Lecture 2-Whitfield Flashcards
list and describe the layers of the tubular GI tract
- mucosa: epithelium that lines the lumen of the GI tract
- submucosa: underlying connecting tissue that contains numerous blood vessels and nerves
- muscularis: has 2 layers of smooth muscle that produce wave-like contractions (peristalsis) to propel food along the GI tract
- serosa: connective tissue outermost covering
define peristalsis
progression of coordinated contraction and relaxations of smooth muscles of the GI tract to propel food
what does the term ileus mean?
no motility/intestinal paralysis
contrast the roles of the autonomic nervous system versus the enteric nervous system in GI motility
the ANS has both parasympathetic and sympathetic innervation
parasympathetic via the vagus nerve that innervates all tract but the terminal colon (innervated by pelvic nerve via sacral spinal cord); neurotransmitter is acetylcholine and has INIDRECT effects on GI tract by stimulating the enteric nervous system first, and then acting on the tubular tract as the ENS tells it what to do
sympathetic: postganglionic fibers follow splanchnic (GI) nerves and vasculature; neurotransmitter is norepinephrine and has both direct (faster) and indirect effects on the GI tract
the enteric nervous system: myenteric plexus innervates longitudinal and circular smooth muscle layers to regulate direction, intensity, and frequency of motility; the submucosal plexus innervates (and partly the myenteric plexus too via crosstalk from interneurons between the plexi) mucosal epithelium to influence glandular secretions, hormones from GI endocrine cells, and blood flow to vascular smooth muscle; ENS crosstalks with ANS and CNS neurons via mechanoreceptors and chemoreceptors (CHECK THIS LAST PART ABOUT receptors)
name the components of the enteric nervous system
this system is independent and intrinsic to the GI tract and has the same number of neurons as neurons in the spinal cord
two systems of ganglia that crosstalk:
1. myenteric (auerbach): between muscle layers, a larger and more complex system than
2. submucosal (meissner): in the submucosa
how is motility regulated?
via efferent neurons from the enteric nervous system that are both inhibitory and stimulatory
acetylcholine, serotonin, and substance P increase GI motility
opioid peptides and dopamine decrease GI motility
what are sphincters?
rings of muscle surrounding and serving to guard or close and opening or tube
list the 6 sphincters of the GI tract in order from mouth to ass
- upper esophageal sphincter
- lower esophageal sphincter: keep stomach contents in stomach, have acid reflux if no work
- pyloric sphincter: closed until food ready to enter SI
- ileocecal valve (or ileocolic junction in dogs and cats)
- internal anal sphincter
- external anal sphincter
describe the ileocolic junction in dogs and cats
the ileum communicates with the ascending colon forming a tube that is joined by the cecum to one side
describe control of the internal anal sphincter versus the external anal sphincter
internal anal sphincter is under parasympathetic control on the ANS which then acts on the ENS to control the sphincter
external anal sphincter is under voluntary control via the pudendal nerve (removal of anal sac carcinomas could hit the pudendal nerve and cause incontinence!
list the 4 main components of the intestinal barrier
- mucus layer(s): physical barrier (an outer right at lumen and an inner deep to that); is right on top of intestinal mucosa
- biochemical
- cells: secrete antimicrobial substances for biochemical part of barrier and also brush border
- tight junctions between cells
describe the intestinal mucosa of the intestinal barrier (4)
- epithelium: 1 cell layer thick, contacts ingesta (can be simple columnar, enterocytes, or colonocytes)
- basement membrane that the cells attach to, an extracellular matrix made up of collagen
- lamina propria: loose collagen with immune cells (loose areolar connective tissue)
- muscularis mucosa/lamina muscularis: thin layer of smooth muscle
give examples of how each component of the intestinal mucosa functions to serve as a barrier
- mucus: forms a physical barrier
- biochemical: chemicals that fight bacteria like IgA and antimicrobial peptides, secreted by cells
- cells: have microvilli to form the brush border, and many types of cells secrete many antimicrobial peptides for the biochemical component; many immune cells found in the lamina propria of the intestinal mucosa
- tight junctions between cells: help contribute to physical barrier, preventing anything from entering between the cells while still allowing for absorption, transport, and movement of enzymes
describe the villi, microvilli situation between the small and large intestine
all cells in both small and large intestines have microvilli that form a brush border, but cells in the small intestine also have villi, that increase the surface area for digestion and absorption
describe mucosal structure
the mucosa of the intestines forms crypts, these crypts contain stem cells that migrate toward the lumen and differentiate, terminally, then die an are sloughed into the lumen of the crypt in about 3-5 days, so this is a constant process
