Motility of the GI tract Flashcards
Purpose of motility control in the gut [3]
- Different parts of the gut have different functions.
- Food has to be delivered at the right time and appropriate composition.
- Food has to stay in a different compartment for the appropriate amount of time.
Mucosa of the gut wall
The innermost layer of the gut wall.
Contains the different layers:
- Epithelium
- Lamina propria
- Muscularis mucosa
Lamina propria
The middle layer of the mucosa in the gut wall.
Contains:
- Blood vessels
- Loose connective tissue
Muscularis mucosa
The bottom layer of the mucosa in the gut wall.
- Composed of a thin layer of smooth muscle.
When the smooth muscle contracts and causes the mucosa to fold.
Layers of the GI tract
Mucosa
Submucosa
Muscularis externa
Serous (adventitia)
Submucosa of the gut wall
The second innermost layer of the gut wall.
Contains:
- Loose CT
- Large blood vessels
- Lymphatics
- Meissners plexus/ submucosal nerve plexus: nerve plexus that regulates blood flow and secretions.
Muscularis externa (propris)
The outer layer of the gut wall composed of three layers:
- Circular muscle: can thicken to form sphincters.
- Myenteric (Auerbach) plexus: regulates motility.
- Longitudinal muscle: 3-5x thinner than circular muscle.
Serosa/ adventitia of the gut wall
The outermost layer of the gut wall.
Serosa: When organ is intraperitoneal.
- Serosa is the visceral peritoneum, composed of mesothelium
Adventitia: Organ is retroperitoneal
- Outer layer is connective tissue.
Enteric nervous system / Intrinsic nervous system:
- Function
- Plexuses
System of nerves that control the GI tract.
Contains two plexuses:
- Myenteric: controls motility.
- Submucosal: secretory functions.
Ganglia in both plexuses are connected by nerves to the extrinsic NS.
A lot of the gut activity is coordinated by short intrinsic reflexes within the ENS.
Extrinsic pathways of the gut
Afferent and efferent fibres that connect the ENS to the CNS.
Composed of both parasympathetic and sympathetic pathways.
Afferent fibres of the extrinsic pathways in the GI.
- Receptors
- Loops
Receive stimuli from chemo/mechanoreceptors in the mucosa.
Fibres send back to the CNS- long loop.
Or form a short loop when it is formed within plexus- reflex arc.
Hirschsprung’s disease
A birth defect where the myenteric plexus is absent in the colon.
Causes severe constipation
Sympathetic innervation of the gut.
- Preganglionic fibres
- Activity
Part of the extrinsic innervation.
Preganglionic fibres in T8-L2.
Activity:
- Inhibits gut motility
- Inhibits secretion
- Constricts sphincters
Postganglionic cell bodies of the sympathetic innervation of the gut.
- Location of ganglia
- Structures innervated.
These are located in these ganglia:
- Celiac
- Inferior
- Superior mesenteric
- Hypogastric
They innervate structures like the smooth muscle and secretory cells.
Parasympathetic innervation of the gut.
- Nerves
- Activity
- Structures innervated
Vagus nerve innervates the foregut and midgut: oesophagus down to the transverse colon.
Pelvic splanchnic nerve innervates the remaining part of the colon via hypogastric plexus:
Activity:
- Stimulate motility
- Stimulates secretions
Smooth muscle in motility
Act as a functional syncytium.
The cells show pacemaker activity with slow spontaneous depolarisations that initiate contracts.
Basic electrical rhythm/ slow wave rhythm
This is the slow, spontaneous depolarisation and repolarisation of pacemaker cells in smooth muscles of the:
- Stomach
- Small intestines
- Colon
The frequency of BER differs according to the area of the gut
Segmentation
- Action
- Function
- Organ
A type of contractile response initiated by circular smooth muscle.
Mainly occurs in the small intestines.
Function:
Mixes food and enxymes
Action:
Closely space contractions of the circular smooth muscle—> then relaxation.
Peristalsis
- Type of SM
- Trigger
- Action
- Function
A contractile response of both longitudinal and circular smooth muscle.
Function: propel food along GI tract.
Contraction triggered by distention of the gut by food.
Action:
- Longitudinal SM [LSM] contraction first.
- Halfway through the first contraction, circular SM contracts.
- LSM relaxes halfway during CSM contraction.
Deglutition
- Voluntary action
- Autonomic control (afferent and efferent)
Swallowing- has voluntary and involuntary control.
Voluntary- mastication forms bolus of food, which is pushed to the oropharynx as the tongue moves up and back against hard palate.
Autonomic control:
Food stimulates mechanoreceptors in the pharynx —-> Glossopharyngeal IX [CN IX] send impulses to the swallowing centre.
