GI: Cephalic, Oral, and Esophageal Phases Flashcards
Describe the function of the cephalic phase.
to prepare the body for food in the absence of a meal
List stimuli for the cephalic phase.
- idea of food
- visual
- auditory
- olfaction
Describe innervations involved in the cephalic phase initiation.
sensory input => cortex/hypothalamus => vagal preganglionic neurons in the medulla/pons => vagal parasympathetic stimulation
Describe the effects of the cephalic phase
- increased salivary secretion (via glossopharyngeal)
- increased pancreatic secretion
- increased gallbladder contraction
- relaxation of sphincter of Oddi
in general, prepares GI tract to receive food
Describe the function of the oral phase.
to further activate the GI phase, but occurs in the presence of food.
What are the only 2 things that can be absorbed in the oral phase?
alcohol
some drugs
Define chewing and its functions.
chewing - mechanical digestion of food into smaller pieces
functions
- mix food with salivary amylases, lingual lipases, and mucin
- allows lubrication and begins breakdown of food
Name the muscles of mastication and their actions
- temporalis (major)
- masseters
- medial and lateral pterygoids.
Action - elevate, protract, and retract the jaw; pterygoids also move mandible laterally
Define xerostomia, its causes, and symptoms.
xerostomia is a congenital or autoimmune condition of dry mouth, with decreased salivary secretions.
the reduced pH in the mouth (acidic environment) can lead to tooth decay, esophageal erosions, and difficulty swallowing.
In general, where do GI secretions come from?
- glands
- gut wall
- intestinal lining
In general, what stimulates GI secretions?
- chemical, osmotic, mechanical signals
- secretagogues act on secretory cells to stimulate secretion
What are the major salivary glands?
- parotid
- sublingual
- submandibular
- some small ones along oral and buccal mucosa
What are the 2 main parts of the salivary glands?
- acinus = blind end where initial saliva is secreted
- collecting ducts = modification of initial saliva => empties secretory juices into gut
Track the movement of salivary secretions (start from acinus).
acinus => initial saliva secretion => intercalated duct => striated ductal cells => final saliva
Describe acinar cells.
- found in the acinus end of salivary glands
- secrete initial isotonic saliva of water, electrolytes, and enzymes
Describe ductal cells.
- found in striated duct
- modify initial saliva by changing its composition
- creates final hypotonic saliva
Describe myoepithelial cells
- found in acinus and intercalated duct
- contain actin and myosin fibers so that contraction propels saliva forward
- under exclusively neural regulation
Describe what type of secretion each salivary gland releases.
parotid = serous (water, electrolytes, mucus) sublingual = mucous (mucin + H2O = mucus) submandibular = mixture
List the functions of saliva.
- lubrication with mucus to aid movement through esophagus
- initial digestion of starches and lipids by salivary amylases and lingual lipases
- aids in speech
- protects by buffering and neutralizing vomit
- washes away and destroys bacteria with lysozymes
Describe the composition of saliva.
Overall: hypotonic (high K, HCO3, low NaCl)
inorganic composition = water, electrolytes, fluoride
organic composition = salivary amylase, lingual lipase, mucin, lysozymes, kallikrein
What is kallikrein?
converts plasma protein into bradykinin for vasodilation
What is step 1 of saliva formation?
acinar cells secrete isotonic initial saliva
Describe step 2 of saliva formation?
Modification occurs through a series of transport mechanisms.
1. Na/H exchanger pumps H into the saliva and reabsorbs Na into the blood
2. Cl/HCO3 exchangers pump HCO3 into the saliva and reabsorbs Cl
3. K/H exchangers pump K into the saliva and reabsorb H
4. water impermeability leaves lots of water in the duct lumen
SUMMARY: reabsorbs NaCl, secretes K, HCO3, same amount of H2O => hypotonic
Describe the effect of flow rate on saliva composition.
At high flow rates (>4 mL/min), saliva resembles isotonic to plasma because there is less time for ductal cells to modify it. The only exception is that HCO3- secretion increases with flow.
In general, what regulates saliva secretion?
exclusively neural
ANS, but mostly parasympathetics
What does neural stimulation of salivary secretion do?
