motility of the GI tract Flashcards
what is the role of the digestive system?
to extract chemical energy, vitamins, minerals and water from ingested products
what are the 7 processes involved in the digestive system?
- ingestion
- secretion
- motility
- mechanical digestion
- chemical digestion
- absorption
- elimination of waste
what is the primary function of the oesophagus?
transport
what is the primary function of the stomach?
- storage
- secretion
- mixing
- digestion
what is the primary function of the small intestine?
- secretion
- mixing
- majority of digestion
- absorption
what is the primary function of the large intestine?
- limited absorption
- faeces formation
- gut microbiota
what is coordinated motility?
motility is governed by involuntary contraction of smooth muscle with pacemaker interstitial cells of Canal (ICC)
what are the 2 areas of the body that don’t use coordinated motility?
- upper oesophagus
- external anal sphincter
how does smooth muscle help with coordinated motility?
- smooth muscle is a single unit-gap junction that allows electrical coupling and contraction as a functional syncytium
- smooth muscle is organised into connected bundles of outer longitudinal and inner circular smooth muscle in muscularis layer
what controls GI motility and secretion independently?
intrinsic enteric nervous system (ENS)
what are the 2 interconnected plexuses in the gut wall?
- myenteric plexus
- submucosal plexus
what allows central modification with motility?
extrinsic autonomic sympathetic and parasympathetic innervation
how does the intrinsic enteric nervous system (ENS) help with GI motility?
- reflect contraction in response to local stimuli
- myenteric plexus in musclaris layer: motility
- submucosal plexus in submucosal layer: secretion and local blood flow
how does the extrinsic autonomic nervous system (ANS) help with GI motility?
- ANS modifies basal activity of the ENS
- parasympathetic innervation: excitatory to motility and secretion
- sympathetic innervation: inhibitory to motility and secretion
where are endocrine hormones secreted and by what?
- secreted by entero-endocrine cells
- in the epithelial layer of the GI mucosa and then they enter the portal blood circulation
what are the 2 types of electrical activity?
- slow waves: cyclical oscillations of membrane potential spontaneously initiated by pacemaker ICCs
- spike potentials: generated once threshold is reached resulting in calcium influx and smooth muscle contraction
what are the 2 types of contraction in the GI tract?
1) segmentation for mixing
2) peristalsis for propulsion
how does the segmentation for mixing contraction work?
- bursts of circular muscle contraction and relaxation
- back and forth pendular movements also occur
how does peristalsis for propulsion work?
- local distention triggers contraction behind bolus and relaxation in front
- waves of contraction
- required functional myenteric plexus
- law of intestines: aborally
what are the 3 stages of swallowing?
1) oral: voluntary initiation of swallowing in the oral cavity
2) pharyngeal: involuntary passage of food through pharynx into oesophagus
3) oesophageal: involuntary passage of food from pharynx to stomach
how does the oral phase of swallowing work?
- under voluntary control
- tongue pushes up against hard palate and contracts to force lubricated bolus into the pharynx
- the pharynx consists of the oropharynx, nasopharynx and larygopharynx
- bolus enters the oropharynx initiating the pharyngeal stage through stimulation of sensory receptors
how does the pharyngeal phase of swallowing work?
- swallowing centre in the medulla oblongata and pons in the brain stem
- motor reference in trigeminal glossopharyngeal and vagal nerves cause series muscles contractions moving bolts through oropharynx int laryngopharynx and into oesophagus
- soft palate elevated over posterior nares to close nasal pharynx
- epiglottis closes larynx
- respiration is inhibited
- upper oesophageal sphincter relaxes
- pharyngeal muscle contraction propels bolus into oesophagus
how does the oesophageal phase of swallowing work?
- primary peristalsis moves bolus downwards
- circular muscle contracts behind bolus, longitudinal muscles contracts in front to shorted fibres and push wall outwards
- mucus lubricate and reduces friction
- relaxation of the lower oesophagus and lower oesophageal sphincter occurs
- secondary peristalsis stimulated by stretch
- coordination is via intrinsic myenteric and extrinsic vagal innervation
what is achalasia?
- oesophageal motility dysfunction
- LOS fails to relax causing food to remain in oesophagus
- cause my be vagal or myenteric defect
- distention, inflammation, infection and ulceration
what is gastro-oesophageal reflux?
