MED2042 WEEK 5 - Gastroenterology (GI motility)

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What are the layers of the CI tract from the lumen outwards?: 4 basic layers (tunics)
LUMEN
MUCOSA
SUBMUCOSA
MUSCULARIS
SEROSA
Describe gut muscularis.: Circular muscle = constricts 'tube'

Longitudinal muscle = shortens length of 'tube' (GI-tract)
Describe how smooth muscle contracts in the gut.: Smooth muscle cells are electrically coupled to each other via GAP JUNCTIONS.

Each muscle layer is a synctytium (action potentials elicited anywhere along the muscle layer travel in all directions along the muscle).
List the steps of muscle contraction to muscle relaxation in the gut.: Depolarisation
--> Ca2+ influx = binds to calmodulin (CAM)
--> Ca2+-CAM binds to & activates myosin kinase (MK)
--> MK phosphorylates & activats myosin
--> Myosin forms crossbridges with actin
--> Contraction
--> Ca2+ pumped out
--> phosphate removed from myosin by myosin phosphatase
--> Myosin inactivated = RELAXATION
Describe the neuronal control of the GI-tract's motility and secretions.: 1. Sensory receptors (in GI-tract wall)
- mechanoreceptors (activated by stretch = food in lumen)
- chemoreceptors
(activated by changes in solute concentration or acidity/pH)

2. Neuronal reflexes:
a) mix lumen contents and move them along the gut
(by stimulating muscle cells)
b) activate or inhibit glands (that secrete digestive juices into lumen or hormones into bloodstream)
What are the levels of organisation in the nervous system?: CNS (brain and spinal cord)
<--> PERIPHERAL NERBOUS SYSTEM (cranial nerves and spinal nerves; communication lines between the CNS and the rest of the body)
<--- SENSORY (afferent) divsion - Somatic and visceral sensory nerve fibers; conducts impulses from receptors to the CNS

--> (from PNS) MOTOR (efferent) division - motor nerve fibers; conducts impulses from the CNS to effectors (muscles and glands))

--> SOMATIC NERVOUS SYSTEM (voluntary (somatic motor; conducts impulses from the CNS to skeletal muscles)

--> (from Motor division) AUTONOMIC NERVOUS SYSTEM - involuntary (visceral motor); conducts impulses from the CNS to cardiac muscles, smooth muscles and glands.

--> SYMPATHETIC DIVISION (mobilises body systems during emergency situations) + PARASYMPATHETIC DIVISION (conserve energy; promote nonemergency functions)

--> Enteric division
Describe the autonomic contrl of motility and secretions.: Parasympathetic division (acetylcholine) = resting & digesting
(D: digestion, defection, diuresis)
STIMULATES contraction and glandgular secretions

Sympathetic division (noradrenaline) = fight-or-flight
(E: exercise, excitement, emergency)
INHIBITS contractions & glandular secretions
Describe the enteric nervous system (ENS/'gut brain').: There are 2 intrinsic nerve plexuses:
1. MYENTERIC PLEXUS - between circular and longitudinal smooth muscle layers (along entire length of GI-tract) = controls gut motility.

2. SUBMUCOSAL PLEXUS - within submucosa connective tissue = controls functions within inner wlal or each gut section (secretions of mucous, digestive juices & hormones; nutrient absorption; bloodflow
What fibers are used in the neuronal control of the GI-tract?: Black fibers = myenteric and submucosal plexuses

Red fibers = extrinsic control of ENS by ANS

Dashed fibers = sensory nerve fibers passing from luminal epithelium & gut wall to ENS then to the spinal cord & CNS
What is the electrical activity in the GI-tract smooth muscle like?: Slow, regular electrical waves in the muscle membrane potential (3/min stomach, 12/min duodenum)
Describe the pacemaking in the GI-tract.: A network of Interstitial cells of Cajal (ICC) generates pacemaker potentials that are conducted into the muscle layers.

Electrical activity in the circular layer of the mouse gastric antrum.
What is peristalsis?: Movements in the GI-tract:
Result from electrical excitation and contraction of the smooth muscle layers. Controlled by nerves & hormones...

Adjacent segments of gut alternatively contract and relax - propelling (squeezing) food along the tract distally
What is segmentation?: Non-adjacent segments of gut alternatively contract and relax - moving food forwards then backward, resulting in mixing.
The oesophagus, stomach, small intsting and large intestine have different methods of GI motility. List.: Oesophagus - peristalsis
Stomach - receptive relaxation, mixing & emptying
Small intestine - segmentation & peristalsis
Large intestine - haustrations & mass movements
What are the phases of deglutition?: Swallowing (deglutition):
1. Voluntary or Buccal phase - initiates the swallowing process
2. Pharyngeal phases - involuntary = passage of food through pharynx into oesophagus
3. Oesophageal phase - involuntary = transports food from oesophagus to stomach
Describe what happens in the Voluntary or Buccal phase.: 1. Tip of tongue is placed on hard palate
2. Tongue contracts to force bolus of food into the oropharynx
3. Bolus stimulats mechanoreceptors and then everything becomes involuntary (controlled by the swallowing centre in the CNS)
Describe what happens in the Involuntary pharyngeal phase.: 1. Larynx and uvula rise to prevent food from entering respiratory passages
2. Relaxation of upper oesophageal sphincter and opening of oesophagus
3. Fast peristaltic wave initiated by innervation of pharynx forces bolus into oesophagus.
Describe what happens in the Involuntary involuntary oesophageal phase.: 1. Pharynx muscles contract, forcing food into esophagus
2. Upper oesophageal sphincter contracts after entry
3. Good is moved through oesophagus to stomach by peristalsis (nervous control = no pacemakers; proximal third striated muscle)
What is heartburn?: Heartburn (gastroesophageal reflux disease)

