Lecture 5: Motility of the GI Tract Flashcards
Phases of the digestive process
Ingestion Propulsion Mechanical digestion Chemical digestion Absorption Defecation
What histological layer is responsible for GI motility? What is unique about this layer in the stomach?
Muscularis externa
3 layers in stomach - longitudinal, circular, and oblique
3 special cell types (and their functions) found in the mucosal layer of the stomach
Parietal cells (release HCl and intrinsic factor)
Chief cells (release pepsinogen)
Enteroendocrine cells (gastrin)
Specialized group of cells in the intestinal wall that are involved in transmission of info from enteric neurons to smooth muscle cells. They are the “pacemaker” cells of GI smooth muscle
Interstitial cells of Cajal (ICCs)
NOTE LOCATION
ICCs act as pacemaker cells via a ______ mechanism which is conducted to smooth muscle cells as a _______ current and action potential mechanism
Slow wave; L-type Ca++
2 basic types of electrical waves found in smooth muscle of GI tract
Slow waves (Basic Electrical Rhythm)
Spikes (Spike potentials) = true APs
Describe slow waves in the GI tract
Oscillating waves of membrane depolarization that are not sufficient to completely depolarize the membrane and stimulate contraction
Not true action potentials, but slow undulating changes in RMP
Make it possible for contractions to be stimulated more easily by raising RMP closer to threshold (less negative)
Where is the rate of slow waves the lowest vs. the highest in the GI tract?
Lowest in stomach (3/min)
Highest in duodenum (12/min)
Describe spike potentials in the GI tract
True APs; occur automatically when the RMP of GI smooth muscle becomes more positive than ~40 mV
Last 10-40x as long in GI muscle as the APs in large nerve fibers
Resting membrane potential in the gut averages about _____ mV
-56
What are 3 ways of depolarizing cells in GI tract?
Stretching of the muscle
Stimulation by ACh
Stimulation by several specific GI hormones
ACh stimulates membrane depolarization in the gut. It is released by ______ axons and acts through ______ receptors, increasing the amplitude and duration of slow waves
Postganglionic; muscarinic
Major mechanism of hyperpolarization in the gut
Norepinephrine or epinephrine stimulation on fiber membrane
Contractions in smooth muscle are the result of _____ ions entering the muscle fiber. Slow waves do NOT cause these ions to enter the smooth muscle, only _____ ions.
Calcium; sodium
Without the presence of calcium, slow waves by themselves usually cause no muscle contraction. IN contrast spike potentials generated at peaks of slow waves allow significant quantities of calcium ions to enter fibers and cause the contraction.
The intensity of these contractions depends on what?
The number of APs that occur when the slow wave potential reaches threshold
2 effects of calcium entry into GI smooth muscle cell
It is responsible for the rising phase of the AP, with the falling phase being brought about by K+ efflux
Triggering contractile response
The greater the number of APs, the _____ the cytosolic Ca++ concentration
Higher
T/F: the approximately 100 million enteric neurons housed in the gut wall communicate among themselves using ALL known major classes of NTs found in the brain
True
Anatomically, stomach has 5 parts: cardia, fundus, body, antrum, pylorus. However, physiologically it behaves as a 2-component structure. What are the 2 components?
Proximal stomach = cardia, fundus, first third of body. Characterized by slow tonic contractions
Distal stomach = distal two thirds of body and antrum. Charcterized by phasic propagating contractions
_____ contractions in the stomach that are cyclic and permit mixing and propelling of GI contents
Phasic
____ contractions in the stomach are continuous and relax only under neural stimulation
Tonic
[upper region of the stomach and the sphincters that control the flow of GI contents from one region to another demonstrate tonic contraction]
T/F: the ENS controls the ENTIRE digestive system and is able to function completely on its own even when cut off from the CNS
True
The intrinsic nn. of the GI system are arranged into 2 main plexuses, how are their functions different?
