestion rate is dependent on the total surface area of the food particle exposed to digestive secretions.

Where are the gastric glands located?

In almost the entire wall of the body of the stomach except along a narrow strip on the lesser curvature of the stomach.

How quickly can the enterogastric inhibitory reflexes become activated?

In as little as 30 seconds.

In which group of people are hunger contractions most intense?

In young, healthy people who have high degrees of GI tonus, and in people with lower than normal blood sugar

What is the function of CCK?

It acts as an inhibitor to block increased stomach motility caused by gastrin

Describe nervous initiation of the pharyngeal stage of swallowing.

It is principally a reflex act. It is almost always initiated by voluntary movement of food into the back of the mouth, which in turn excites involuntary pharyngeal sensory receptors to elicit the swallowing reflex.

What is the lower esophageal sphincter normally like?

It normally remains tonically constricted with an intraluminal pressure at this point in the esophagus of ~30mm Hg.

What is the pressure like in the stomach as food is added?

It remains low until its limit is approached.

What is secondary peristalsis?

It results from the distension of the esophagus by the retained food if the primary peristaltic wave fails to move all the food into the stomach.

Summarize the control of stomach emptying.

Mainly controlled by inhibitory feedback signals from the duodenum, including both enterogastric inhibitory nervous feedback reflexes and hormonal feedback by CCK. These mechanisms work to slow rate of emptying when (1) too much chyme in small intestine or (2) chyme is excessively acidic, has too much unprocessed protein or fat, is hypotonic or hypertonic, or is irritating. Rate of stomach emptying limited to amnt. of chyme that small intestine can process.

Which type of foods is chewing especially important for?

Most fruits and raw vegetables bc these have indigestible cellulose membranes around their nutrient portions that must be broken before food can be digested.

What happens when food enters the duodenum?

Multiple nervous reflexes are initiated from the duodenal wall and pass to the stomach to slow or stop stomach emptying if the volume of chyme in the duodenum becomes too much.

Does increased storage pressure of the food in the stomach cause increase emptying?

No bc in the usual range of volume, the icrease in volume does not increase the pressure much.

How do the intense antral peristaltic contractions that occur during stomach emptying affect water pressure?

Often create 50-70 cm of water pressure, ~ 6x more powerful than normal mixing peristaltic waves.

Which part of food particles do digestive enzymes act on?

Only the surfaces.

What happens when a portion of the small intestine becomes distended with chyme?

Stretching of the intestinal wall elicits localized concentric contractions spaced at intervals along the intestine and lasting only a fraction of a minute. The contractions cause "segmentation" of the small intestine.

Why is food transported through the esophagus faster than the peristaltic wave itself?

The additional effect of gravity pulling the food downward.

What happens to the intense antral peristaltic contractions that occur during stomach emptying as the stomach becomes progressively more empty?

The constrictions begin farther up the body of the stomach, gradually pinching off the food in the body of the stomach and adding it to the chyme in the antrum

Explain the intense antral peristaltic contractions that occur during stomach emptying.

The contractions become intense, beginning in midstomach and spreading through caudad stomach. They are strong peristaltic, very tight ringlike constrictions that can cause stomach emptying.

What happens when the vagus nerves to the esophagus are cut?

The myenteric nerve plexus of the esophagus becomes excitable enough after several days to cause strong secondary peristaltic waves even without support from the vagal reflexes.

What is the distal opening of the stomach?

The pylorus

Why does swallowing not have a large effect on respiration?

The swallowing center inhibits the respiratory center of the medulla to allow swallowing to proceed but it is so short that it interrupts respiration for only a fraction of a usual respiratory cycle and is hardly noticeable.

How does the lower esophageal sphincter function in relation to reflux of stomach contents?

The tonic constriction of the sphincter helps prevent significant reflux except in abnormal conditions, which protects the esophageal mucosa that is not capable of resisting the digestive action of gastric secretions for long.

What are the mechanics of the pharyngeal stage of swallowing?

The trachea is closed, the esophagus is opened, and a fast peristaltic wave initiated by the nervous system of the pharynx forces the bolus of food into the upper esophagus, the entire process occurring in less than 2 seconds.

What is an additional prevention of esophageal reflux?

There is a valvelike mechanism of a short portion of the esophagus that extends slightly into the stomach. Increased intra-abdominal pressure caves the esophagus inward at this point. The valvelike closure prevents high intra-abdominal pressure from forcing stomach contents backward into the esophagus.

What happens to the lower esophageal sphincter when a peristaltic swallowing wave travels down the esophagus?

There is receptive relaxation ahead of the peristaltic wave, which allows easy propulsion of the swallowed food into the stomach

What happens as mixing/constrictor waves progress from the body of the stomach to the antrum?

They become more intense and some are so intense that they provide powerful peristaltic action potential-driven constrictor rings that force the antral contents under higher and higher pressure toward the pylorus.

How do hypotonic or hypertonic fluids affect stomach emptying?

They elicit the inhibitory enterogastric reflexes to prevent the too rapid flow of nonisotonic fluids into the small intestine, which prevents rapid changes in electrolytes in the whole-body extracellular fluid during absorption of intestinal contents.

What happens when fats enter the duodenum?

They extract several different hormones from the duodenal and jejunal epithelium, either by binding with "receptors" on the epithelial cells or in some other way.

What happens after hormones are carried by blood to the stomach from the duodenum?

They inhibit the pyloric pump and at the same time increase the strength of contraction of the pyloric sphincter. These are important bc fats are much slower to be digested than other foods.

What is the primary function of the esophagus?

To conduct food rapidly from the pharynx to the stomach

Why is it said that the peristaltic waves are also a "pyloric pump" ?

