GI Tract Motility Flashcards
Briefly describe the mechanism of chewing
- involves teeth (incisors cut and molars grind the food)
- the nerve that controls the process comes out of brain stem nuclei
- mixes food with saliva lubricating it
- decreases the size of the food particles to form a food bolus to facilitate swallowing
- mixes food components with digestive enzymes
Describe the mechanism of swallowing
consists of:
- voluntary stage: initiates swallowing process
- pharyngeal stage: involuntary passage of food through pharynx to oesophagus
- oesophageal stage: involuntary transport of food from pharynx to stomach
Describe the oral/voluntary phase of swallowing and the nerves that control it
- controlled by glossopharyngeal and vagus nerves
1. food voluntarily moves posteriorly into pharynx by tongue
2. trachea is closed
3. oesophagus opened
4. fast peristaltic wave initiated by NS of pharynx forces bolus of food into upper oesophagus
(all in less than 2 seconds)
Describe the pharyngeal phase of swallowing
- automatic reflex
- initiated by voluntary movement of food to back of mouth
- excitation of involuntary pharyngeal sensory receptors to elicit swallowing reflex
- next stage automatic by neuronal areas of medulla and lower pons
- sends motor impulses from swallowing centre to pharynx and upper oesophagus are transmitted
(by cranial nerves)
(all in less than 6 seconds)
How does the pharyngeal phase of swallowing affect respiration?
- interrupts respiration for a fraction of usual respiratory cycle
- swallowing centre specifically inhibits respiratory centre of medulla during time for a brief period
Describe the oesophageal phase and peristalsis of swallowing
primary peristalsis:
- continuation of peristaltic wave that begins in pharynx and spreads into oesophagus during pharyngeal stage
- continuous wave from pharynx to stomach in 8-10 seconds
Secondary peristalsis:
- results from distention of oesophagus from retained food
- waves continue until all food emptied into stomach
What initiates secondary peristalsis?
- intrinsic neural circuits in myenteric NS
- reflexes that begin in pharynx
Describe the muscle type and innervation of the oesophagus
Upper region:
- striated
- controlled by glossopharyngeal (CN X) and vagus nerves (CN X)
Lower 2/3rds:
- smooth muscle
- controlled by vagus centres (CN X) that act through connections with oesophageal myenteric nervous plexus
Describe how the LES and stomach prepare for food
- relaxation wave transmitterd by myenteric inhibitory neurones before peristalsis to stomach
- last 3 cm oesophageal circular muscle is LES which is normally tonically constricted
What are the 3 motor functions of the stomach?
- storage
- mixing
- emptying
Describe how the stomach can store food
- food entering stomach forms ceoncentric circles in stomach
- newest food closest to oesophageal opening and older food near outer wall of stomach
- food stretches the stomach causing a vagovagal reflex
- decreases tone in stomach body muscular wall causes the wall to bulge outwards allowing greater quantities of food to be accomodated
Describe how the stomach mixes food
- mixes food with gastric secretions to form chyme
- mixing waves (weak peristaltic constrictor waves): begin in upper/mid portions of stomach wall and move towards antrum every 15-20s
- antral contents squeezed upstream through peristaltic ring towards body of stomach
Describe how the stomach empties its contents
- slow emptying of chyme from stomach into small intestine at rate suitable for proper digestion and absorption by small intestine
- promoted by intense peristaltic contractions in stomach antrum
- opposed by pylorus (constriction under influence of nervous and hormonal signals from stomach and duodenum)
What are the regulating factors of stomach emptying?
Gastric and duodenal:
- stomach volume (higher the volume, increases emptying)
- enterogastric inhibitory reflex:
- distension of duodenum
- irritation of duodenal mucosa
- acidity/osmolality of duodenal chyme
- presence of digestion products in chyme
Hormonal:
- stimulus mainly fats
- CCK most potent hormone
Describe the segmentation contractions of the small intestine
- chyme induced extension of GI wall causing stretch causes localised concentric contractions
- frequency of segmentation determined by frequency of slow waves
- requires background excitation from myenteric nerve plexus
