T1 L4 Motility of GI tract Flashcards
What are the 4 functions of the GI system?
1) Digestion
2) Secretion
3) Absorption
4) Motility
Definition of secretion
Delivery of appropriate fluid & enzyme solutions
Definition of absorption
Nutrient molecules are transported into the circulatory system
Definition of digestion
Macromolecules in food are hydrolysed
Definition of enteric nervous system
Large intrinsic network of neutrons in the walls of the gastrointestinal tract
Definition of transit time
Time taken for food to reach each indicated point after ingestion
Sphincters
Structures that separate the digestive tracts from each other
Made if smooth muscle
Act as valve of a reservoir for holding luminal content before emptying into next segment
Dysregulation of sphincters
GI motility disorders Gastroparesis Dumping syndrome Achalasia GERD
Dysregulation of GI motility or secretion
GERD
Peptic ulcer
Diarrhoea
General structure of gut wall
Mucosa
Submucosa
Muscularis externa
Serosa
Structure of mucosa
Epithelial layer with exocrine & endocrine gland cells
Lamina propria
Muscularis mucosa
What do exocrine gland cells in mucosa of gut wall do?
Secrete mucus & digestive enzymes
What do endocrine cells in mucosa of gut wall do?
Release GI hormones into the blood
What makes up the lamina propria?
Small blood vessels
Nerve fibres
Gut-associated lymphatic tissue which secretes antibodies to specific food or bacterial antigens & triggers immune response
Loose connective tissue
Structure and function of muscular mucosa?
Thin layers of smooth muscle
Control mucosal blood flow & GI secretion
Structure of submucosa
Loose connective tissue Large blood vessels Lymphatic vessels Glands Submucosal nerve plexus - Meissners plexus
Structure of muscular externa
Thick muscle - 2 layers of smooth muscle cells. Inner circular & outer longitudinal
Myenteric nerve plexus - Auerbach’s, which lies between muscle layers & regulates motility
Structure & function of serosa
Connective tissue
Connects to abdominal wall & supports GI tract in abdominal cavity
Major structures enter through serosa - blood vessels, extrinsic nerves & ducts of large accessory exocrine glands
Myenteric (Auerbach’s) plexus
Controls gut motility
Found between circular & longitudinal muscle layers
Thin layer of ganglia, ganglion cells & inter-ganglionic nerve tracts
Innervates longitudinal & outer lamella of circular smooth muscle layers
Some neutrons have projections into adjacent muscle layers
Some are interneurons
Submucosal (Meissner’s) plexus
Intestinal absorption & secretion
Neuronal array found between submucosal layers & circular muscle
Neurons are functionally distinct
Appear to project to inner lamella of circular muscle layer
Innervate glandular epithelium, intestinal endocrine cells & submucosal blood vessels
Hirschsprung’s disease
Congenital absence of myenteric plexus, usually involving part of distal colon
Pathologic section of the large bowel lacks peristalsis & undergoes continuous spasm
Leads to functional obstruction & severe constipation
Intrinsic pathway
Innervation needed for motility & secretory action
Enteric nervous system
Consists of submucosal & myenteric plexus
Extrinsic pathway
Autonomic nervous system
Linked to CNS via sensory & motor nerves of parasympathetic & sympathetic nervous system
Sympathetic innervation
Extrinsic pathway Inhibits gut motility & secretion Constricts sphincters Preganglionic fibres from T8-L2 Postganglionic cell bodies in coeliac, inferior & superior mesenteric, hypogastric ganglia
Parasympathetic innervation
Stimulates motility & secretions by increasing the activity of ENS neurons
Vagus nerve
Pelvic nerve
What does the vagus nerve supply?
Oesophagus Stomach Small intestine Liver Pancreas Cecum Appendix Ascending colon Transverse colon
What does the pelvic nerve supply?
Remainder of colon via hypogastric plexus
Types of gastrointestinal movement
1) Segmentation
2) Tonic contraction
3) Peristalsis
What is the function of segmentation?
Mixes food with enzymes
Mainly occurs in small intestines
Closely spaced contractions of circular smooth movements followed by relaxation
No net forward movement
What is the function of tonic contraction?
Separation
What is the function of peristalsis?
Propulsion
What is the mechanism of peristalsis?
Longitudinal smooth muscle contracts
Circular muscle contracts
Longitudinal relaxes during latter half of circular contraction
What is the migrating motor complex?
Pattern of motility that occurs every 90 minutes between meals
Intervals of strong, propulsive contractions which pass down the distal stomach & small intestine
Intrinsic property that doesn’t require external innervation
What is the function of the migrating motor complex?
Sweeps the stomach of indigestible materials
What is paralytic ileus?
Temporary cessation of gut motility that is commonly caused by abdominal surgery
Causes of paralytic ileus
Infection or inflammation in abdominal cavity
Electrolyte abnormalities
Drug ingestion
What are the signs & symptoms of paralytic ileus?
Nausea & vomiting
Abdominal distention
Absent bowel sounds
What are the 3 stages of swallowing?
1) Voluntary stage
2) Pharyngeal stage
3) Oesophageal stage
What is the voluntary stage of swallowing?
Bolus is formed in the mouth by mastication & passed into oropharynx
The tongue moves upward & backwards against the palate
What is the pharyngeal stage of swallowing?
Involuntary passage of bolus through the pharynx into the oesophagus
1) Bolus stimulates mechanoreceptors in pharynx, which send impulses to deglutition centre in medulla oblongata & lower pons of brain stem
2) Returning imposes cause soft palate & uvula to move upwards & superior constrictor of pharynx contracts to close off nasopharynx
3) Larynx rises so epiglottis covers trachea
4) Respiration is inhibited
5) Bolus moves through oropharynx & laryngopharynx
6) upper oesophageal sphincter relaxes & bolus moves into oesophagus
What is the oesophageal stage of swallowing?