When pyloric tone is normal, each strong peristaltic wave forces up to several ml of chyme into the duodenum.

What is an example of when the enterogastric inhibitory reflexes become quickly activated?

When the pH of the chyme in the duodenum falls below 3.5-4, the reflexes quickly block further release of acidic stomach contents into the duodenum until the duodenal chyme can be neutralized by pancreatic and other secretions

After paralysis of the brainstem swallowing reflex, can food fed by a tube or some other way into the esophagus still pass into the stomach?

Yes because secondary peristalsis can be generated by the myenteric plexus without vagal reflexes.

What is the function of secretin?

a general but weak effect of decreasing GI motility

What determines the amount of food that a person preferentially seeks?

appetite

What is the most potent hormone that causes hormonal feedback inhibition of the stomach?

cholecystokinin (CCK)

What is the resulting mixture that is passed down the gut from the stomach?

chyme

What sends impulses to the brainstem to initiate a series of automatic pharyngeal muscle contractions for swallowing?

epithelial swallowing receptor areas all around the opening of the pharynx, especially on the tonsillar pillars

What is the stimulus for releasing inhibitory hormones?

fats entering the duodenum, although other types of food can increase the hormones too

What is the main thing that determines the amount of food a person ingests?

hunger - intrinsic desire for food

What are intense stomach contractions that occur when the stomach has been empty for several hours or more?

hunger contractions - rhythmical peristaltic contractions in the body of the stomach

In starvation, when do hunger pangs reach their greatest intensity?

in 3-4 days and they gradually weaken in succeeding days

What is the function of GIP?

inhibits GI motility under some conditions, and stimulates secretion of insulin by the pancreas

What promotes stomach emptying?

intense peristaltic contractions in the stomach antrum

How long does the entire pharyngeal stage of swallowing take?

less than 6 seconds

What are the two main mechanics of ingestion?

mastication and swallowing

What are the movements of the small intestine?

mixing contractions and propulsive contractions

What controls the chewing process?

nuclei in the brainstem

What are hunger pangs?

pain experienced in the pit of the stomach when hunger contractions occur which usually do not begin until 12-24 hours after the last ingestion of food

What are the two types of peristaltic movements that the esophagus exhibits?

primary peristalsis and secondary peristalsis

What is the upstream squeezing action of antral contents that are moved upstream through the peristaltic ring rather than through the pylorus?

retropulsion, an important mixing mechanism in the stomach

What are other hormones (besides CCK) that are inhibitors of stomach emptying?

secretin and gastric inhibitory peptide (GIP) which is also called glucose-dependent insulinotropic peptide

What does the release of gastrin from the antral mucosa cause?

secretion of highly acidic gastric juice by stomach glands

What are mixing contractions of the small intestine also called?

segmentation contractions

What regulates the rate at which the stomach empties?

signals from both the stomach and the duodenum, but the duodenum provides more potent signals and controls the emptying of chyme into the duodenum at a rate so that the chyme can be digested and absorbed by small intestine

What controls the peristaltic waves in the pharyngeal wall and upper 1/3 of the esophagus?

skeletal nerve impulses from the glossopharyngeal and vagus nerves

What is the musculature in the lower 2/3 of the esophagus?

smooth muscle

What is the musculature of the pharyngeal wall and upper 1/3 of the esophagus?

striated muscle

Which teeth provide a strong cutting action?

the anterior teeth (incisors)

What are the areas in the medulla and lower pons that control swallowing collectively called?

the deglutition or swallowing center

Where is secretin released from?

the duodenal mucosa in response to gastric acid passed from the stomach through the pylorus.

What secretes the digestive juices of the stomach?

the gastric glands

What is the lower esophageal sphincter called?

the gastroesophageal sphincter

What initiates the mixing waves?

the gut wall basic electrical rhythm (slow waves)

What initiates the secondary peristaltic waves?

the intrinsic neural circuits in the myenteric nervous system and reflexes that begin in the pharynx and are transmitted upward through vagal afferent fibers to the medulla and back again to the esophagus through glossopharyngeal and vagal efferent nerve fibers

Which nerve innervates most of the muscles of chewing?

the motor branch of the fifth cranial nerve

Where is cholecystokinin released from?

the mucosa of the jejunum in response to fatty acids in the chyme

Which teeth provide a grinding action?

the posterior teeth (molars)

What is the pyloric circular muscle called?

the pyloric sphincter

Stimulation of which areas of the brain will cause chewing?

the specific reticular areas in the brain stem and taste centers, also the hypothalamus, amygdala, and even the cerebral cortex near the sensory areas for taste and smell

Where is GIP released from?

the upper small intestine in response mainly to fat in the chyme, but also to carbohydrates

What controls the lower 2/3 of the esophagus?

the vagus nerves acting through connections with the esophageal myenteric nervous system

What is the role of hormonal feedback from the duodenum?

to inhibit stomach emptying

What receives essentially all sensory impulses from the mouth?

tractus solitarius

What opposes stomach emptying?

varying degrees of resistance to passage of chyme at the pylorus

What type of reflex occurs when food stretches the stomach?

vasovagal reflex from the stomach to the brainstem and then back to the stomach that reduces the tone in the muscular wall of the body of the stomach so that the wall bulges progressively outward, accommodating greater quantities of food.

Is chewing a voluntary or involuntary action?

voluntary and mostly reflexive

What type of waves occur as long as food is in the stomach?

weak peristaltic constrictor waves, or mixing waves

How often do intense antral peristaltic contractions occur during stomach emptying?

~ 20% of the time whilde food is in the stomach

How often do mixing waves occur in the stomach while food is present?

~ once every 15-20 seconds

How long does the primary peristaltic wave take to pass from the pharynx to the stomach?

~8-10 seconds