Lecture 4 Flashcards

(46 cards)

1
Q

What are sphincters

A

Sphincters are made up of smooth muscle act as the “valve of a reservoir” for holding luminal
content adequately before emptying the content into next segment by their highly
coordinated activity.

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2
Q

Name the four main layers of the gut wall

A

mucosa, submucosa, muscularis externa, serosa

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3
Q

Describe the layers of the mucosa

A

Epithelial layer – exocrine gland cells(secrete mucous and digestive enzymes), endocrine cells (release GI hormones into the blood; constitute gut endocrine system, e.g. the hormone cholecystokinin is released in response to fat and protein in the gut lumen),

Lamina propria – (small blood vessels(BVs), nerve fibers, lymphatic cells/tissue (GALT-gut associated lymphatic tissue) loose connective tissue(CT),

Muscularis mucosa
thin layers smooth muscle
responsible for controlling mucosal blood flow and GI secretion
(contraction throws mucosa into folds/ridges)

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4
Q

Describe the function of the gut associated lymphatic tissue (GALT)

A

Secretes antibodies to specific food or bacterial antigens

triggers immunological reactions against them leading to mucosal inflammation and damage. (e.g. Activation of this local GI immune system involved in celiac disease and inflammatory bowel diseases (IBD) such as ulcerative colitis and Crohn’s disease.)

provides permission of immunological tolerance to potentially immunologic dietary substances and bacteria that normally reside in the gut (intestinal microflora)

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5
Q

Describe the submucosa and its layers

A

loose CT, large BVs, lymphatic vessels

Glands in some GI regions

Submucosal nerve plexus - Meissners plexus - regulates blood flow and secretion

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6
Q

Describe the muscularis externa and its layers

A

thick muscle whose contraction contributes to major gut motility (segmentation and peristalsis)

  • two substantial layers of smooth muscle cells: Inner circular and outer longitudinal layer. (circular layer 3-5x as thick as outer longitudinal)

Myenteric nerve plexus - Auerbach’s lies between muscle layers and regulates motility

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7
Q

Describe the serosa

A

Connective tissue and connects to the abdominal wall, supporting GI tract in the abdominal cavity. Several major structures enter through the serosa, including blood vessels, extrinsic nerves, and the ducts of the large accessory exocrine glands

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8
Q

Describe the GI intrinsic pathway

A

Intrinsic pathway:
The enteric nervous system (ENS). The ENS is functionally organized as the submucosal plexus and the myenteric plexus. The myenteric (Auerbach’s) plexus is mainly involved with control of gut motility and innervates the longitudinal and circular smooth muscle layers. The submucosal (Meissners) plexus coordinates intestinal absorption and secretion through its innervation of the glandular epithelium, intestinal endocrine cells, and submucosal blood vessels.

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9
Q

Describe the GI extrinsic pathway

A

Extrinsic pathway
The gut-brain axis. The ENS is linked to the central nervous system (CNS) via the sensory and motor nerves of the parasympathetic nervous system and the sympathetic nervous system

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10
Q

Describe the extrinsic parasympathetic innervation

A

Preganglionic Vagus nerve innervates oesophagus, stomach, small intestine, liver, pancreas, caecum, appendix, ascending colon, traverse colon
pelvic nerve innervates remainder of the colon via hypogastric plexus.
Parasympathetic activity stimulates motility and secretions.

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11
Q

Describe the intrinsic sympathetic innervation

A

Preganglionic fibres from T8-L2. Postganglionic cell bodies in celiac, inferior and superior mesenteric ganglia. (innervate e.g. secretory cells, and circular smooth muscles)
Sympathetic activity inhibits gut motility and secretion;
constrict sphincters

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12
Q

Describe the Myenteric/Auerbach’s plexus

A

between the circular and longitudinal muscle layers,
a thin layer array of ganglia, ganglion cells, and inter-ganglionic nerve tracts that serve to interconnect the plexus.

innervate longitudinal muscles and the outer lamella of the circular muscle layer.

