Physiology Review Flashcards
Histological layers of GI tract
Epithelium Lamina propria Muscularis mucosae Submucosa Submucosal plexus Circular smooth muscle Myenteric plexus Longitudinal smooth muscle Serosa
What are the components of the ENS and where does it receive info from?
Comprises the myenteric and submucosal plexuses
Receives info from parasympathetic and sympathetic nervous systems and CNS, as well as mechanoreceptors and chemoreceptors in the mucosa
Roles of CNS in regulation of GI function
Vagovagal reflex (e.g., gastric receptive relaxation reflex)
Modulates ENS responses
Centers that control food intake are located in the brain (NTS communicates with gut via vagal innervation — sensory ganglion of vagal nerve is nodose ganglion)
Parasympathetic innervation of the GI tract is via the ____and ____ nerves.
Preganglionic cell bodies are located in the _____ or the _____
Postganglionic neurons lie in the wall of the organ (enteric neurons)
Synapse between pre and post ganglionic cells is ______
Vagus; pelvic
Brainstem; sacral SC
Nicotinic (nAChRs)
Sympathetic innervation of the GI tract is via nerves running between the SC and the prevertebral ganglia, and between these ganglia and GI organs.
Preganglionic efferent fibers arise within the SC and end in prevertebral ganglia
Postganglionic fibers from the prevertebral ganglia innervate the myenteric and submucosal plexuses
Mostly, preganglionic efferent fibers release ___, while postganglionic efferent fibers release ____
ACh; NE
Action of peptides like somatostatin or messenger molecules like histamine in the GI tract is via _____ signaling
Paracrine
______ cells in the gut contain secretory granules filled with hormones that are released upon stimulation
Hormones then enter the _____ circulation, passing through the liver then entering systemic circulation and traveling to the target cell
Enteroendocrine
Portal
Stimuli of secretion, site of secretion, and action of the following GI hormone:
Gastrin
Stimuli of secretion: small peptides and aa, distention of stomach, vagal stimulation (via GRP)
Site of secretion: G cells of stomach
Actions: increased gastric H+ secretion; stimulates growth of gastric mucosa
Stimuli of secretion, site of secretion, and action of the following GI hormone:
CCK
Stimuli of secretion: small peptides and aa, fatty acids
Site of secretion: I cells of duodenum and jejunum
Actions: increased pancreatic enzyme secretion; increased pancreatic bicarb section, stimulates contraction of gallbladder and relaxation of sphincter of oddi, stimulates growth of exocrine pancreas and gallbaldder, inhibits gastric emptying
Stimuli of secretion, site of secretion, and action of the following GI hormone:
Secretin
Stimuli of secretion: H+ in duodenum, fatty acids in duodenum
Site of secretion: S cells of the duodenum
Actions: increased pancreatic bicarb secretion, increased biliary HCO3 secretion, decreased gastric H+ secretion, inhibits trophic effect of gastrin on gastric mucosa
Stimuli of secretion, site of secretion, and action of the following GI hormone:
Glucose-dependent insulinotropic peptide (GIP)
Stimuli of secretion: fatty acids, aa, oral glucose
Site of secretion: duodenum and jejunum
Actions: increased insulin secretion from pancreatic beta cells (incretin effect), decreased gastric H+ secretion
What is the incretin effect?
The ability of a GI hormone to promote secretion of insulin (GIP is classified as an incretin)
____ and ____ mediate the incretin effect
GLP-1; GIP
Source and actions of the following on the GI tract:
ACh
Source: cholinergic neurons
Actions: contraction of smooth muscle, relaxation of sphincters, increased salivary secretion, increased gastric acid secretion, increased pancreatic secretion
Source and actions of the following on the GI tract:
NE
Source: adrenergic neurons
Actions: relaxation of smooth muscle wall, contraction of sphincters, increased salivary secretion
Source and actions of the following on the GI tract:
Vasoactive intestinal peptide (VIP)
Source: neurons of the ENS
Actions: relaxation of smooth muscle, increased intestinal secretion, increased pancreatic secretion
Source and actions of the following on the GI tract:
NO
Source: neurons of the ENS
Action: relaxation of smooth muscle
Source and actions of the following on the GI tract:
GRP
Source: vagal neurons of gastric mucosa
Actions: increased gastrin secretion
Source and actions of the following on the GI tract:
Enkephalins
Source: neurons of the ENS
Actions: contraction of smooth muscle, decreased intestinal secretion
Source and actions of the following on the GI tract:
Neuropeptide Y
Source: neurons of the ENS
Actions: relaxation of smooth muscle, decreased intestinal secretion
Source and actions of the following on the GI tract:
Substance P
Source: co-release with ACh by neurons of the ENS
Actions: contraction of smooth muscle, increased salivary secretion
Slow waves vs. APs
Slow waves are NOT APs - they consist of depolarization and repolarization of membrane potential
APs occur when the depolarization moves the membrane potential to or above threshold (to more positive membrane potentials) —> mechanical response
[modulated by neural and hormone activity]
Phasic vs. tonic contractions in GI tract
Phasic: periodic contractions followed by relaxation [esophagus, stomach (antrum), small intestine, and all tissues involved in mixing and propulsion]
Tonic: maintain a constant level of contraction without regular periods of relaxation [stomach (orad), lower esophageal, ileocecal, and internal anal sphincters]
What factors increase vs. decrease the amplitude of slow waves and # of APs?
