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