- oesophageal motility dysfunction
- LOS tone lost leading to flow of acidic gastric contents into oesophagus
- inflammation, ulceration
- may be linked to hiatus hernia where portion of the stomach protrudes through diaphragm into thorax causing gastric reflux
what are the 3 primary motor functions of the stomach?
- storage
- mixing
- emptying
how does storage work as a primary motor function of the stomach?
- the vasovagal reflex mediated receptive relaxation reducing muscle tone and allowing reservoir function
- the funds functions primarily as a reservoir for storage of stomach contents
how does mixing work as a primary motor function of the stomach?
- fragmentation of food and mixing with secreted gastric juice for digestion
- slow peristaltic waves are initiated in the body of the stomach moving stomach contents towards pyloric antrum
- food is forced back for further mixing and digestion
- this process of propulsion and retropulsion occurs in cycles to produce chyme
how does emptying work as a primary motor function of the stomach?
- emptying contents int the duodenum at a controlled rate
- highly regulated with primary inhibitory feedback signals from small intestine
- more powerful peristaltic contractions build to force chyme into the duodenum
- regulation of emptying: excitatory (ENS/ANS neuronal stimulation and hormones) and inhibitory (ANS regulation, duodenal, enterogastric reflexes and hormones)
what is dumping syndrome?
- gastric motility dysfunction
- rapid emptying of gastric contents into small intestine
- occurs following ingestion of large meal after gastrectomy
- characterised by nausea, pillow, sweating, cramps, vertigo, and sometimes fainting within minutes
- may be caused by hypertonic duodenal contents causing rapid entrance of fluid
what is gastroparesis?
- gastric motility dysfunction
- stomach fails to empty
- prevents proper digestion
- causes bloating and nausea
- may be caused by gastric cancer or peptic ulcers
- occasionally observed through impaired vagal stimulation to the stomach in severely diabetic patients who develop autonomic neuropathy
what are some key adaptations that the small intestine has?
- large surface area for absorption
- circular folds
- villi projections of the mucosa
- ‘brush border’ microvilli on the epithelial cell apical surface
what are the 2 types of motility linked to the small intestine?
1) mixing and circulation of maximum exposure to absorptive epithelium
2) propulsion of chyme aborally
what controls motility in the small intestine?
intrinsic motor patterns modified by hormonal and ANS neural stimuli
how does segmental for mixing work with the small intestine?
- stretch receptors trigger myenteric stimulation of muscle contraction
- no net movement
how does propulsive peristalsis work with the small intestine?
- stretch
- hormones: excitation (gastrin, CCK, insulin, motilin, seretonin) and inhibition (secreting and glucagon)
what are the 3 types propulsive peristaltic reflexes?
- gastroenteritis reflex: gastric distention activates myenteric plexus to promote SI peristalsis
- gastroileal reflex: gastric distention promotes peristalsis in the ileum to force chyme through ileocaecal valve into caecum
- migrating motor complex (MMC): series of peristaltic contractions between meals every 90 mins sweeps contents into colon. Intrinsic enteric control, hormone motilin. Absence can lead to bacterial overgrowth.
what are the 3 main disruptions to peristalsis?
- peristaltic rush: mucosal irritation, ENS and ANS neural reflexes rapidly sweep contents of SI into colon
- paralytic ileum: loss of peristalsis following mechanical trauma
- vomiting: reverse peristalsis initiated in distal small intestine to expel intestinal and gastric contents
why is the motility in the large intestine more sluggish?
to allow optimal:
- absorption of water and electrolytes (proximal)
- formation and storage of faeces (distal)
- communal microbiome aids digestion, synthesis B and K vitamins
what is the motility in the large intestine like?
- motility is via mixing and propulsion under intrinsic enteric control modified by neural and hormonal stimuli
- mixing contractions via haustral churning
- peristalsis: mass movement occurs 2-3x a day - gastro-colic and duodeno-colic reflexes; mass movements occur after meals on stretching via ANS
what is the defecation reflex and how does it work?
- initiates defection to expel faeces containing residues of digestion, bacteria, bile pigment, mucosal debris
- mass movements push faecal matter into normally empty rectum
- stretch receptors are stimulated and activate the ENS and parasympathetic SNA
- involuntary contraction of longitudinal muscle in the rectum pens the internal anal sphincter
- the constricted external sphincter is voluntary relaxed to allow defecation
