Motility of the GI Tract Flashcards
What is the landmark between the upper and lower GI Tract?
Ligament of Treitz
Suspensory muscle of the duodenum
The GI Tract and its accessory organs
7 Primary Functions of the GI Tract:
- Ingestion
- Digestion
- Motility
- Secretion
- Absorption
- Storage/Elimination
- Protective Functions
IDMSASEP
GI Tract I Primary Functions
GI Tract II Primary Functions
7 Primary Functions of the GI Tract: Ingestion:
Ingestion of food/formation of bolus
7 Primary Functions of the GI Tract: Digestion:
mechanical/chemical digestion of foodstuff breaks up food particles
7 Primary Functions of the GI Tract: Motility:
movements of organs and the bolus
mechanical digestion of food particles
7 Primary Functions of the GI Tract: Secretion:
secretion of digestive agents
permits chemical digestion of food particles
7 Primary Functions of the GI Tract: Absorption:
absorption of the products of digestion to blood or lymphatic vessels
7 Primary Functions of the GI Tract: Storage/Elimination:
undigested food particles
7 Primary Functions of the GI Tract: Protective Functions:
mechanical, chemical, immunological
not only of the GI Tract organs but also the body, against the potential harmful food components
GI Tract Basic Four Layer Structure
Structural Differences are linked to variations in function:
Motility depends on
co-ordinated motility
Motility is governed by
motility is governed by the contraction of smooth muscle
Exception to motility being governed by smooth muscle contraction is
- upper oesophagus and external anal sphincter
- which are governed by voluntary control of striated
muscle
Arrangement of smooth muscle cells to allow coordinated motility
Smooth muscle consisting of a single unit - gap junctions allow electrical coupling and contraction as a functional SYNCYTIUM
Smooth muscle is organised into connected bundles of outer longitudinal and inner circular smooth muscle in the muscularis layer allowing involuntary contraction
Smooth muscle
What are the two types of smooth muscle contraction?
- Segmentation for mixing
- Peristalsis for propulsion
Two broad types of smooth muscle contraction: segmentation for mixing:
- bursts of circular muscle contraction and relaxation
- back and forth pendular movements occur
Two broad types of smooth muscle contraction: peristalsis for propulsion:
- local distension triggers contraction behind the bolus and relaxation in-front
- requires functional myenteric plexus
What type of smooth muscle contraction is depicted in the diagram below?
peristalsis for propulsion
What type of smooth muscle contraction is depicted in the diagram below?
segmentation for mixing
Three stages of swallowing: deglutition:
1) Oral: voluntary initiation of swallowing in the oral
cavity
2) Pharyngeal: involuntary passage of food through the
pharynx into the oesophagus
3) Oesophageal: involuntary passage of food through
pharynx to stomach
Oral phase of swallowing: deglutition:
- under voluntary control (all other mechanisms are
under involuntary control) - the tongue pushes against hard palate and contracts
to force the lubricated bolus into oropharynx
Oral Phase and Pharyngeal phase
Pharyngeal phase of swallowing: deglutition:
- bolus causes glossopharyngeal stimulation of
swallowing center in the medulla oblongata - motor efferents in trigeminal, glossopharyngeal and
vagal nerves (CN V, IX, X) causes a series of muscular
contractions: - soft palate elevates over posterior nares
- epiglottis closes of larynx
- respiration is temporarily inhibited
- upper oesophageal sphincter relaxes
- pharyngeal muscle contraction propels the bolus into
the oesophagus
Oesophageal phase of swallowing: deglutition:
- Primary peristaltic wave: striated upper 1/3 and
smooth muscle contraction - Secondary peristaltic wave (stretch): intrinsic
myenteric nerves - Vagal excitation
- Vagal inhibition for receptive relaxation of lower
oesophagus and lower oesophageal sphincter - Peristalsis is facilitated by the ENS and intrinsic
myenteric plexus and extrinsic vagal control
Oesophageal phase
What lubricates the bolus to aid movement through the oesophagus?
- multiple mucous glands in the submucosal laer
produce a lubricating mucous to aid movement of the
bolus
Where are sphincters located in the oesophagus?
at both the superior and inferior end of the oesopagus
What is the purpose of the sphincters at either end of the oesophagus?
the sphincters close the oesophagus at times that food is not being swallowed
prevent backward movement “up” the GI Tract
Propulsion of bolus through the oesophagus:
Consists of circular and longitudinal smooth muscles which line the oesophagus; these circular muscles are embedded within the longitudinal muscles causing the lumen to reduce.
The bolus stimulates stretch receptors within the oesophagus permitting the ENS to identify the location of bolus; this coordinates peristalsis and propels the bolus toward the lower oesophageal sphincter [LOS].
As the bolus approaches the LOS, pressure in the oesophagus begins to overcome the pressure gradient between the LOS permitting the bolus entry into the stomach. Without this change in pressure, the LOS remains closed to prevent acid reflux
Smooth muscle valves separate the regions of the GI Tract through tonic contraction.
Why does the GI Tract require sphincters?
To prevent backward movement of the contents of the GI Tract
GI Sphincters
Enteric Nervous System
- located only in GI tissue
- Two complex plexi:
- Myenteric (Auerbach’s) plexus
- Submucosal (Meissner’s) plexus
- a complete reflex circuit, which can function
independent of the CNS; truly autonomous, can
function even after all central connections are severed
The ENS is an ——– control mechanism
intrinsic
Myenteric (Auerbach’s) plexus
controls motility/contraction
located in the muscular layer
Submucosal (Meissner’s) plexus
controls glandular secretions/blood flow
located in the submucosal layer
Enteric Nervous System
The myenteric (Auerbach’s) plexus and Submucosal (Meissner’s) plexus are interconnected in the diaphragm.