= burning pain that occurs when acidic gastric juice regurgitates into the oesophagus (symptoms similar to heart attack)

= most likely to occur after excessive drinking and/or eating, plus by conditions that force abdominal contents superiorly (e.g. pregnancy, extremem obesity, running)
= can be treated with antacid preparations (weak bases that neutralise gastric acid e.g. sodium bicarbonate
What motility occurs in the stomach?: 1. STORAGE of large quantities of food until it can be processed in the stomach (receptive & adaptive relaxation)
2. MIXING of food with gastric secretions until it forms a mixture called chyme
3. EMPTYING of chyme into the duodenum at a rate suitable for digestion and absorption by the small intestine.
Describe the relaxation of stomach in preparation for food.: 1. As bolus moves by peristalsis through oesophagus, stomach muscles relax (receptive relaxation = swallowing centre & vagus)

2. Gastroesophageal sphincter relaxes (normally tonic constriction prevents reflux)

3. Stomach also dilates in response to gastric filling (stretch receptor activate ENS = adaptive relaxation involving NO?)
What are hunger contractions?: - occur when stomach has been empty for a few hours
- rhythmical peristaltic contractions of the gastric body
- when successive contractions strong enough they can fuse to cause a continuing tetanic contraction (up to 3 mins)
- triggered by low blood glucose = 'hunger [ains' result from vagal nerve activation
- stomach noises (borborygmus)
= rumbling sound made by gas & fluids moving through the intestines.
Describe the pyloric pump (antral peristaltic contractions).: Rhythmical peristaltic gastric contractions (3/min) cause mixing and force ~3ml of chyme into duodenum (contractions initiated by ICC)

This also closes the pyloric valve, forcing the rest of the chyme into the stomach (for further mixing)
Describe emptying of the stomach.: - chyme empties slowly from stomach into small intestine
- gastrin released from antrum in response to stretch and certain digestive products (e.g. peptides) enhances pyloric pump activity
- regulated by inhibitory neuronal and hormonal signals from the duodenum

Rate of gastric emptying depends on duodenal contents:
- carbohydrate-rich meal moves rapidly through duodenum
- fats cause food to remain in stomach for longer
Describe the duodenal hormonal inhibitory signals that regulate stomach emptying.: - fats entering duodenum stimulate mainly cholescystokinin release

- CCK carried in bloodstream to stomach where it inhibits pyloric pump and enhances constriction of pyloric sphincter.
Describe the duodenal neuronal inhibitory reflexes that regulate stomach emptying.: 3 routes:
1. duodenum - stomach (ENS)
2. Extrinsic nerves - prevertebral sympathetic ganglia - activation sympathetic nerves
3. Vagus - brainstem - inhibition of parasympathetic nerves

Effects = inhibit pyloric pump, increase constriction of pyloric sphincter
Describe large intestine motility.: - longitudinal muscle layer reduced to 3 bands (taeniae coli)
- primarily propulse actions = mass movements (1-2/day)
- slow haustrations (every 30 min)
What does the colon do?: 1. ABSORBS water and electrolytes from ~1.5litres of liquid chyme (leaving solid faeces)
2. STORES faecal matter until excreted

movement ~5cm.hour
Proximal half - mainly absorption
Distal half - storage
What is Hirschsprung's disease?: Congenital aganglionic megacolon

Enlargement of the colon caused by bowel obstruction resulting from an aganglionic section of the bowel

Diagnosis = failure to pass a bowel movement (meconium) within the first 48 hours of life; staining histological sections with cholinesterase

Treatment = surgical removal of aganglionic section
What is the defecation reflex?: 1. Distention/stretch of rectal wall activates sensory (afferent) nerves which synapse with spinal cord neurones.
2. Parasympathetic motor (efferent) nerves stimulate contraction of rectal wall and relaxation of internal anal sphincter.
3. Voluntary (conscious) signals stimulate relaxation of external anal sphincter

Delay = reflex contractions end within second and rectal walls relax
Describe paralysis of defecation in spinal cord injuries.: Spinal cord injured between conus medullaris and the brain = voluntary part of defecation reflex blocked

Spinal cord mediated defecation reflex (involuntary) can still occur = give a small enema to cause adequate defecation
What does aganglionic mean?: The normal enteric nerves are absent.

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