Many of these neurons have projections into adjacent muscle layers, where they are either excitatory or inhibitory, but some are interneurons involved in integrative functions.
control of gut motility

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13
Q

Describe the submucosal/Meissner’s plexus

A

between the submucosal layers and circular muscle. – neurons that are functionally distinct from those of the myenteric plexus and, relative to intestinal motor function,

appear to be projecting mainly to the inner lamella of the circular muscle layer.
coordinates intestinal absorption and secretion

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14
Q

Name and describe a GI innervation dysfunction

A

Hirschsprung’s disease is a congenital absence of the myenteric plexus, usually involving a portion of the distal colon. The pathologic aganglionic section of large bowel lacks peristalsis and undergoes continuous spasm, leading to a functional obstruction and severe constipation.

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15
Q

Describe basic electrical rhythm /slow wave rhythm

A

Display pacemaker activity - Slow waves - spontaneous oscillations in the membrane potential. (The distal stomach is the first location in the smooth muscle to exhibit slow waves)

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16
Q

Describe spike potentials

A

triggered if the peak of a slow wave depolarizes the membrane to a threshold potential. Spike potentials are slow action potentials caused by the opening of Ca2+ channels.
Ca2+ entry into smooth muscle cells occurs during spike potentials and triggers muscle contraction

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17
Q

What is the major function of BER/SWR

A

Major function of BER to determine when contractions can occur in a certain area of a bowel.

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18
Q

What is the force of contraction determined by?

A

Force of contraction is determined by the number of spikes fired within each wave, which in turn depend on neural and hormonal inputto give rise to 2 major types of contractile responses: segmentation and peristalsis

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19
Q

What is the origin of the pacemaker activity?

A

The origin of the basic electrical rhythm is a network of fibroblast-like cells called the interstitial cells of Cajal, which are positioned between the longitudinal and the circular smooth muscle layers.

20
Q

What affects the appearance and frequency of spike potentials

A

The appearance and frequency of spike potentials are greatly affected by hormonal agents and neurotransmitters. The greater the number of spike potentials there are per BER cycle, the greater is the degree of muscular contraction.

Excitatory transmitters often cause nonselective cation channels in the smooth muscle cells to open; the resting membrane potential is depolarized and more slow waves cross the threshold for the generation of a spike potential.
Inhibitory transmitters often act by opening the K+ channels in smooth muscle cells, hyperpolarizing the membrane
potential and preventing the slow waves from reaching threshold.

Anticholinergic drugs such as amitriptyline, a tricyclic antidepressant, inhibit the effects of acetylcholine systemically, resulting in a myriad of side effects. Gastrointestinal side effects include xerostomia (dry mouth), constipation, ileus, and nausea and vomiting

21
Q

What are the three types of GI movement

A

Segmentation, tonic contraction and peristalsis.

22
Q

Describe segmentation

A

Mainly small intestine, for mixing food with enzymes.
Closely spaced contractions of circular smooth followed by relaxation.
No net forward movement.

23
Q

Describe peristalsis

A

Longitudinal smooth muscle contracts first, followed half way through its contraction by the circular muscle. Longitudinal relaxes during latter half of circular contraction …leads to a progressive wave. Distension of gut by food is trigger for peristalsis. Net effect – propel food along GI tract

24
Q

describe the migrating motor complex

A

a pattern of motility that occurs about every 90 minutes between meals.

intervals of strong propulsive contractions, which pass down the distal stomach and small intestine

sweep the stomach and small intestine of indigestible materials. During a meal, the stomach only allows small particles to pass into the small intestine, leaving behind larger particles (e.g., dietary fiber).

an intrinsic property of the gastrointestinal tract that does not require external innervation. The appearance of migrating motor complexes in infants indicates the developmental maturity of the intestines and can be absent in premature neonates.

Migrating motor complexes rarely disappear as a person ages or in pathologic states; however, they have been found to disappear, for example, in patients after treatment of cancer with radiotherapy. Loss of the migrating motor complex can cause bacterial overgrowth in the small intestine, suggesting that it normally prevents bacterial colonization of the upper intestine.