Myenteric = inhibitory and excitatory nn. control the function of muscular layers which control motility
Submucosal = secretomotor neurons promote vasodilation, regulate secretion of fluid and electrolytes and contractions of the muscularis mucosa
2 mechanical processes of digestion in oral cavity
Mastication
Deglutination
During mastication, teeth break up food and salivary enzymes begin hydrolysis of ______, buffers neutralize _____, and antibacterial agents kill bacteria on food
Starch; acids
What is deglutination
Swallowing - moves bolus toward pharynx
Components of saliva and their functions
Water (majority)
Electrolytes: Na, K, Cl, PO4
Mucin - protein that forms thick slimy mucous
IgA Abs - immune defense
Lysozyme - antibacterial
Salivary amylase - breakdown of carbs
Functions of salivary amylase other than beginning the breakdown of dietary carbs
Lubricates and cleanses oral cavity
Dissolves chemicals
Suppresses bacterial growth
Pressure receptors and chemoreceptors in the mouth are involved in a ______ reflex pathway to the salivary center in the ______ of the brain, which sends signals via _____ nerves to the salivary glands which increase their secretion
Simple; medulla; autonomic
Thinking of food, seeing food, and smelling food can elicit a ______ reflex via the ______ in the brain, which signals the salivary center in the medulla in the same way pressure receptors do, leading to autonomic nerve stimulation, salivary gland stimulation, and increased salivary secretion
Conditioned; cerebral cortex
Most of the muscles of chewing are innervated by the ____ branch of the ______ nerve
Chewing is regulated by _____ nuclei
Motor; trigeminal (CN V)
Brainstem
3 major functions of chewing
Reduces size of ingested particles to facilitate swallowing
Mixes food with saliva for digestive enzymes and lubrication
Increases surface area of ingested material to increase digestion rate
In terms of neural control of swallowing, what is the voluntary phase, and when does it become involuntary?
During the voluntary oral phase, the tongue pushes a bolus of food to the back of the mouth and into the pharynx.
From there on, the process is INVOLUNTARY (aka pharyngeal phase)
Describe pharyngeal phase of swallowing in terms of neural control
Food bolus stimulates touch receptors in the pharynx
Sensory signals pass by the glossopharyngeal, vagal, and trigeminal nn. to the swallowing center in the medulla and pons
Motor impulses pass through CNs to control an involuntary process that directs food into the esophagus and away from the airway
3 phases of swallowing
Oral phase - voluntary
Pharyngeal phase - involuntary
Esophageal phase - begins after UES
During the pharyngeal phase of swallowing, the soft palate is pulled ______ and the _________ folds move inward toward one another, opening a narrow passage into the pharynx.
The larynx is moved forward and upward against the ______; preventing food entry into the ______ and helping to open the ______
The UES _______ to receive the bolus and the constrictor muscles contract strongly to force the bolus through the UES
Upward; palatopharyngeal
Epiglottis; trachea; UES
Relaxes
Difference between primary and secondary peristalsis during esophageal phase of swallowing
Primary: simply a continuation of peristaltic wave that begins in pharynx and spreads into esophagus during pharyngeal phase. Passes from pharynx to stomach in about 8-10 seconds. Regulated by medulla.
Secondary: occurs in primary fails to move food from esophagus to stomach and continues until complete esophageal emptying. Regulated by medulla and myenteric nervous system
Would peristaltic waves still exist if vagus nerves to the esophagus are cut?
Yes, secondary peristaltic waves would. Even after paralysis of brainstem swallowing reflex, food fed by tube or other way into esophagus sstill passes readily into stomach
Resting pressures are _____ at the UES and LES because both sphincters exhibit continuous resting smooth muscle tone. In the lumen of the body of the esophagus above the diaphragm, pressure is ________ because the esophagus is passing through the intrathoracic space
High; subatmospheric
Receptive relaxation is a _____ reflex that causes the muscles of the proximal stomach to relax, which facilitates entry of bolus. It allows the stomach to expand without _____ intragastric pressure
Vagovagal; increasing
During receptive relaxation, esophageal pressure _____ to match the pressure in the proximal stomach, indicating opening of the LES
The LES opens d/t the vagovagal reflex mediated by _____ neurons releasing vasoactive intestinal peptide and ______
Drops
Myenteric; NO
Muscular differences between UES and LES
UES = distinct striated circular m.
LES = smooth muscle
Between swallows, the LES is contracted, in large part by _____ _____ mechanisms
During swallowing, vagal inhibitory fibers allows the lower esophageal sphincter to relax, possibly because of release of inhibitory NTs from enteric nerves, such as ____ and ______
Vagal cholinergic
NO; VIP
Factors that increase LES tone
ACh Increased intraabdominal and intragastric pressure Gastrin Motilin Protein-rich food
Factors that decrease LES tone
NO VIP CCK GIP B-adrenergic receptor agonists Secretin Progesterone Prostaglandin E Fat-rich food
What causes GERD
Inappropriate relaxation of LES
(Due to loss of LES tone, increased frequency of transient relaxation, loss of secondary peristalsis after a transient relaxation, increased stomach volume or pressure, increased production of acid)
What condition results from degeneration of neurons in the myenteric plexuses, leading to the LES not opening fully in concert with the perstaltic wave that sweeps the bolus along the length of the esophagus so that food becomes retained at the level of the LES?
