Fr1- Gut 1

Question 1

How much ingested food is made available for the body’s use?

Answer 1

About 95% of the ingested food is made available for the body’s use

Question 2

/What is the difference between motilty and tone?

Answer 2

The term motility refers to the muscular contractions that mix and move forward the contents within the tract. Although the smooth muscle in the walls of the digestive tract is phasic smooth muscle that displays action potential–induced bursts of contraction , it also maintains a constant low level of contraction known as tone

Question 3

Why is tone important?

Answer 3

Tone is important in maintaining a steady pressure on the contents of the digestive tract and in preventing its walls from remaining permanently stretched following distension

Question 4

Two basic types of phasic digestive motility are superimposed on ongoing tonic activity

Answer 4

• Propulsive movements and mixing movements. Propulsive movements propel or push the contents forward through the digestive tract.
• Mixing movements have a twofold function.
• First, by mixing food with the digestive juices, these movements promote digestion of the food.
• Second, they facilitate absorption by exposing all parts of the intestinal contents to the absorbing surfaces of the digestive tract

Question 5

Which parts of the digestive tract does motility involve skeletal muscle rather than smooth muscle activity?

Answer 5

The mouth through the early part of the esophagus at the beginning and the external anal sphincter at the end

Accordingly, the acts of chewing, swallowing, and defecation have voluntary components because skeletal muscle is under voluntary control

Question 6

Why does secretion require energy?

Answer 6

for active transport of some of the raw materials into the cell (others diffuse in passively) and for synthesis of secretory products

Question 7

The endocrine tissue of the gastrointestinal tract is organized as single, individual cells scattered throughout the length of the tract. These specialized epithelial cells produce a range of signal proteins. How are these proteins classified?

Answer 7

As either GI hormones or GI peptides

(Regardless of their classification, these endocrine secretions regulate digestive function)

Question 8

What is the purpose of digestion?

Answer 8

to chemically break down the structurally complex foodstuffs of the diet into smaller, absorbable units

Question 9

Indigestible dietary polysaccharides found in plant walls include

Answer 9

insoluble fiber such as cellulose and soluble fiber such as pectin, which cannot be digested into their constituent monosaccharides by digestive juices humans secrete; thus, indigestible fiber represents the “bulk” of our diet

Question 10

Draw the hydrolysis of a triglyceride

Answer 10

Question 11

Draw the hydrolysis of carbohydrates

Answer 11

Question 12

Draw the hydeolysis of amino acids (polypeptides)

Answer 12

Question 13

State all the organs of the digestive tract

Why are these organs considered as separate entities?

Answer 13

mouth, pharynx (throat), esophagus, stomach, small intestine (consisting of the duodenum, jejunum, and ileum), large intestine (the cecum, appendix, colon, and rectum), and anus

• Although these organs are continuous with one another, they are considered as separate entities because of their regional modifications, which allow them to specialize in particular digestive activities

Question 14

Give examples of conditions essential to the digestive process that can be tolerated in the digestive tract lumen but could not be tolerated in the body

Answer 14

■ The pH of the stomach contents falls as low as 2 as a result of gastric secretion of hydrochloric acid (HCl), yet in the body fluids the range of pH compatible with life is 6.8 to 8.0.

■ The digestive enzymes that hydrolyze the protein in food could also destroy the body tissues that produce them. (Protein is the main structural component of cells.) Therefore, once these enzymes are synthesized in inactive form, they are not activated until they reach the lumen, where they actually attack the food outside the body (that is, within the lumen), thereby protecting the body tissues against self-digestion.

■ In the lower part of the intestine exist quadrillions of living microorganisms that are normally harmless and even beneficial, yet if these same microorganisms enter the body proper (as may happen with a ruptured appendix), they may be extremely harmful or even lethal.

■ Foodstuffs are complex foreign particles that would be attacked by the immune system if they were in contact with the body proper. However, the foodstuffs are digested within the lumen into absorbable units such as glucose, amino acids, and fatty acids that are indistinguishable from these simple energyrich molecules already present in the body

Question 15

The mucosa lines the luminal surface of the digestive tract. It is divided into three layers:

Answer 15

■ The primary component of the mucosa is a mucous membrane, an inner epithelial layer that serves as a protective surface. It is also modified in particular areas for secretion and absorption. The mucous membrane contains exocrine gland cells for secretion of digestive juices, endocrine gland cells for secretion of blood-borne GI hormones, and epithelial cells specialized for absorbing digested nutrients

■ The lamina propria is a thin middle layer of connective tissue on which the epithelium rests. It houses the gutassociated lymphoid tissue (GALT), which is important in defense against disease-causing intestinal bacteria.