25
Name and describe a motility dysfunction
Paralytic ileus is a temporary cessation of gut motility that is most commonly caused by abdominal surgery. Other common causes that result in an ileus are infection or inflammation in the abdominal cavity (e.g., appendicitis), electrolyte abnormalities (e.g., hypokalemia), and drug ingestion (e.g., narcotics). Signs and symptoms of paralytic ileus include nausea and vomiting, abdominal distension, and absent bowel sounds.
26
describe deglutition
Voluntary control- bolus of food formed in the mouth by mastication, propelled to oropharynx as the tongue moves up and back against hard palate (A) Remainder of swallowing under reflex autonomic control Bolus stimulates mechanoreceptors in pharynx - glossopharyngeal (IX) afferent impulses to swallowing centre Efferent impulses from vagus(X) to pharynx, oesophagus and palate for co-ordinated muscle contraction Soft palate elevates and superior constrictor of pharynx contracts to close off nasopharynx (B) Respiration inhibited Larynx rises so that epiglottis covers trachea (B-C) Upper Esophageal Sphincter (UES) relaxes and bolus enters Peristaltic wave initiated in pharynx continues length of oesophagus This wave 7-10s; if insufficient vago-vagal reflex triggers second
27
What happens in the oesophagus upon swallowing
The upper esophageal sphincter briefly relaxes, allowing the food bolus to pass into the esophagus. A contractile (peristaltic) wave sweeps down the esophagus. The lower esophageal sphincter and the proximal stomach relax to allow the bolus to enter the stomach. No pacemakers in oesophageal muscle (no spontaneous contractions)
28
State two esophageal motility dysfunctions
Achalasia and Gastro-esophageal reflux disease
29
Describe Achalasia
condition characterized by : dysphagia (ie difficulty in swallowing) results from failure of LES to relax, thereby causing a functional obstruction. loss of peristalsis of the esophageal body Cause unknown, but evidence shows that many patients lose ganglionic cells of myenteric plexus, or neural defects in the vagal dorsal nucleus of the brainstem)
30
Describe Gastro-esophageal reflux disease (GERD)
occurs when the LES is incompetent, allowing the flow of gastric juices and contents back into the esophagus. Since gastric juices are corrosive to the esophageal musoca, the distal esophagus becomes inflamed and sometimes ulcerated (leading to extreme heartburn)
31
State the three functions of the stomach motility
1. Storage. Ingest food faster than can be digested, aided by receptive relaxation. 2. Physical and chemical disruption - mixing. 3. Deliver resultant chyme to intestine at optimal rate – gastric emptying These functions require complex patterns of motility Pacemaker located in longitudinal layer, greater curvature
32
Describe the role of pacemaker cells in the regulation of gastric peristalsis
Peristalsis, which is the major motor activity of the distal stomach, is initiated and regulated by specialized pacemaker cells in the mid-portion of the greater curvature. The probability of spike potentials is increased by vagal or gastrin stimulation
33
describe the events that take place during gastric motility
As the bolus of food enters the stomach, the fundus and upper body of the stomach relax to receive it. Through peristalsis, the distal stomach mixes the gastric contents with gastric secretions and slowly empties the gastric chyme into the small intestine. As peristalsis proceeds through the gastric antrum, the pyloric sphincter partially closes, resulting in retropulsion of antral contents as the antrum contracts. This process facilities the breakup of particulate matter and the mixing of food with gastric secretions. Small amounts of liquidized and digested food enter the small intestine through the narrow pyloric channel
34
Describe receptive relaxation
Increases in stomach pressure triggers dumping and reflux. Following ingestion, first ‘movement’ is relaxation of muscle - increase in fibre length without change in tone This permits increase in size without increase in intragastric pressure Receptive relaxation mediated by vagus as part of end of swallowing reflex Pressure sensors maintain pressure at abdominal levels Occurs in proximal unit
35
Describe the mixing process in the stomach
Peristalsis through strong coordinated contraction of three muscle layers in distal motor unit - pylorus and antrum Cells in longitudinal layer act as pacemakers - 3x/min after a meal Activity originates mid-stomach As spreads away distally, force and speed of contraction increases (more forceful contractions of thicker muscle walls) Little chyme forced into duodenum, but sphincter only transiently open thus most contents return back under pressure into distal regions - retropulsion (mixes and mechanical breakdown) along with grinding and churning Regulation: Distension of stomach activates mechanoreceptors, ENS/extrinsic Gastrin release in response to food in stomach also stimulates stomach motility