Stretch, ACh, and parasympathetics increase
NE and sympathetics decrease
Pacemaker cells for GI smooth muscle
Interstitial cells of Cajal (ICC) in myenteric plexus
[ICCs generate and propagate slow waves, which occur spontaneously in the ICC and spread rapidly to smooth muscle via gap junctions, driving the frequency of contractions]
Phases of swallowing
Oral phase = voluntary
Pharyngeal phase = involuntary (soft palate pulled up, epiglottis moves, UES relaxes, contraction initiated in pharynx, food propelled through UES)
Esophageal phase = involuntary (control by swallowing reflex and ENS, consists of primary and secondary peristaltic waves)
What region of the brain controls the involuntary swallowing reflex?
Medulla
[food in the pharynx —> afferent sensory input via vagus/glossopharyngeal n.—>medulla—>brainstem nuclei—>efferent input to pharynx]
Differentiate primary from secondary peristaltic waves; which one can still occur after vagotomy?
Primary: continuation of pharyngeal peristalsis, controlled by medulla, CANNOT occur after vagotomy
Secondary: occurs if primary wave fails to empty esophagus or if gastric contents reflux into esophagus; medulla and ENS involved, can occur in absence of oral and pharyngeal phases - occurs even after vagotomy
Opening of the LES is mediated by peptidergic fibers in the ____ nerve and is accompanied by release of ______
Vagal; VIP
Receptive relaxation occurs in the __ region of the stomach, which is the process of decreasing pressure and increasing volume of this region — aka the ____ reflex
Orad; vagovagal
Large particles of undigested residue remaining in the stomach are emptied by ______ ____ ____, which are periodic, bursting peristaltic contractions that occur at 90 min intervals during fasting; ____ plays a significant role in mediating the complex and these contractions are inhibited during feeding
Migrating myoelectric complex (MMC); motilin
Does the rate of gastric emptying increase by increasing or decreasing the following?
_____ distensibility of the orad stomach
______ force of peristaltic contractions of the caudad stomach
_____ tone of the pylorus
____ the diameter and inhibition of segmenting contractions of the proximal duodenum
Decreased
Increased
Decreased
Increased
[gastric emptying takes ~3 hrs]
What is the enterogastric reflex
Negative feedback from the duodenum slows down the rate of gastric emptying
—Acid in duodenum —> stimulate secretin release —> inhibit stomach motility via gastrin inhibition
—Fats in duodenum —> stimulate CCK and GIP —> inhibit stomach motility
—Hypertonicity in duodenum —> (unknown hormone) —> inhibit gastric emptying
Describe electrical activity of ICCs and smooth muscle cells in intestine
Slow wave activity always present whether contractions occurring or not
Unlike the stomach, slow waves themselves do NOT initiate contractions in the SI; spike potentials (AP) are necessary for muscle contraction to occur and slow wave frequency sets max frequency of contractions
What hormones stimulate vs. inhibit contractions in the intestine?
Stimulate: serotonin, prostaglandins, gastrin, CCK, motilin, insulin
Inhibit: epinephrine, secretin, glucagon
What is the rectosphincteric reflex?
As the rectum fills with feces, SM wall of the rectum contracts and internal anal sphincter relaxes
It is under neural control, primarily by ENS and reinforced by activity of neurons in SC
The vomiting reflex is coordinated in the _____; nerve impulses are transmitted by vagus and _______ afferents to multiple brainstem nuclei
Medulla; sympathetic
Events in vomiting reflex
Reverse peristalsis in the small intestine
Stomach and pylorus relaxation
Forced inspiration to increase abdominal pressure
Movement of the larynx
LES relaxation
Glottis closes
Forceful expulsion of gastric contents