■ The muscularis mucosa is a sparse outermost mucosal layer of smooth muscle

Question 16

Describe the submucosa

Answer 16

• The submucosa (“under the mucosa”) is a thick layer of connective tissue that provides the digestive tract with its distensibility and elasticity.
• It contains the larger blood and lymph vessels, both of which send branches inward to the mucosal layer and outward to the surrounding thick muscle layer.
• Also, a nerve network known as the submucosal plexus lies within the submucos

Question 17

Explain the structure of the muscularis externa and how it helps regulate local gut activity

Answer 17

• The muscularis externa, the major smooth muscle coat of the digestive tube, surrounds the submucosa. In most parts of the tract, the muscularis externa consists of two layers:
• an inner circular layer and an outer longitudinal layer. The fibers of the inner smooth muscle layer (adjacent to the submucosa) run circularly around the tube. Contraction of these circular fibers decreases the diameter of the lumen, constricting the tube at the point of contraction. Contraction of the fibers in the outer layer, which run longitudinally along the length of the tube, shortens the tube.
• Together, contractile activity of these smooth muscle layers produces the propulsive and mixing movements.
• Another nerve network, the myenteric plexus, lies between the two muscle layers (myo means “muscle”; enteric means “intestine”).
• Together the submucosal and myenteric plexuses, along with GI hormones and local chemical mediators, help regulate local gut activity

Question 18

Explain the structure of the serosa

Answer 18

• The outer connective tissue covering of the digestive tract is the serosa, which secretes a watery, slippery fluid (serous fluid) that lubricates and prevents friction between the digestive organs and the surrounding viscera.
• Throughout much of the tract, the serosa is continuous with the mesentery, which suspends the digestive organs from the inner wall of the abdominal cavity like a sling.
• This attachment provides relative fixation, supporting the digestive organs in proper position, while still allowing them freedom for mixing and propulsive movements.

Question 19

What are slow-wave potentials?

Answer 19

Smooth muscle of the digestive tract undergoes spontaneous, rhythmic cycles of depolarization and repolarization. The prominent type of self-induced electrical activity in digestive smooth muscle is slow-wave potentials lternatively referred to as the digestive tract’s basic electrical rhythm (BER)

Question 20

Where are interstitial cells of Cajal (pacemaker cells) located?

Answer 20

They are located throughout the layers of the muscularis externa

These pacemakers generate the slow-wave potentials that propagate via gap junctions to adjacent smooth muscle cells

Question 21

Are slow waves action potentials?

Answer 21

• Slow waves are not action potentials and do not directly induce muscle contraction; they are rhythmic, wavelike fluctuations in membrane potential that cyclically bring the membrane closer to or farther from threshold potential
• If these waves reach threshold at the peaks of depolarization, a volley of action potentials is triggered at each peak, resulting in rhythmic cycles of muscle contraction

Question 22

What determines wheter threshold is reached?

What does the intensity of contractions depend on?

Answer 22

• The effect of various mechanical, neural, and hormonal factors that influence the starting point around which the slow-wave rhythm oscillates
• The intensity (strength) of these contractions depends on the number of action potentials that occur when the slow-wave potential reaches threshold, which in turn depends on how long threshold is sustained

Question 23

Explain how the level of contractility can range from low-level tone to vigorous mixing and propulsive movements by varying the cytosolic Ca²⁺ concentration

Answer 23

• At threshold, voltage-gated Ca21 channels are activate resulting in Ca²⁺ influx into the smooth muscle cell. The resultant Ca²⁺ entry has two effects:
• (1) It is responsible for the rising phase of an action potential, with the falling phase being brought about as usual by K1 efflux; and (2) it triggers a contractile response
• The greater the number of action potentials, the higher the cytosolic Ca21 concentration, the greater the cross-bridge activity, and the stronger the contraction.
• Other factors that influence contractile activity also do so by altering the cytosolic Ca²⁺concentration

Question 24

The intrinsic nerve plexuses are the two major nerve fiber networks

Answer 24

The submucosal plexus and the myenteric plexus—that lie entirely within the digestive tract wall and run its entire length

Question 25

Why does the digestive tract have a considerable degree of self-regulation?

Answer 25

Unlike any other body system, the digestive tract has its own intramural (“within-wall”) nervous system

Question 26

What are the main components of the enteric nervous system?

Answer 26

The two plexuses (submucosal plexus and the myenteric plexus) are termed the enteric nervous system

Question 27

What main types of neurons are present in the intrinsic plexuses?