True or False?
Are interconnected in the gut wall
What allows central modification the intrinsic enteric nervous system regulation of the GI Tract?
Extrinsic autonomic sympathetic and parasympathetic innervation allows central modification
Intrinsic NS regulation of the Gi Tract
Extrinsic Nervous System Regulation of the GI Tract
- Autonomic Nervous System is extrinsic
Extrinsic nervous system regulation of the GI Tract: Parasympathetic pathway:
postganglionic fibres innervate smooth muscle and secretory cells. Ach is dominant neurotransmittor that stimulates motor and secretory activity. Primarily via vagus nerve (CNX)
Extrinsic Nervous System Regulation of the GI Tract: Sympathetic Pathway:
inhibits motor and secretory activity, stimulates sphincter and blood vessel contraction via the prevertebral ganglia
3 primary motor functions of the stomach:
- storage
- mixing
- emptying
3 Primary Motor Functions of Stomach: Storage:
the vagovagal reflex (interplay between afferent/efferent nerves and CNX) mediates receptive relaxation reducing muscle tone and allowing reservoir
3 Primary Motor Functions of Stomach: Mixing:
Fragmentation of food and mixing with secreted gastric juice for digestion
3 Primary Motor Functions of Stomach: Emptying:
contents transfer into the duodenum at a controlled rate (regulatory mechanism in situ)
Stomach
What functions primarily as a reservoir for storage of stomach contents?
The fundus
Stomach storage
How is chyme produced in the stomach?
Mixing
slow peristaltic waves are initiated in the body of the stomach moving the stomach contents toward the pyloric antrum. Foodstuff is forced back for further mixing and digestion. This process of propulsion and retropulsion occurs in cycles to produce chyme.
Stomach: Mixing:
How is chyme emptied into the duodenum?
highly regulated with primary inhibitory feedback signals from the small intestine
more powerful peristaltic contractions build to force chyme into the duodenum
Regulation of emptying chyme from the stomach into the duodenum:
Excitatory:
- ENS/ANS neuronal stimulation and hormonal (ie
motlin)
Inhibitory:
- ANS regulation, duodenal enterogastric reflexes,
and hormonal CCK
Small Intestine Diagram
Two broad types of motility in the small intestine:
- mixing and circulation for max exposure to absoprtive
epithelium - propulsion of chyme aborally
Motility patterns allow the majority of digestion and absorption of nutrients here over (how many hours?)
3-5 hours
Small intestine adaption for increased absorption
Large surface area provided by circular folds, villi projections of the mucosa and “brushborder” microvilli on the epithelial cell apical surface
Motility in the small intestine is controlled by
intrinsic motor patterns modified by hormonal and ANS neural stimuli
Segmentation for mixing in small intestine:
- stretch receptors trigger myenteric stimulation of
muscle contraction - no net movement
Propulsive peristalsis in small intestine:
- stretch receptors trigger myenteric stimulation of
muscle contraction causing a net movement
propulsive peristaltic reflexes (3):
- gastroenteric reflex
- gastroileal reflex
- migrating motor complex MMC
Gastroenteric reflux:
gastric distention activates the myenteric plexus to promote peristalsis throughout the small intestine
Gastroileal reflex
gastric distension promotes peristalsis in the ileum to force chyme through the ileocecal valve into caecum
Migrating Motor Complex (MMC):
- series of frequent peristaltic contractions, between
meals to move small intestine content to large
intestine - intrinsic enteric control, hormone motilin
- absence can lead to bacterial overgrowth
ileocecal valve controls
emptying of chyme into colon
Large intestine
proximal transverse colon function
absorption of water and electrolytes
distal transverse colon function
formation and storage of faeces
Motility in the large intestine is controlled bby what nervous system
- intrinsic enteric control modified by neural and
hormonal stimuli
Motility in the large intestine (3):
- mixing contractions via haustral churning
- propulsive peristalsis
- mass movements
How often do mass movements occur in the large intestine?
2x-3x a day
What are mass movements in the large intestine?
forceful peristaltic contractions, which force contents into the sigmoid colon and rectum
motility in the large intestine
Defecation Reflex
initiates defecation to expel faeces containing residues of digestion, bacteria, bile pigment and mucosal debris
mass movements push faecal matter into the rectum where stretch receptors are stimulated leading to activation of the ENS and parasympathetic ANS
involuntary contraction of longitudinal muscle in the rectum opens the internal anal sphincter
the constricted external anal sphincter is voluntarily relaxed to allow defecation
Defecation reflex
Emesis (vomiting)
the forceful ejection of the contents of the stomach as a valuable physiological mechanism to remove harmful or toxic substancesq
Three phases of emesis:
- Nausea: poorly defined experience of unpleasant
feelings, unease and distress along with the feeling of
a need to vomit - retching: laboured respiratory movements with
contractions of the muscles of the abdomen, thoracic
wall and diaphragm. Can occur with or without
vomiting. Movement of the diaphragm generates a
pressure gradient to permit emesis - expulsive phase: involuntary emptying of gastric
contents due to pressure gradient which is secondary
to abdominal and diaphragm contraction leading to
the opening of LOS. Epiglottis covers trachea/soft
palate cover nasopharynx
