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Module 104: Theme 1 > Motility of the GI tract > Flashcards

Motility of the GI tract Flashcards

1
Q

Purpose of motility control in the gut [3]

A
  • 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.
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2
Q

Mucosa of the gut wall

A

The innermost layer of the gut wall.

Contains the different layers:

  • Epithelium
  • Lamina propria
  • Muscularis mucosa
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3
Q

Lamina propria

A

The middle layer of the mucosa in the gut wall.

Contains:

  • Blood vessels
  • Loose connective tissue
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4
Q

Muscularis mucosa

A

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.

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5
Q

Layers of the GI tract

A

Mucosa

Submucosa

Muscularis externa

Serous (adventitia)

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6
Q

Submucosa of the gut wall

A

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.
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7
Q

Muscularis externa (propris)

A

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.
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8
Q

Serosa/ adventitia of the gut wall

A

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.

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9
Q

Enteric nervous system / Intrinsic nervous system:

  • Function
  • Plexuses
A

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.

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10
Q

Extrinsic pathways of the gut

A

Afferent and efferent fibres that connect the ENS to the CNS.

Composed of both parasympathetic and sympathetic pathways.

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11
Q

Afferent fibres of the extrinsic pathways in the GI.

  • Receptors
  • Loops
A

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.

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12
Q

Hirschsprung’s disease

A

A birth defect where the myenteric plexus is absent in the colon.

Causes severe constipation

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13
Q

Sympathetic innervation of the gut.

  • Preganglionic fibres
  • Activity
A

Part of the extrinsic innervation.

Preganglionic fibres in T8-L2.

Activity:

  • Inhibits gut motility
  • Inhibits secretion
  • Constricts sphincters
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14
Q

Postganglionic cell bodies of the sympathetic innervation of the gut.

  • Location of ganglia
  • Structures innervated.
A

These are located in these ganglia:

  • Celiac
  • Inferior
  • Superior mesenteric
  • Hypogastric

They innervate structures like the smooth muscle and secretory cells.

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15
Q

Parasympathetic innervation of the gut.

  • Nerves
  • Activity
  • Structures innervated
A

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
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16
Q

Smooth muscle in motility

A

Act as a functional syncytium.

The cells show pacemaker activity with slow spontaneous depolarisations that initiate contracts.

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17
Q

Basic electrical rhythm/ slow wave rhythm

A

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

18
Q

Segmentation

  • Action
  • Function
  • Organ
A

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.

19
Q

Peristalsis

  • Type of SM
  • Trigger
  • Action
  • Function
A

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:

  1. Longitudinal SM [LSM] contraction first.
  2. Halfway through the first contraction, circular SM contracts.
  3. LSM relaxes halfway during CSM contraction.
20
Q

Deglutition

  • Voluntary action
  • Autonomic control (afferent and efferent)
A

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.

21
Q

Oesophagus

A

Structure:

  • 25 cm long
  • Upper 1/3 skeletal muscle, the rest smooth muscle.

Swallowing:
Sphincters relax during swallowing.

22
Q

Upper oesophageal sphincter (UES)

A

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.

23
Q

Lower oesophageal sphincter (LES)

A

Sphincter at the gastro-oesophageal muscle of the oesophagus.

When closed: prevents reflux of gastric contents.

Relaxes during swallowing

24
Q

3 functions and motilities of the stomach

A

Storage:
Ingests food faster than digestion- receptive relaxation

Physical and chemical disruption- through mixing.

Delivers resultant chyme to intestine at optimal rate- emptying

25
Q

Receptive relaxation

  • Innervation
  • Area of stomach
A

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
26
Q

Regulation of receptive relaxation

A

Mediated by vagus nerve

Pressure sensors at abdominal levels maintain pressure whilst stomach size changes.

27
Q

Mixing in the stomach

A

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.

28
Q

Regulation of mixing in the stomach

A

ENS/ extrinsic:
Activated by mechanoreceptors in the stomach that sense distension.

Gastrin release:
Activated by food in the stomach= stimulates stomach motility

29
Q

Emptying of the stomach

A

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
30
Q

How gastric contents control stomach emptying

A

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.

31
Q

How events in the duodenum control stomach emptying

A

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.

32
Q

Hormones that decrease gastric emptying

A

Secretin- stimulated by low pH

CCK- stimulated by fats

Gastric inhibitory peptide- stimulated by fats

Gastrin- stimulate by amino acids/ peptides

33
Q

Motility in small intestines

A

Segmentation: varying rate according to area of intestines

Peristalsis: BER contractions

Villus movements: mixes and drains lymphatics of fat absorption

34
Q

MMC

A

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.

35
Q

Control of small intestine motility

A

Extrinsic input and enteric system

36
Q

Motility in the ascending and transverse colon.

A

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

37
Q

Muscle layers in the colon

A

Different from usual:

  • Pacemaker in submucosal muscle
  • Longitudinal muscle is now taeniae coli. Tone of this muscle forms haustra (small sacs in the colon).
38
Q

Motility in the descending/transverse and sigmoid colon.

A

Propulsive movement stimulated by short range peristalsis.

Mass movement:
- Extended peristaltic contraction (5cm/min)

  • Gastrocolic and duodenocolic reflexes causes extended peristaltic contractions.
39
Q

Rectum and defecation

A

Mass movement that propels faeces into the rectum distends stretch receptors
- defecation reflex

Afferent stimulation (parasympathetic)= relax of internal sphincter

40
Q

Voluntary relaxation of external sphincter

A

This occurs via the pudendal nerve.

If this relaxation does not occur, this reverses peristalsis to the colon.

41
Q

Sphincters in the anus

A

Involuntary:
Internal anal sphincter

Voluntary:
External anal sphincter (skeletal muscle)

These sphincters are normally contracted.

Module 104: Theme 1 (18 decks)

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