Answer 27

• Sensory neurons called intrinsic primary afferent neurons respond to specific local stimuli in the digestive tract.
• Intrinsic efferent neurons innervate and control smooth muscle and exocrine and endocrine cells of the digestive tract

Question 28

How can the intrinsic efferent neurons can directly affect digestive tract motility, secretion of digestive juices, and secretion of GI hormones?

Answer 28

Through excitatory or inhibitory interactions. For example, neurons that release acetylcholine (ACh) as a neurotransmitter promote contraction of digestive tract smooth muscle, whereas the neurotransmitters nitric oxide and vasoactive intestinal peptide act in concert to cause its relaxation

Question 29

What are the extrinisc nerves?

Answer 29

The extrinsic nerves are the nerve fibers from both branches of the autonomic nervous system that originate outside the digestive tract and regulate digestive tract function

Question 30

What are the effects of the parasympathetic nerve fibers supplying the digestive tract?

Answer 30

• They arrive primarily by way of the vagus nerve, tend to increase smooth muscle motility and promote secretion of digestive enzymes and hormones.
• Unique to the parasympathetic nerve supply to the digestive tract, the postganglionic parasympathetic nerve fibers are actually a part of the intrinsic nerve plexuses.
• They are the ACh-secreting output neurons within the plexuses. Thus, ACh is released in response to local reflexes coordinated entirely by the intrinsic plexuses as well as to vagal stimulation, which acts through the intrinsic plexuses

Question 31

The digestive tract wall contains three types of sensory receptors that respond to local changes in the digestive tract:

Answer 31

(1) chemoreceptors sensitive to chemical components within the lumen
(2) mechanoreceptors (pressure receptors) sensitive to stretch or tension within the wall
(3) osmoreceptors sensitive to the osmolarity of the luminal contents

Question 32

What is a short reflex?

Answer 32

A short reflex takes place when all elements of the reflex are located within the wall of the digestive tract itself—that is, when the intrinsic nerve networks influence local motility or secretion in response to specific local stimulation

Question 33

What is a long reflex?

Answer 33

Because the autonomic reflexes involve long pathways between the central nervous system and digestive system, they are known as long reflexes

Question 34

What is the first step in the digestive process?

Is the degree of occlusion more important than the force of the bite?

Answer 34

• The first step in the digestive process is mastication, or chewing, the mouth motility that involves the slicing, tearing, grinding, and mixing of ingested food by the teeth. The teeth are firmly embedded in and protrude from the jawbones. The teeth can exert forces much greater than those necessary to eat ordinary food
• The degree of occlusion (how well the upper and lower teeth fit together when the jaw is closed) is more important than the force of the bite in determining the efficiency of chewing.

Question 35

What is malocclusion?

Answer 35

When the upper and lower teeth do not make proper contact with one another, they cannot accomplish their normal cutting and grinding action adequately. Such malocclusion is usually caused either by overcrowding of teeth too large for the available jaw space or by one jaw being displaced in relation to the other

Question 36

Is chewing totally voluntary?

Answer 36

The act of chewing can be voluntary, but most chewing during a meal is a rhythmic reflexlike activity accomplished with out conscious effort

Question 37

State the composition of saliva

Answer 37

Saliva is about 99.5% H2O and 0.5% electrolytes and protein

Question 38

What are the most important salivary proteins?

Answer 38

amylase, mucus, and lysozyme

Question 39

What are the main functions of saliva?

Answer 39

1. Saliva begins digestion of dietary starches through action of the enzyme salivary amylase. The products of digestion include maltose, a disaccharide consisting of two glucose molecules and alpha-limit dextrins, a branched polysaccharide resulting from amylopectin digestion.
2. Saliva facilitates swallowing by moistening food particles, thereby holding them together, and by providing lubrication through the presence of mucus, which is thick and slippery.
3. Saliva exerts some antibacterial action by a fourfold effect—first, by lysozyme, a salivary enzyme that lyses, or destroys, certain bacteria by breaking down their cell walls; second, by salivary IgA antibodies third, by salivary lactoferrin, which tightly binds to iron that bacteria need to multiply and fourth, by rinsing away material that may be a food source for bacteria.
4. Saliva serves as a solvent for molecules that stimulate the taste buds
5. Saliva aids speech by facilitating movements of the lips and tongue. It is difficult to talk when your mouth feels dry.

