L22 - GI Motility

• Swallowing (phagia)- what is it?- what are the three phases?

  - passage of food bolus from mouth to stomach- partly voluntary, partly reflexive- oral, pharyngeal, oesophageal phases

  
• Swallowing Reflex- when is it stimulated?- where is swallowing centre in brain?

  - food bolus stimulates pressure receptors in back of throat and pharynx- signals to swallowing centre in medulla (periphery to CNS)- innervated skeletal muscles in pharynx and upper oesophagus- involuntary contraction in pharyngeal muscles pushes material into oesophagus
• Define pharynx

  throat - connects mouth and nose to oesophagus
• What motility occurs in stomach?What does movement depend on?What's produced?What exits faster (solids/liquids)?

  - peristalsis and mixing (churning) of food bolus from oesophagus- depends on viscosity of contents- chyme is produced- liquids exit faster than solids

  
• How does movement and mixing occur in stomach?Fast or slow?

  - liquid moves down towards pyloric sphincter, solid centre of chyme moves back up- churning mixes food with HCl and breaks it down, so bigger SA for enzyme action, eg pepsin- peristaltic contractions (stronger than mixing wave) push a few mls of fluid chyme through pyloric sphincter into duodenum- most chyme forces back into stomach body for further mixing- slow stage - don't want to overwhelm small intestine

  
• What motility occurs in small intestine?What regulates this?

  segmentation aids mixing and peristalsis aids moving contents forwardsRegulated by the ENS (regulates contraction of smooth muscle)
• Segmentation in small intestine - what happens?

  concentric rings of smooth muscle contract to break up food bolus and increase SAstationary, rhythmic contraction and relaxationlow net movement

  
• Peristalsis in small intestine- what happens?

  forward movement: wavelike contraction of smooth muscle- circular muscles contract (beind) and longitudinal muscles relax (infront) of food bolus, so bolus is pushed forwards

  
• ENS- What is it? - Function?- What's it under control of?

  Enteric Nervous System regulates GI system independently from brain- under control of para/sympathetic nervous system after direction by vagus nerve
• What are the two main networks of ENS?

  1 - Myenteric plexus (in muscularis layer)controls GI tract motility2 - Submucosal plexuscontrols local secretion, blood flowResponsible for detecting chemical changes in GI system

  
• How do enteric neurons control peristalsis?

  Food bolus causes distention of tubes in GI system- this signals to ENS which releases neurotransmitters behind food bolus to induce contraction (eg Ach/histamine)- neurotransmitters released ahead of food bolus initiate relaxation

  
• How is GI smooth muscle contraction induced?

  - Acetylcholine (Ach) induces contraction via activation of muscarinin receptors (GPCRs)- Histamine induces contraction vie actiation of H1 receptors (GPCRs)- triggers cascade of events resulting in muscle contraction

  
• What effect does the sympathetic nervous system have on ENS?

  - inhibits ENS: digestion slows so more energy can be directed towards figh or flight response- decreased peristalsis, blood flow, secretion, absorption
• What effect does the parasympathetic nervous system have on ENS?

  - stimulates ENS in rest and digest- increases peristalsis, blood flow, secretion, absorption
• What hormones affect ENS activity?

  Stimulates/Increases ENS activity:- cholecystokinin- insulin- gastrin- serotonin (5-HT)Inhibits/Decreases ENS activity:- secretin- glucagon (turns glucose to glycogen for storage)
• Migrating Motility Complex- what is it?- when does it occur?- what does it cause?- how is it regulated?

  = cleansing mechanism which expels undigested contents out of intestine- occurs when fasting- causes stomach rumbing- regulated by electrical activity
• Large Intestine Motility- what is it? - when does it occur?- what are haustra?- what activates it?


  = haustral shuttling- occurs all the time- Haustra = saccules in colon giving a segmented appearance- haustral contraction activated by chyme presence to cause bulk movement

  
• Large intestine motilty 2-what controls haustral shuttling- what's stored in haustra


  - controlled by electrical cells which signal at dif rates- signal faster at end of intestines- faeces stored in haustra

  
• Defaecation- what does it do?- where is waste stored?- how is process initiated?

  - eliminates waste temporarily stored in rectum- initiated by stretch receptor activation

  
• Defaecation 2- What's the process?- which parts are (in)/voluntary?

  1. Faeces activate stretch receptors in rectum2. Rectum contracts3. Internal anal sphincter relaxes (involuntary)4. External sphincter remains contracted until voluntarily relaxed

  
• Vomiting- what is it- where's the vomiting centre?- what are the three phases?


  = forcible expulsion of gastric contents through mouth. involuntary reflex- vomiting centre in medulla oblongata (brain stem)1. Pre-ejection2. Ejection3. Post-ejection
• Pre-ejection phase of vomiting- what parasympathetic responses occur?

  - nausea- autonomic changes: tachycardia (fast heart rate) and increased salivation (protects teeth from stomach acid)- reverse peristalsis as pyloric sphincter relaxes, allowing food content to fill stomach
• Ejection phase of vomiting- what reflex occurs?- what muscles contract?- what does epiglottis do?

  Retching = involuntary reflexrhythmic contraction of diaphragm and abdominal muscles forces stomach contents into oesophagusepiglottis closes to prevent food travelling to lungs
• Post-ejection phase of vomiting

  - fatigue and muscle weakness- epiglottis remains closed