Control of the Digestive Tract

Question 1

Neural control

Answer 1

Intrinsic control from the Enteric nervous system 
• Submucosalplexus
• Myentericplexus
• Autonomic control from CNS
• Parasympathetic‘restanddigest’

Question 2

Paracrine

Answer 2

• agents released for local diffusion to target on nearby cells

Question 3

Endocrine

Answer 3

-hormones released into blood stream to reach target

Question 4

Gut smooth muscle exhibits

Answer 4

spontaneous “slow wave activity

Question 5

slow wave activity

Answer 5

Basic Electric Rhythm of depolarisation and re-polarisation

The Basal or Basic electrical rhythm (BER) or electrical control activity (ECA) determines the frequency of the contractions in the gastrointestinal (GI) tract.

Smooth muscle within the GI tract causes the involuntary peristaltic motion that moves consumed food down the esophagus and towards the rectum.[1] The smooth muscle throughout most of the GI tract is divided into two layers: an outer longitudinal layer and an inner circular layer.[1] Both layers of muscle are located within the muscularis externa. The stomach has a third layer: an innermost oblique layer.

The physical contractions of the smooth muscle cells can be caused by action potentials in efferent motor neurons of the enteric nervous system, or by receptor mediated calcium influx.[1] The inner circular layer is innervated by both excitatory and inhibitory motor neurons, while the outer longitudinal layer is innervated by mainly excitatory neurons. These action potentials cause the smooth muscle cells to contract or relax, depending on the particular stimulation the cells receive. Longitudinal muscle fibers depend on calcium influx into the cell for excitation-contraction coupling, while circular muscle fibers rely on intracellular calcium release. Contraction of the smooth muscle can occur when the BER reaches its plateau (an absolute value less than -45mV)[citation needed] while a simultaneous stimulatory action potential occurs. A contraction will not occur unless an action potential occurs.

Question 6

Autonomic Control –Extrinsic control

Answer 6

• Parasympathetic division- Vagus nerve (CNX) to gut.
REST AND DIGEST!
• Sympathetic division- splanchnic nerves from thoracic spinal cord. SLOWS DIGESTION- (FLIGHT AND FIGHT)

Question 7

Paracrine Control

Answer 7

• paracrine agents are
  released for local diffusion to target on nearby cells
  • Paracrine agents are released locally and don’t have far to travel so can initiate a fast localised response.
  • Examples of paracrine agents are somatostatin, which acts on G cells to increase the release of the hormone gastrin and histamine, which acts on the stomach to increase secretion of gastrin and increase contractility of the stomach wall.

Question 8

Endocrine Control

Answer 8

-hormones released into blood stream to reach target
• Hormones are circulated around the body so have a far reaching effect. They initiate slower responses than neural control but are less costly energetically.
• Gastrin in an example of a hormone released from the stomach to act back on the stomach to increase release of HCl and pepsinogen.
• GIP is an example of a hormone released from the intestinal tract. It promotes intestinal secretions and slows stomach secretions (negative feedback) It also sends message to other body systems about satiety and energy balance.
• There are many other hormones that act on the digestive tract.

Question 9

Types of motility

Answer 9

• Peristalsis
• Mechanical digestion
• Mixing

Question 10

motility

Answer 10

the ability of the muscles of the digestive tract to undergo contraction

Question 11

Peristalsis

Answer 11

involuntary movements of the longitudinal and circular muscles, primarily in the digestive tract but occasionally in other hollow tubes of the body, that occur in progressive wavelike contractions. Peristaltic waves occur in the esophagus, stomach, and intestines.

Question 12

Mechanical digestion

Answer 12

involves physically breaking the food into smaller pieces. Mechanical digestion begins in the mouth as the food is chewed.

Question 13

Two forms of small intestine motility

Answer 13

• Peristalsis relies upon the arrangement of the 2 muscle layers
• Segmental mixing and packing uses rhythmic contractions of circular muscle

Question 14

Two forms of large intestine motility;-

Answer 14

• Peristalsis, including mass movements

* Haustration –similar to segmentation but with the formation of sacs which store and propel contents

Question 15

Haustration

Answer 15

The presence of a segment or recess, esp. in the bowel.

Question 16

Digestive and inter-digestive phases of peristalsis

Answer 16

• Digestive phase peristaltic contractions pass over short segments of the intestine
• Interdigestive phase peristaltic contractions pass over large segments of the
intestine-
MIGRATING MOTILITY COMPLEX
• Clears out undigested material
• Controls bacterial population

Question 17

Composition of secretions

Answer 17

• Water
• Ions
• Mucous
• Enzymes

Question 18

Basic structure of a gland

Answer 18

Exocrine glands consist of two main parts, a secretory unit and a duct. The secretory unit consists of a group of epithelial cells, which release their secretions into a lumen. A duct is lined with epithelium and is involved in transporting the secretions from the secretory unit to an epithelium-lined surface.

Question 19

Types of secretions

Answer 19

• Saliva
• Gastric secretions
• Intestinal secretions
• Pancreatic secretions
• Liver secretions
• Mucous is secreted for lubrication throughout the tract

Question 20

Stomach secretions

Answer 20

HCl acid for protection and activation of pepsinogen
• Pepsinogen, precursor of pepsin for protein and peptide digestion.
• Mucous for lubrication and protection.
• Saliva that Is swallowed adds other enzymes, fluids and bicarbonate, an important buffer.

Question 21

Three phases of regulation of gastric secretions

Answer 21

• Cephalic
• Gastric

Intestinal

Question 22

Cephalic

Answer 22

Conditioned reflexor sight/smell/taste of food
• Viavagus,gastrin secretion and histamine
• Increase stomach motility
• Increase HCl and pepsinogen secretion

Question 23

Gastric

Answer 23

• Stimulationofchemoandmechanoreceptorsinstomachleadtothereleaseofgastrin
and histamine
• AlocalnegativefeedbackloopoperatesifgastricpHis<3

Question 24

Intestinal

Answer 24

• The presence of chyme in the duodenum inhibits acid secretion and motility via the release of secretin, Gastric inhibitory peptide, (GIP), and Choleycystokinin, (CCK)

Question 25

Secretions of the small intestine

Answer 25

• Succus entericus
• Enterokinase,
• Carboxy-andaminopeptidases, • Maltase
• Lactase
• Sucrase
• Lipase
• Nucleases
• pH7-8
• Alkaline mucoid fluid (no enzymes) from Brunner’s glands (in first part of duodenum, extending into submucosa)

Question 26

Composition of bile

Answer 26

• Bile acids are produced from cholesterol in smooth ER of hepatocytes (also takes phospholipids and cholesterol from the cell membrane)
• Hydrophilic and hydrophobic sides-detergent
• Secreted into canaliculi as sodium salts-this draws water into bile by
osmosis and electrolytes
• Bicarbonate is secreted from duct cells into bile
• Bile pigments are excreted through this route- the main one being bilirubin (from red blood cell turnover)

Question 27

Enterohepatic circulation

Answer 27

90% of bile salts are resorbed from the small intestine and returned to the hepatocytes via the portal circulation for re-excretion.

Question 28

Secretions of the exocrine pancreas

Answer 28

Enzymes
– Trypsinogen
– Chymotrypsinogen
– Carboxy and aminopeptidase
– Lipase
– Amylase
– Maltase
– Nucleases
• Bicarbonate, pH7-8
• Stimulated by acetylcholine, cholecystokinin and secretin

Question 29

Secretions of the endocrine pancreas

Answer 29

• Insulin

* Glucagon