Efferent impulse:
Vagus nerve send impulse to pharynx, oesophagus and palate for coordinated muscle contraction.
Oesophagus
Structure:
- 25 cm long
- Upper 1/3 skeletal muscle, the rest smooth muscle.
Swallowing:
Sphincters relax during swallowing.
Upper oesophageal sphincter (UES)
Also called the cricopharyngeal muscle.
Located in the upper end of the oesophagus.
Usually always closed to prevent entry of air.
During swallowing it relaxes.
Lower oesophageal sphincter (LES)
Sphincter at the gastro-oesophageal muscle of the oesophagus.
When closed: prevents reflux of gastric contents.
Relaxes during swallowing
3 functions and motilities of the stomach
Storage:
Ingests food faster than digestion- receptive relaxation
Physical and chemical disruption- through mixing.
Delivers resultant chyme to intestine at optimal rate- emptying
Receptive relaxation
- Innervation
- Area of stomach
The type of motility is mediated by the vagus nerve- occurs in the proximal motor unit.
- After ingestion, stomach muscles relax to increase fibre length.
- This increases stomach size without changing intragastric pressure
Regulation of receptive relaxation
Mediated by vagus nerve
Pressure sensors at abdominal levels maintain pressure whilst stomach size changes.
Mixing in the stomach
Peristalsis:
- Occurs in the pylorus and Antrum.
- Longitudinal muscle cells act as a pacemaker: 3x/min after meal
- Contraction originates in the mid-stomach and spreads distally.
- Force and speed on contraction increases.
Retropulsion:
Some chyme is pushed into the duodenum then renters the antrum.
This mixes food and enzymes and helps to mechanically breakdown food.
Regulation of mixing in the stomach
ENS/ extrinsic:
Activated by mechanoreceptors in the stomach that sense distension.
Gastrin release:
Activated by food in the stomach= stimulates stomach motility
Emptying of the stomach
Stimulated by the increase of chyme in the stomach.
Peristaltic waves stimulate antral contraction and the opening of the pyloric sphincter.
The ejects small amount of chyme into duodenum
- liquids leave first
- then solids
How gastric contents control stomach emptying
Emptying is influenced by the volume, pH, physical and chemical nature of the gastric contents.
Volume: increased volume is sensed by stretch receptors nd promotes emptying.
Solute concentration:
Increase in isotonicity causes more rapid emptying.
How events in the duodenum control stomach emptying
Enterogastric reflex: slows emptying
- presence of fatty acids in duodenum
- low pH= secretin release
Nerves: ENS from duodenum to stomach wall
Hormones: CCK GIP gastrin secretin
Increase tone of pyloric sphincter and stimulate pyloric contractions.
Hormones that decrease gastric emptying
Secretin- stimulated by low pH
CCK- stimulated by fats
Gastric inhibitory peptide- stimulated by fats
Gastrin- stimulate by amino acids/ peptides
Motility in small intestines
Segmentation: varying rate according to area of intestines
Peristalsis: BER contractions
Villus movements: mixes and drains lymphatics of fat absorption
MMC
Migrating motor complexes
Waves of electrical activity that sweep throughout small intestines regularly during fasting—> peristalsis
Moves indigestible food and bacteria to the colon.
Responsible for the rumbling heard during hunger.
Control of small intestine motility
Extrinsic input and enteric system
Motility in the ascending and transverse colon.
Here chyme is converted to feces:
- Ascending and transverse mainly liquid.
Mixing movements:
Mainly in ascending and transverse- haustral shuffling, which is segmentation.
- This exposes fecal material for water and electrolyte absorption
Secretion of mucus—> excretes mucus
Muscle layers in the colon
Different from usual:
- Pacemaker in submucosal muscle
- Longitudinal muscle is now taeniae coli. Tone of this muscle forms haustra (small sacs in the colon).
Motility in the descending/transverse and sigmoid colon.
Propulsive movement stimulated by short range peristalsis.
Mass movement:
- Extended peristaltic contraction (5cm/min)
- Gastrocolic and duodenocolic reflexes causes extended peristaltic contractions.
Rectum and defecation
Mass movement that propels faeces into the rectum distends stretch receptors
- defecation reflex
Afferent stimulation (parasympathetic)= relax of internal sphincter
Voluntary relaxation of external sphincter
This occurs via the pudendal nerve.
If this relaxation does not occur, this reverses peristalsis to the colon.
Sphincters in the anus
Involuntary:
Internal anal sphincter
Voluntary:
External anal sphincter (skeletal muscle)
These sphincters are normally contracted.
