Defecation Reflex Mechanism

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Questions and Answers

During defecation, which of the following occurs?

Internal anal sphincter is voluntarily relaxed

Internal and external anal sphincters are voluntarily relaxed (correct)

External anal sphincter is involuntarily relaxed

Internal and external anal sphincters are involuntarily relaxed

What is the role of the parasympathetic reflex in defecation?

To inhibit the parasympathetic nerve fibers

To relax the internal anal sphincter (correct)

To stimulate the external anal sphincter

To stimulate the sacral segments of the spinal cord

What is the function of secretory glands in the GIT?

To stimulate the parasympathetic nerve fibers

To absorb nutrients from food

To secrete digestive enzymes and provide mucus for lubrication and protection (correct)

To inhibit the sympathetic nerve fibers

How does food stimulate the GIT glands?

Through direct contact stimulation of the surface glandular cells Signup and view all the answers

What is the composition of esophageal secretions?

Entirely mucous Signup and view all the answers

Which of the following stimulates the secretion of GIT glands?

Both parasympathetic and sympathetic stimulation Signup and view all the answers

What is the effect of sympathetic stimulation on GIT glands?

It has a dual effect on the secretion rate Signup and view all the answers

What is the relationship between pernicious anemia and gastric secretions?

Pernicious anemia is related to the secretion of oxyntic glands and pyloric glands Signup and view all the answers

What is the primary function of the alimentary tract?

To absorb nutrients and water Signup and view all the answers

What is the normal resting membrane potential in the smooth muscle fibers of the gut?

Between -50 and -60 millivolts Signup and view all the answers

What is the effect of stretching the muscle on the membrane potential?

It makes the membrane potential more positive Signup and view all the answers

What is the role of slow waves in gastrointestinal smooth muscle?

They only allow sodium ions to enter the muscle fiber Signup and view all the answers

What is the effect of acetylcholine on the smooth muscle fibers of the gut?

It depolarizes the membrane Signup and view all the answers

What is the effect of norepinephrine (NE) on the smooth muscle fibers of the gut?

It hyperpolarizes the membrane Signup and view all the answers

What is the primary mechanism by which the duodenum inhibits stomach emptying?

Enterogastric inhibitory reflexes Signup and view all the answers

Which of the following factors can initiate enterogastric inhibitory reflexes?

Presence of certain breakdown products in the chyme Signup and view all the answers

What is the main function of the enzymes secreted by the brush border of the villi?

Digestion of specific food substances Signup and view all the answers

What is the primary site where most digestion and absorption take place?

Small intestine Signup and view all the answers

What is the name of the reflex that is mediated by the local enteric nervous system in the rectal wall and initiates peristaltic waves in the descending colon, sigmoid, and rectum?

Intrinsic defecation reflex Signup and view all the answers

What is the effect of the degree of distention of the duodenum on stomach emptying?

Inhibits stomach emptying Signup and view all the answers

What is the function of the peptidases secreted by the enterocytes of the mucosa?

Splitting small polypeptides into amino acids Signup and view all the answers

What is the primary function of the small intestine in relation to digestion?

Chemical digestion and absorption of nutrients Signup and view all the answers

Study Notes

Defecation Reflexes

There are two types of defecation reflexes
Intrinsic reflex mediated by the local enteric nervous system in the rectal wall • Distention of the rectal wall initiates afferent signals through the myenteric plexus • Initiates peristaltic waves in the descending colon, sigmoid, and rectum, forcing feces toward the anus
External anal sphincter is consciously and voluntarily relaxed at the same time of defecation
Internal anal sphincter is relaxed by inhibitory signals from the myenteric plexus

Secretory Functions of the GIT

Secretory glands have two primary functions: • Secrete digestive enzymes in most areas of the GIT • Provide mucus for lubrication and protection of all parts of the GIT
Basic mechanisms of stimulation of GIT glands: • Contact of food with the epithelium stimulates secretion • Parasympathetic stimulation increases secretion rate • Sympathetic stimulation has a dual effect

Esophageal Secretions

Entirely mucous and provide lubrication for swallowing
Main body of the esophagus is lined with many simple mucous glands

General Principles of Gastrointestinal Function

The alimentary tract provides the body with a continual supply of: • Water • Electrolytes • Vitamins • Nutrients
Achieving this requires: • Movement of food through the GIT • Secretion of digestive juices and digestion of the food • Absorption of water, electrolytes, vitamins, and digestive products • Circulation of blood through the GI organs to carry away the absorbed substances • Control of all these functions by local, nervous, and hormonal systems

Physiologic Anatomy of the GI Wall

Characteristics of GIT wall: • Layers from outside towards the lumen:
- Serosa
- Longitudinal muscle layer
- Circular muscle layer
- Submucosa
- Mucosa

Electrical Activity of Gastrointestinal Smooth Muscle

Slow waves: • Slow, undulating changes in the resting membrane potential • Intensity usually varies between 5 and 15 millivolts
Spike potentials: • True action potentials that occur automatically when the resting membrane potential becomes more positive than about −40 millivolts • Normal resting membrane potential in the smooth muscle fibers of the gut is between −50 and −60 millivolts
Factors that depolarize the membrane: • Stretching of the muscle • Stimulation by acetylcholine • Stimulation by several GI hormones
Factors that hyperpolarize the membrane: • Effect of NE or EP on the fiber membrane • Stimulation of the sympathetic nerves that secrete mainly NE at their endings

Small Intestine Function

Digestion: • Enzymes are intracellular (disaccharidases and aminopeptidases)
Absorption: • Completed in small intestine • Large surface area (as large as a tennis court area) • Site where most digestion and absorption take place
Secretion: • Secretion of digestive enzymes by the brush border of the villi • Enterocytes of the mucosa contain digestive enzymes that digest specific food substances while they are being absorbed through the epithelium
- Several peptidases for the splitting of small polypeptides into AA
- Disaccharidases that break disaccharides into monosaccharides
- Small amounts of lipases for the digestion of fat

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Description

This quiz deals with the physiological process of defecation, including the relaxation of the internal and external anal sphincters and the role of the parasympathetic reflex.