Lecture 4 Flashcards
(46 cards)
What are sphincters
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.
Name the four main layers of the gut wall
mucosa, submucosa, muscularis externa, serosa
Describe the layers of the mucosa
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)
Describe the function of the gut associated lymphatic tissue (GALT)
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)
Describe the submucosa and its layers
loose CT, large BVs, lymphatic vessels
Glands in some GI regions
Submucosal nerve plexus - Meissners plexus - regulates blood flow and secretion
Describe the muscularis externa and its layers
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
Describe the serosa
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
Describe the GI intrinsic pathway
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.
Describe the GI extrinsic pathway
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
Describe the extrinsic parasympathetic innervation
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.
Describe the intrinsic sympathetic innervation
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
Describe the Myenteric/Auerbach’s plexus
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
Describe the submucosal/Meissner’s plexus
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
Name and describe a GI innervation dysfunction
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.
Describe basic electrical rhythm /slow wave rhythm
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)
Describe spike potentials
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
What is the major function of BER/SWR
Major function of BER to determine when contractions can occur in a certain area of a bowel.
What is the force of contraction determined by?
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
What is the origin of the pacemaker activity?
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.
What affects the appearance and frequency of spike potentials
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
What are the three types of GI movement
Segmentation, tonic contraction and peristalsis.
Describe segmentation
Mainly small intestine, for mixing food with enzymes.
Closely spaced contractions of circular smooth followed by relaxation.
No net forward movement.
Describe peristalsis
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
describe the migrating motor complex
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.