- increase saliva secretion
- increase enzymatic secretion
- increase HCO3 secretion
- contract myoepithelial cells to propel saliva forward
Describe the mechanisms of parasympathetic innervation on saliva secretion.
- stimulus = food; inhibition = dehydration, fear, sleep
- facial and glossopharyngeal nerve postganglionics => ACh on muscarinic receptors
- production of IP3 => increased intracellular Ca => increased secretion
Describe the mechanism of sympathetic innervation on saliva secretion.
- T1-T3 preganglionics synapse on superior cervical ganglion
- postganglionic release NE on B-adrenergic receptors
- production of cAMP => increased secretion
What phase does swallowing occur in?
between oral and esophageal
Is swallowing voluntary?
to initiate = voluntary
but after that it is a reflex
In general, what does the swallowing reflex do?
- propels food to the back of the pharynx
- inhibits respiration to prevent food in trachea
Describe the mechanism of the swallowing reflex.
Afferent Limb
- touch receptors in pharynx send impulse to medulla/pons
Efferent Limb
- motor fibers from the medulla/pons go to pharynx/UE (cranial) and LE (vagal) to trigger swallowing
What are the 3 phases of swallowing.
- oral
- pharyngeal
- esophageal
Describe the oral phase of swallowing.
- voluntary
- pushing bolus to the back of the pharynx activates touch receptors
- initiates swallowing reflex
What 5 steps happens during the pharyngeal phase of swallowing?
- involuntary
1. soft palate goes up, palatopharyngeal folds go in => narrow passage to prevent reflux into nasopharynx
2. epiglottis covers the larynx to prevent food entering the trachea
3. UES relaxes to receive food
4. pharynx contracts to push food into the esophagus
5. UES contracts again to prevent reflux into pharynx
What happens during the esophageal phase of swallowing?
- propels food through the esophagus into the stomach
- sphincters protect the airways
- sphincters protect from acid reflux
Describe the primary and secondary peristaltic contractions. What phase do they occur in?
occur in esophageal phase
- primary = due to swallowing reflex
- secondary = clears esophagus of food; via ENS; stimulated by distension
What happens in the esophageal phase after the swallowing reflex?
- peristalsis causes LES to relax (VIP-releasing vagal neurons)
- orad of stomach relaxes (receptive relaxation)
- bolus enters stomach (which had a decreased pressure to encourage food to drop)
- LES contracts back to high resting tone
What is the purpose of esophageal pressure being less than abdominal pressure?
- keeps air out of upper esophagus
- keeps acidic content out of lower esophagus
What happens when abdominal pressure increases past esophageal pressure?
GERD
Define GERD
GERD = increased abdominal pressure causes loosening of the LES, allowing acidic reflux into the unprotected esophagus => pain
risk: pregnancy, obesity
What are treatment options for GERD?
- H2 receptor antagonists (ranitidine) to reduce gastric secretions
- proton pump inhibitors (omeprazole) to reduce gastric secretions
Define hiatal hernias: risks, prevalence, symptoms.
Hiatal hernia = widening of the esophageal hiatus in the diaphragm, causing stomach to protrude through the diaphragm
- symptoms = pain, reflux, LES impairment
- risks = genetic, obesity, constipation, weight lifting
- prevalence = 60% of those above 60 years
Define achalasia: symptoms, treatment.
Achalasia occurs as a result of impaired peristalsis and LES relaxation.
- symptoms = dysphagia, regurgitation, chest pain
- Tx = surgery, Ca Channel blockers, BOTOX, muscle relaxants
List causes of vomiting.
- motion sickness
- elevated intracranial pressure
- stimulation of back of throat
- distension of the stomach
- pain
- emotions
What is retching?
precedes vomiting
- UES is closed
- content in esophagus cannot enter the pharynx
What autonomic “reflexes” occur during vomiting?
- sweating
- salivation
- dilation of pupils
- pallor
- irregular heartbeat
Describe the mechanism of vomiting.
- pyloric sphincter and stomach relax
- forced inspiration against closed glottis
- forced abdominal muscle contraction => increased abdominal pressure, decreased thoracic pressure
- UES, LES, esophageal relaxation