Achalasia
The nerves affected by achalasia utilize _____ to produce their inhibitory effects. Patients with achalasia lack _________ along with a decrease in other inhibitory NT _______
NO; NO synthase; VIP
Symptoms and treatment aims for achalasia
Regurgitation of food, CP, difficulty swallowing liquids and solids, cough, and weight loss
Drug tx is aimed at reducing the tone of the LES
The stomach is guarded by what 2 sphincters
LES
Pyloric sphincter
2 regions of the stomach and their functions
Orad region = fundus + proximal portion of body. Serves as reservoir and to move gastric contents to distal stomach.
Caudad region = distal portion of body + antrum, serves to grind and triturate the meal
Describe musculature and contractions in orad area of stomach, what is the consequence of this?
Thin musculature; weak contractions
Minimal contractile activity = little mixing of ingested contents in orad stomach (contents often remain relatively undisturbed layers for 1+ hours after eating)
What effect does CCK have on the orad region of the stomach?
CCK decreases contractions and increases gastric distensibility
Describe retropulsion in the caudad stomach
As contraction pushes contents toward gastroduodenal junction, peristaltic wave increases in velocity and most of the contents are propelled back into the main body of the stomach, where they remain until the next contraction sequence
Causes a thorough mixing of gastric contents and mechanically reduces size of food particles
The duration of each contraction in the caudad region of the stomach ranges between ___ and ___ seconds, and the max frequency is ____/min.
Between contractions, pressures in the caudad region are near ______ levels
2-20; 3
Intraabdominal
What factors increase contractions in the stomach?
Vagal nerve (parasympathetic) stim increases number and force
Gastrin and motilin
What factors decrease contractions in the stomach?
Sympathetic nerve activity
Secretin and GIP
Increases in gastric emptying lead to:
_____ in distensibility of orad stomach
______ in force of peristaltic contractions of caudad stomach
_____ in diameter and inhibition of segmenting contractions of proximal duodenum
Decrease
Increase
Increase
Which of the following would have the fastest time for gastric emptying: protein solution, glucose solution, or solid meal?
Glucose solution (then protein, then solid meal which takes about 3 hours)
Inhibition of emptying occurs when contractile activities of the stomach are reversed. This leads to:
_____ of orad region of stomach
____ in number and force of contractions of caudad region
Contraction of the _____
_______ in segmenting contractions of the duodenum
Relaxation
Decrease
Pylorus
Increase
Acidic chyme in the duodenum stimulates the release of _____, which reduces gastric motility and ______ the tone of the pyloric sphincter.
The products of lipid digestion stimulate the release of _____ and _____, which also reduce gastric motility
The products of protein digestion stimulate the release of ____, _____, and _____, which all slow gastric emptying
Secretin; increases
CCK; GIP
Gastrin, CCK, GIP
One of the factors in the duodenum that inhibits further emptying is the _____ of the fluid in the duodenum
Hypertonicity/hyperosmolarity
What is the function of the migrating motor complex associated with the stomach and duodenum?
Restores environment in between meals; removes mucous, sloughed cells, and bacteria from small intestine, helping to prevent bacterial overgrowth
Collection of disorders of varied etiologies in which gastric emptying is impaired or delayed without evidence of obstruction. Early symptoms include early satiety, nausea, vomiting, bloating, and upper abdominal discomfort
Gastroparesis
Primary cause of gastroparesis
Idiopathic is most common
May also result from systemic disease resulting in abnormalities of neuromuscular function, like diabetes or scleroderma
May occur as a result of surgical or medical tx that injure vagus n.
Peristaltic waves can occur in any part of the SI and move toward the anus at a rate of 0.5 to 2.0 cm/sec - ______ in the proximal intestine and _______ in the terminal intestine
Waves rarely travel farther than 10 cm
3-5 hours are required for passage of chyme from pylorus to ________ valve
Faster; slower
Ileocecal
The stimulus for peristalsis is _______, which is the myenteric reflex
Stretch releases ______, which activates IPANS that stimulate the myenteric plexus
Distension
Serotonin
What hormones enhance intestinal motility?
Gastrin CCK 5-HT Thyroxine Insulin
What hormone decrease intestinal motility?
Secretin
Glucagon
A vomiting center in the _______ coordinates the vomiting reflex. _______ information comes to the vomiting center from the ______ system, the back of the throat, the GI tract, and the chemoreceptor trigger zone in the fourth ventricle.
Medulla; afferent; vestibular
What 2 factors contribute to the ileocecal valve/sphincter’s ability to act as a barrier between the small and large intestines?