Involuntary passage of bolus through oesophagus into stomach
Starts once bolus enters oesophagus
Peristalsis wave is initiated in pharynx & continues length of oesophagus to push bolus onwards
What is the composition of the upper 1/3 of the oesophagus
Skeletal, striated muscle
What is the composition of the lower 2/3 of the oesophagus?
Smooth muscle
Achalasia
Condition characterised by dysphagia resulting from a failure of the lower oesophageal sphincter to relax leading to a functional obstruction.
Loss of peristalsis of oesophageal body
Unknown cause
Gastro-oesophageal reflux disease
Occurs when lower oesophageal sphincter is incompetent & allows the flow of gastric juices & content back into the oesophagus
Gastric juices are corrosive to the oesophageal mucosa leading to the distal oesophagus becoming inflamed & ulcerated causing heartburn
What are the 4 regions of the stomach?
Cardia - surrounds opening of oesophagus into stomach
Fundus - primary function is for storage
Body - inferior to fundus
Pyloric part
What regions is the pyloric part of the stomach divided into?
Pyloric Antrum - connects to body of stomach
Pyloric canal
Pylorus - connects to duodenum
What are the functions of the stomach?
Storage
Physical & chemical disruption - mixing
Deliver resultant chyme to intestine at optimal rate
What is the reason for the stomach having a storage function?
Food is ingested faster than it can be digested
Mainly in the funds
What is the movement of food through the stomach?
1) Bolus of food enters the stomach & funds & upper body relax to receive it
2) Through peristalsis, the distal stomach mixes the gastric contents with gastric secretions
3) The gastric chyme is slowly emptied into the small intestine
4) Peristalsis proceeds through the gastric antrum, the pyloric sphincter partially closes leading to retropulsion of astral contents as the Antrum contracts
What is receptive relaxation in the stomach?
The increase in stomach pressure triggers dumping & reflux
After ingestion, the first movement is relaxation of muscle by an increase in fibre length without a change in tone
Mediated by vagus as part of the end of swallowing reflex
Pressure sensors maintain pressure at abdominal levels
Occurs in proximal motor unit - body & funds of the stomach
What is mixing in the stomach?
Peristalsis through strong coordinated contraction of 3 muscle layers in the distal motor unit - pylorus & Antrum
Activity originates in mid stomach & spreads away distally
A little chyme enters duodenum but most of the contents are returned under pressure to the distal regions - retropulsion
What is emptying in the stomach?
Terminal part - pyloric Antrum has thickened muscle layers
Pyloric sphincter controls exit
Increase in chyme In stomach induces antral contractions & opening of sphincter
Small amount of chyme is ejected into the duodenum & pyloric sphincter contracts
Liquids leave first, solids after a lag time for mechanical break up
What is the enterogastric reflex?
Inhibits emptying & consists of complex hormonal & neural signals
Stimulates pyloric contractions & increases tone of pyloric sphincter to prevent emptying
Prevents upper small intestine being overwhelmed by material from the stomach
What is the hormonal regulatory pathway for gastric emptying?
Presence of fatty acids/monoglycerides in the duodenum, low pH
Leads to release of hormones - secretin, GIP, CCK
What is the neural regulatory pathway for gastric emptying?
Via ENS and/or extrinsic nerves from duodenum to stomach wall
Fear, anger, depression produce a change in gastric motor ability
What are the symptoms of dumping syndrome?
Nausea, pallor, sweating, vertigo & fainting within minutes after a meal or ingestion of a hypertonic solution
What is gastroparesis?
Impaired or absent ability of the stomach to empty
Occasionally observe in severely diabetic patients who develop autonomic neuropathy
Loss of vagal stimulation in the stomach impairs antral systole, which prevents proper digestion & emptying of stomach contents
What are the symptoms of gastroparesis?
Early satiety
Abnormal bloating
Nausea
What are the functions of the small intestine?
Mixing
Peristalsis
How does the small intestine protect itself as it is exposed to hostile environments?
The gut can develop non-immunological defence mechanisms: Gastric acid secretions Intestinal mucin Epithelial cell permeability barrier Gut peristalsis
What is the consequence of impaired peristalsis in the small intestine?
Abnormally high levels of bacteria leading to diarrhoea and/or steatorrhoea
What are the taeniae coli?
3 thick bands of longitudinal muscle in the large intestine
What is the main role of the large intestine?
Squeeze & roll to expose faecal matter for reabsorption of water & electrolytes.
Secretes mucus to lubricate for expulsion
What is the mechanism for defecation?
1) Mass movement propels faeces into the rectum
2) Faeces distend the stretch receptors which transmit signals to the spinal cord to provoke defecation reflex
3) Spinal reflex stimulates contraction of the rectum & relaxes internal anal sphincter
4) Impulses from the brain prevent defecation by keeping external anal sphincter closed
5) Defecation occurs once external anal sphincter relaxes
If voluntary relaxation of the external sphincter doesn’t occur via the pudenal nerve then reverse peristalsis returns material to colon
What is the basic electrical rhythm?
Slow wave rhythm in gastrointestinal smooth muscle
How is smooth muscle contraction initiated?
Calcium entry occurs during spike potentials, which are trigger if the peak of a slow wave depolarises the membrane to threshold potential
ENS determines when slow waves will produce spike potentials & give rise to a contraction
What do inhibitory transmitters do?
Open potassium channels in smooth muscle cells
Hyperpolarise membrane potential to prevent slow wave reaching threshold
Regulates smooth muscle contraction
What side effects do anticholinergic drugs causes?
Xerostomia
Constipation
Ileus
Nausea & vomiting
