Lect. 19 GI/UG Reflexes

Lect.17. Digestive system.	Lab. 16 Abd. Cavity	Quiz 3
Lect. 18. Urogenital System	Lab 17. UG Organs
Lect. 19 GI/UG Reflexes	Lab 18. GI/UG Reflexes

THE FUNCTIONAL ANATOMY OF DIGESTIVE

AND UROGENITAL REFLEXES

Blair Turner, Ph.D.

Robert Cowie, Ph.D.

John Young, Ph.D.

OBJECTIVES:

Using the cadaver and these lecture notes (& handouts), the student should be able to:

1. List and diagram the anatomy of the muscular layers and the intrinsic and extrinsic neural controls for the stomach, small intestine, large intestine, and urinary bladder.

2. Explain the evoked movement(s) of each GI or UG effector organ following local (and CNS) specific sensory stimulation.

3. Discuss the specific digestive or elimination process which is produced in each of the previous cases.

LECTURE:

The digestive system consists, structurally, of the gastrointestinal (GI) tract. It is a tube divided into compartments, each with a gate. There is a specific function for each compartment. These compartments and their functions are:

STRUCTURE	FUNCTION

Oral cavity	Ingestion and mastication of food, and initiation of digestion with saliva

Pharynx	Swallowing-transfer of food from oral cavity to esophagus

Esophagus	Transports bolus to stomach

Stomach	Churns bolus with gastric juice, and initiates digestion

Small intestine	Digestion and absorption of nutrients

Large intestine	Reabsorption of water and electrolytes.

These compartments are shown, in order, below.

A bolus of masticated food is moved caudally along the length of the GI tract by peristalsis. This is a neural process, partially voluntary at first (and hence controlled by the somatomotor nervous system), but by the time the bolus reaches the lower esophagus it is entirely automatic, and is controlled by the enteric (see neuroglossary for definition) nervous system. The neural mechanism organizing peristalsis is shown later, below. After a meal empties into the stomach, it is nudged toward the periphery so it contacts the stomach wall, coming into contact with cells which reflexly release hydrochloric acid and digestive enzymes.

DIGESTION: GASTRIC PHASE

The Histological Organization of the Stomach is similar to that of the esophagus. That is, it consists of concentric layers, including Auerbach's and Meissner's plexuses (enteric nervous system) and concentrically and longitudinally arranged layers of smooth muscle. It differs from the esophagus in that its inner, mucosal surface is embedded with gastric glands which secrete hydrochloric acid (HCl) and digestive enzymes.

The Neural Control of the Stomach is the same as for the esophagus. That is, gastric movements are primarily controlled intrinsically and continue to occur after vagal section. Extrinsic, vagal (i.e., parasympathetic) stimulation causes increased contractions of the stomach when tone is low and decreased contractions when tone is high. Extrinsic, splanchnic (i.e., sympathetic) stimulation causes diminution of tone and decreased contractions.

THE STOMACH

Structure Stretched	Evoked Movement	Evoked Digestive Process

Cardiac part of stomach	Lumpy food mass stored here until stretch receptors initiate tonic contraction, moving mass outward toward fundus, body and pylorus. These and later gastric movements are regulated by the myenteric plexus and the vagus nerve.	Digestion begun in mouth with salivary enzymes continues.

Body of stomach	Stretch receptors initiate slow peristaltic waves, moving mass outward toward fundus, body and pylorus.	Food contacts mucous membranes lining stomach, activating release from gastric glands of pepsinogen (from chief cells), HCl from parietal cells, and mucous (from mucous cells). Protein breakdown begins.

Pylorus + pyloric valve	Stretch receptors initiate vigorous contractions, mixing gastric secretions with the food mass into a paste known as chyme, and moving it into the pyloric region.	Protein breakdown continues.

Entrance into the small intestine	Food pushed through the partially open pyloric sphincter into the duodenum evokes the enterogastric reflex (via stretch and chemoreception) and causes inhibition of contraction of the pyloric sphincter, enhancing flow of the chyme into the small intestine.

Next, the acid chyme is pushed into the duodenum, where it is mixed with further digestive enzymes from the liver (and gall bladder) and exocrine pancreas.