Question 40

Explain the simple and conditioned salivary reflexes

Answer 40

• The simple salivary reflex occurs when chemoreceptors and pressure receptors within the oral cavity respond to the presence of food. On activation, these receptors initiate impulses in afferent nerve fibers that carry the information to the salivary center, which is located in the medulla of the brain stem, as are all the brain centers that control digestive activities. The salivary center, in turn, sends impulses via the extrinsic autonomic nerves to the salivary glands to promote increased salivation. Dental procedures, by activating pressure receptors in the mouth, promote salivary secretion in the absence of food.
• With the conditioned, or acquired, salivary reflex, salivation occurs without oral stimulation. Just thinking about, seeing, smelling, or hearing the preparation of pleasant food initiates salivation through this reflex. This reflex is a learned response based on previous experience. The cerebral cortex stimulates the medullary salivary center when it receives inputs that arise outside the mouth and are mentally associated with the pleasure of eating

Question 41

Unlike the autonomic nervous system elsewhere in the body, how do sympathetic and parasympathetic responses in the salivary glands differ?

Is salivary secretion under hormonal control?

Answer 41

• The responses are not antagonistic

Both sympathetic and parasympathetic stimulation increase salivary secretion, but the quantity and characteristics differ

• Salivary secretion is the only digestive secretion entirely under neural control. All other digestive secretions are regulated by both nervous system reflexes and hormones.

Question 42

No absorption of nutrients occurs in the mouth

What drug can be absorbed by the oral mucosa?

Answer 42

nitroglycerin, a vasodilator drug sometimes used by cardiac patients to relieve angina attacks associated with myocardial ischemia

Question 43

Swallowing is a sequentially programmed all-or-none reflex

Answer 43

• Swallowing is initiated when a bolus, or ball of chewed or liquid food, is voluntarily forced by the tongue to the rear of the mouth and into the pharynx
• The pressure of the bolus stimulates pharyngeal pressure receptors, which send afferent impulses to the swallowing center located in the medulla of the brain stem.
• The swallowing center then reflexly activates in the appropriate sequence the muscles involved in swallowing.
• Swallowing is the most complex reflex in the body, with multiple highly coordinated responses being triggered in a specific all-or-none pattern over a period of time.
• Swallowing is initiated voluntarily, but once begun it cannot be stopped

Question 44

The following coordinated activities prevent the bolus from entering the respiratory passageways and direct it into the esophagus:

Answer 44

■ The swallowing center temporarily inhibits the respiratory center thus the person does not attempt futile respiratory efforts while the airways are briefly sealed off.

■ The uvula is elevated and lodges against the back of the throat, sealing off the nasal passage from the pharynx so that food does not enter the nose

■ The tongue’s position against the hard palate keeps food from reentering the mouth during swallowing

■ Food is prevented from entering the trachea primarily by elevation of the larynx and tight closure of the vocal folds across the laryngeal opening, or glottis

■ The epiglottis (epi means “upon”), a flap of cartilaginous tissue anterior to the glottis, folds backward down over the closed glottis as further protection from food entering the respiratory airway

■ With the glottis closed, pharyngeal muscles contract to force the bolus into the esophagus (step 8 )

Question 45

What sphincter prevents air from entering the digestive tract?

Answer 45

The pharyngoesophageal sphincter

Question 46

What is a sphincter?

Answer 46

A ringlike muscular structure that, when closed, prevents passage through the tube it guards.

Question 47

What is the upper and lower esophageal sphincter?

Answer 47

• The upper esophageal sphincter is the pharyngoesophageal sphincter
• Lower esophageal sphincter is the gastroesophageal sphincter

Question 48

When is the only time a pressure gradient doesnt exist between the atmosphere and the eosophagus?

Answer 48

During a swallow

the pharyngoesophageal sphincter remains closed as a result of neurally induced contraction of the sphincter’s circular skeletal muscle.

Tonic contraction of this upper esophageal sphincter prevents large volumes of air from entering the esophagus and stomach during breathing, thus averting excessive eructation (burping).

Instead, air is directed only into the respiratory airways. During swallowing, this sphincter opens and allows the bolus to pass into the esophagus

Question 49

Peristaltic waves push food through the esophagus

Answer 49

The esophageal stage of the swallow now begins. The swallowing center triggers a primary peristaltic wave that sweeps from the beginning to the end of the esophagus, forcing the bolus ahead of it toward the stomach.

The term peristalsis refers to ringlike contractions of the circular smooth muscle that move progressively forward, pushing the bolus into a relaxed area ahead of the contraction.

The peristaltic wave takes about 5 to 9 seconds to reach the lower end of the esophagus. Progression of the wave is controlled by the swallowing center, with innervation by means of the vagus