Anatomic arrangement - valve-like folds protrude from ileum into lumen of cecum
Thickening of ileal wall at last several cm, forming a sphincter that is under neural and horonal control
______ of the large intestine actively change location as a result of contraction of the circular smooth muscle layer
Haustra
Differences in musculature of internal vs. external anal sphincters
Internal = formed by circular layer of muscle fibers continuous from cecum to anal canal
External = formed by layers of striated m.
Parasympathetic innervation of the colon is divided into cranial (vagus nerve) and sacral (pelvic nerves S2-4) divisions
What are the boundaries of these divisions?
Vagus nerve = foregut and midgut ending at splenic flexure
Pelvic nerves = hindgut - descending and sigmoid colon and anorectum
Sympathetic innervation of the GI tract originates in the _____ outflow (T5-L2), and it works by inhibitory effect of _____ on the enteric nerves
Thoracolumbar; noradrenaline
The mass movements through the large intestine are a special type of _____ contraction facilitating transit. These occur 3-4 times/day generally after meals and each contraction lasts for about ___ mins
Mass movements force fecal material rapidly in mass down the colon, moving into the rectum and the _____ there initiates the defecation reflex
Peristaltic; 3
Distension
A mass movement in the LI can be initiated by ____ or duodenocolic reflexes, intense stimulation of the _____ nerves or _______ of a segment of colon
Gastrocolic; parasympathetic; overdistension
Where does 90% of water absorption take place in the GI tract?
Small intestine; but the large intestine absorbs enough to make it an important organ in maintaining the body’s water balance
By the time chyme has remained in the large intestine 3-10 hours, it has become solid or semisolid because of water absorption and is now called feces.
What is the chemical composition of feces?
Water Inorganic salts Sloughed off epithelial cells from mucosa of GI tract Bacteria Products of bacterial decomposition Unabsorbed digested materials Indigestible parts of food
Smooth muscle of the rectum and anal canal is controlled by what nerves?
Parasympathetics - S2-S4 levels of spinal cord stimulate your rectum and anal canal to contract or tighten, assisting in defecation
Sympathetics - T11-L2 levels of spinal cord. Hypogastric nerve stimulates your rectum and anal canal to relax
Nervous control of internal anal sphincter
Parasympathetics: S2, S3, and S4 levels of spinal cord cause internal anal sphincter to relax, when your rectum and anal canal contract
Sympathetics: T11-12 levels of the spinal cord cause sphincter to contract or tighten
External anal sphincter neural control
Spinal nerves from S2, S3, and S4 levels of your spinal cord
Defecation reflex when feces (stool) enters rectum, spinal cord reflex is triggered
Distension of the rectum with feces initiates reflex contractions of its musculature and the desire to defecate
Defecation involves both _____ and reflex activity
The urge to defecate first occurs when rectal pressure increases to about ____ mm Hg; when this increases to ____ mm Hg the external and internal sphincter relaxes and there is reflex expulsion contents of the rectum
Voluntary
18; 55
_____ GI reflexes = reflexes from the gut to prevertebral sympathetic ganglia and back to the gut. These type of reflexes are helpful for transmitting reflexes to far areas of the gut such as reflex from stomach to ileum or from stomach or duodenum to colon, etc.
Short
Examples of short GI reflexes
Ileogastric reflex
Enterogastric reflex
Gastrocolic reflex
Colonoileal reflex
_____ GI reflexes = travel all the way from gut to spinal cord OR brainstem and back to GI tract
Long
Examples of long GI reflexes
Vomiting reflexes
Pain reflexes
Vagovagal reflexes
Defecation reflexes
Brain center that controls swallowing reflex
Medulla
The rectosphincteric reflex and act of defecation are under neural control. Part of the control lies in the ENS, but is reinforced by activity of neurons within the spinal cord. What might the effect of spinal cord injury be on defecation?
Destruction of the nerves to the anorectal area can result in fecal retention
The sensation of rectal distension, as well as voluntary control of external anal sphincter is mediated by pathways within the spinal cord that lead to the cerebral cortex. Destruction of these pathways causes a loss of voluntary control of defecation
Megacolon physiology
Characterized by absence of ENS in distal colon (always includes internal anal sphincter and typically rectum as well)
Involved segment exhibits decreased tone, very narrow lumen, devoid of propulsive activity. As a result, the colon proximal to diseased segment becomes dilated.
Tx = resection of diseased segment
Temporal sequence events initiated by the vomiting reflex
Reverse peristalsis begins in the small intestine
Relaxation of the stomach and pylorus
Forced inspiration to increase abdominal pressure
Movement of the larynx upward and forward and relaxation of LES
Closure of the glottis
Forceful expulsion of gastric, and sometimes duodenal contents
In retching, UES remains closed, and because the LES is open, the gastric contents return to the stomach when the retch is over