DIGESTION: INTESTINAL PHASE

The Histological Organization of the Small Intestine is similar to that of the previous GI structures. That is, it consists of concentric epithelial, neural (myenteric [Auerbach's] and submucosal [Meissner's] neural plexuses) and muscular layers. (Consult the Neuroglossary on the home page of this site for a definition of the enteric nervous system. Distinguish it from the autonomic and somatic nervous systems). The layers of intestinal smooth muscle differ from the esophagus and stomach in that they have a helical arrangement.

The Neural Control of the Small Intestine is the same as for the previous GI structures. Movements of the small intestine are primarily controlled by the intrinsic plexuses, and occur even after vagal section. However, extrinsic parasympathetic (i.e., vagal ) stimulation causes increased contractions of the intestine when tone is low and decreased contractions when tone is high. Vagal stimulation is also important in causing release of pancreatic enzymes into the intestine. Extrinsic, splanchnic (i.e., sympathetic) stimulation causes diminution of tone and decreased contractions.

THE SMALL INTESTINE

Structure Stretched	Evoked Movement	Evoked Digestive Process

Duodenum	Non-propulsive peristalsis (contraction with no preliminary relaxation) and rhythmic segmenting movements involve the circular muscle and are regulated by the myenteric plexus. Pendular (swaying) movements alternate with the above, and involve the longitudinal muscle. These move-ments thoroughly mix the chyme with intestinal, pancreatic & hepatic secretions.	Secretion and Breakdown. Food contacts mucous membranes lining intestine, activating release of secretin + cholesystokinin. These, along with vagal activation, trigger release into duodenum of 1) pancreatic enzymes via pancreatic duct; 2) bile from gall bladder via hepatic duct. Breakdown of protein, fat and carbohydrates into amino acids, monosaccharides and fatty acids is carried out.

Jejunum	Propulsive peristalsis (contraction preceded by relaxation) is regulated by the myenteric plexus acting on both circular and longitudinal muscle. This moves the contents toward ileum.	Absorption. Breakdown products are actively transported from intestinal lumen to blood and lymph vessels by epithelial cells in the luminal brush border.
Ileum	Propulsive peristalsis same as above Reflex relaxation of the ileocecal valve, contents pushed into cecum of large intestine.	Absorption. Similar, but less as chyme moves toward cecum. Remember: Ilium has fewer/lower plicae circularis, and more/shorter vasa recta than does jejunum.

The duodenal chyme must be thorougly mixed with the digestive enzymes in order for efficient breakdown to the simplest chemical constituents. This is accomplished by another intestinal movement-rhythmic segmentation. It consists of a series of alternating, non-propulsive contractions of the small intestine. The idea is illustrated below.

The movements of the enteric nervous system of the intestines are "hard-wired" and can function in the absence of control by the autonomic nervous system, as shown in the following experiments.

Next, a schematic illustration of the histology of the GI tract, in preparation for an understanding of the neural mechanism underlying peristalsis.

THE LARGE INTESTINE

Structure Stretched	Evoked Movement	Function

Cecum	Pulls valve edges together, closing it.	Contents prevented from reflux into ileum.

Ascending, transverse, and descending colon	Rhythmic, segmenting movements (under control of myenteric plexus and circular muscle) are weak due to low intrinsic and extrinsic (vagal & pelvic splanhnic) innervation. Peristaltic movements are strong, under control of myenteric plexus; the longitudinal muscle (=teniae coli), shortens and widens the colon; and the circular muscle pushes the colonic contents caudally.	Contents exposed to mucosal surface, and water is reabsorbed, leaving the waste products of digestion. Empty the contents of one section into the next.
Rectum	Involuntary reflex. Stretching causes opening (i.e. inhibition) of the internal anal sphincter (circular smooth muscle). Autonomic innervation (via parasympathetic preganglionics of the sacral cord, and postganglionics of the myenteric plexus) aids inhibition. Voluntary control. Somatic motor neurons in the sacral cord inhibit contraction of the external anal sphincter (striated muscle) via the pudendal nerves.	Fecal contents accumulate, gradually raising pressure on the external anal sphincter. Fecal contents expelled.