36
Describe the emptying process in the stomach
Terminal part - pyloric antrum markedly thickened muscle layers Pyloric sphincter controls exit Increase of chyme in stomach induces antral contractions and opening of sphincter as peristaltic wave approaches Small amount chyme ejected into duodenum and pyloric sphincter contracts Liquids leave first, solids after lag time for mechanical break-up (half time liquids 20min, solids 120min) Emptying regulated to allow for optimal intestinal digestion
37
How is stomach emptying controlled?
Small intestine releases different hormones to inhibit gastric emptying (i.e. under complex neural and hormonal control related to state of gastric contents AND events in upper duodenum) Gastric contents: Empties at rate proportional to volume, pH, physical and chemical nature Volume (stretch receptors) in stomach promotes emptying as contents become isotonic, empties more rapidly Enterogastric reflex inhibits emptying and consists of complex hormonal: Secretin, GIP (Gastric inhibitory Peptide), CCK Cholecystokinin and neural signals Via ENS and/or extrinsic nerves from duodenum to stomach wall. It stimulates pyloric contractions and also increases tone of pyloric sphincter to prevent emptying, thereby preventing the upper small intestine from being overwhelmed by material from the stomach.
38
Name two gastric motility dysfunctions
Dumping syndrome and gastroparesis
39
Describe dumping syndrome
Rapid emptying of gastric contents into the small intestine, for instance characterized by nausea, pallor, sweating, vertigo, and sometimes fainting within minutes after a meal or ingestion of a hypertonic solution.
40
Describe gastroparesis
condition characterized by impaired or absent ability of the stomach to empty. This condition is occasionally observed in severely diabetic patients who develop autonomic neuropathy. The loss of vagal stimulation to the stomach markedly impairs antral systole, preventing the proper digestion and emptying of gastric contents. These patients often complain of early satiety (feeling of being full), abnormal bloating, and nausea.
41
What are the two major motor functions along the small intestine
Mixing (segmentation) and peristalsis (propulsion)
42
Describe segmentation in the small intestine
multiple short contactions frequency varies along a proxiaml to distal gradient. The decreasing gradient of BER frequency promotes the distal movement of intestinal chimes.
43
Describe villus movements
swaying and piston-like movements - to mix and to drain lymphatics of fat absorption
44
Name and describe a small intestine motility dysfunction
Impaired small intestinal peristalsis can lead to abnormally high levels of bacteria and lead to diarrhea and/or streatorrhea (fecal fat excretion)- clinical condition called Intestinal Blind Loop Syndrome
45
describe the motility in the large intestine
Movements are slow and irregular, and serve mainly to increase contact with the absorbing surface Colon lacks continuous layer of longitudinal muscles, instead muscles are organized in 3 thick bands - taeniae coli (movement generated smilar to accordian) Segmental contractions of circular muscles divide the colon into segments called haustrations and represent the main motor activity of the colon (tone generates haustra) Mixing movements by segmentation or haustral shuffling’ The main role is to squeeze and roll to expose fecal material for reabsorption of water and electrolytes, and secretion of mucus to lubricate for expulsion Propulsive movements by short range peristalsis propels material slowly, 5-10cm/h, espacially transverse and descending Mass movement occurs when segmental contraction of the left colon disappears and a simultaneous contraction of the right colon propels its content distally. Occurs several times a day but especially after meal – referred to Gastric colic reflex When mass movement empties faecal material into rectum… induces sensation to defecate.
46
Describe the rectum and defecation
Mass movement propels faeces into rectum and distends stretch receptors to provoke defecation reflex 2 sphincters normally contracted: Internal anal sphincter involuntary External anal sphincter voluntary – skeletal muscle Afferent stimulation - leads to parasympathetic signal to relax internal sphincter If voluntary relaxation of external sphincter does not occur via pudendal nerve, reverse peristalsis returns to colon