Wk 1 - Physiology: Mouth, Stomach and Secretion Flashcards
What two plexuses make up the enteric nervous system?
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Myenteric plexus
- Located between the inner and outer layers of muscularis externa
- Responsible for increasing tone of gut
- Controls velocity and intensity of contractions
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Submucosal plexus
- Located in the submucosal layer
- Responsible for secretions and absorption in the gut
- Controls local muscle movements as well
Where is the myenteric plexus located?
Between the inner and outer layers of the muscularis externa
Where is the submucosal plexus located?
In the submucosal layer
List the structures that make up the enteric nervous system.
- myenteric plexus
- submucosal plexus
- mucosal plexus
- deep muscle plexus
- sensory neurones
- internurones
- motor neurones
What is the function of the enteric nervous system?
- Constantly receiving sensory info from the GI tract
- Responds via local control of motor functions + secretions
- Feedback mechanism in a way
List the structures that make up the sympathetic nervous system.
Post ganglionic fibres from :
- coeliac
- superior mesenteric plexi
- inferior mesenteric plexi
- hypogastric
Some direct to blood vessels
Some to secretory cells
Many synapse with enteric plexi
What is the function of the sympathetic nervous system on the GI tract?
Mostly inhibitory on GI function
List the structures that make up the parasympathetic nervous system.
- Vagus as far as transverse colon
- Pelvic nerves via Hypogastric plexi
What is the function of the parasympathetic nervous system on gut function?
Mostly stimulatory
What are the main salivary glands?
- Submandibular gland (mixed secretion) produces 75% of secretions
- Parotid gland (serous) produces 20% of secretions
- Sublingual gland (mixed) produces 5% of secretions - produces lipase
What is the total flow from glands per day?
About 1000ml (1L) per day (nb at maximum stimulation a gland can produce 1ml per minute per gram of gland)
Describe the structure of the glands.
- Salivary glands are made up of secretory acini (acini - means a rounded secretory unit) and ducts. The acini can either be serous, mucous, or a mixture of serous and mucous.
- The secretory units merge into intercalated ducts, lined by simple low cuboidal epithelium, and surrounded by myoepithelial cells.
- These ducts continue on as striated ducts. These have a folded basal membrane, to enable active transport of substances out of the duct. Water resorption, and ion secretion takes place in the striated ducts, to make saliva hypotonic (reduced Na,Cl ions and increased carbonate, and potassium ions).
- The striated ducts lead into interlobular (excretory) ducts, lined with a tall columnar epithelium.
What controls the ionic composition of saliva?
Saliva flow
What is saliva composed of?
- Enzymes: amylase (breakdown of starch), lipase (breakdown of fat imp in children whose pancreas has not yet fully developed) and lysosomes (kill bacteria - mouth imp source on infection)
- Inorganic: Na, K, Cl and HCOs + little bit of phosphate and Ca
- Organic: urea, citrate, amino acids, steroid hormones at same conc as plasma (exact equilibrium of free hormone)*, ABO antigens, mucin (ie mucous), kallikrein (indicator that vasodilation system is working - produce bradykinin)
*Useful to detect hormones eg ovulation in women salivary progesterone
What are the 4 functions of saliva?
- Moistens food
- Digests food (amylase and lipase - which continue to digest the food bolus into the stomach)
- Lysozyme activity (protect teeth and mouth from infection)
- Buffer action (but saliva generally more acidic)
What controls salivary gland secretion?
- Parasympathetic nervous system: stimulation of saliva flow associated with powerful vasodilation
- Sympathetic nervous system: if stimulated in isolation, leads to profound vasoconstriction - leading to dry mouth (characteristic of stressful situations. If alongside PNS, it stimulates enzyme secretion
- If sympathetic innervation is stimulated same time as parasympathetic then PNS vasodilation OVERRIDES the SNS vasoconstriction
Describe the different phases of saliva secretion.
- Cephalic phase: triggered by sight, smell or thought of food. This is a learned (conditional) response
- Reflex phase: triggered by presence of food in mouth (by rubbing gums and/or chewing. This is a hardwired (unconditional) reponse
Resting flow falls during sleep - imp as otherwise would choke on own saliva.
Describe the muscles found in the oesophagus.
- Upper portion (5%) is voluntary muscle
- Middle portion (35-40%) is a mixture of voluntary and smooth muscle
- Lowest portion (55-60%) is smooth muscle
What are the two sphincters of the oesophagus?
- Upper oesophageal sphincter (UOS) is an “anatomical” structure
- Lower oesophageal sphincter (LOS) is physiological (cannot see it anatomically)
What is the function of the upper oesophageal sphincter?
Prevents air swallowing
What is the function of the lower oesophageal sphincter?
Prevents acid reflux (acid reflux usually worse when lying down). More of a problem as would lead to inflammation and maybe Barrett’s oesophagus.
How does food pass down through the oesophagus?
Through primary (and if necessary secondary) peristaltic wave - the wave goes down the whole of the oesophagus until the stomach.
Explain the mechanism of swallowing food.
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Voluntary phase:
- Mastication leads to a bolus of food being produced, during this stage the back of the tongue is elevated and the soft palate pulled anteriorly against it.
- Following this, inspiration is inhibited (i.e. close nasal opening) and the bolus of food is moved to the pharynx by the tongue.
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Pharyngeal phase:
- Pressure receptors are activated in the palate and anterior pharynx.
- This signals swallowing centre in brainstem which inhibits respiration, raises the larynx, closes the glottis, opens the upper oesophageal sphincter.
- The true vocal cords close to prevent aspiration.
- The bolus is moved towards the oesophagus via peristalsis of the pharyngeal constrictor muscles.
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Oesophageal phase:
- At the beginning of this phase, the larynx lowers, returning to its normal position.
- The cricopharyngeus muscle then contracts to prevent reflux and respiration begins again.
- Bolus is moved down the oesophagus via peristalsis (initiated by UOS contraction), which is coordinated by extrinsic nerve.
List the main functions of the stomach.
- Sterilisation of food (with fluid pH of 2/3)
- Storage of food (receptive relaxation - tone of stomach decreases as more food goes in)
- Mechanical disruption of food
- Digestion
- Control of release of material to duodenum (water-permeable so if pyloric sphincter is not working –> gastric dumping –> blood volume drops. People with this condition faint after eating)
- Secretion of intrinsic factor (imp in absorption of Vit B12)
What is the ‘pacemaker’ zone in the stomach?
The pacemaker cells in the fundus of the stomach establish a basal electrical rhythm continuously that spread down to the pyloric sphincter, creating a rate of approximately three to eight contractions per minute.
Describe how parietal cells (aka oxyntic cells) produce acid and secrete it into the stomach.
- Acid producing cells + ATP-dependent secretion into lumen (pH is about 2 –> vs body is pH 7 so need large amount of energy)
- Secreting HCl into stomach - secretes protons into stomach lumen in exchange for potassium
- Protons come from CO2 - comes into cell from blood side - reacts with OH from the water found in these cells to release H
- For each H (HCl) into stomach, there is HCO3 going into blood
This exchange is associated with metabolic alkalosis. Vomiting is major cause of metabolic alkalosis (as losing acid and need to produce more so have more of this exchange)
How is the acid (gastric) secretion in parietal cells controlled?
Gastric secretion is stimulated chiefly by three chemicals: acetylcholine (ACh), histamine (secreted by ECL cells), and gastrin:
- Acetylcholine (ACh). This is secreted by the parasympathetic nerve fibers of both the short and long reflex pathways.
- Histamine. This is a paracrine secretion from the enteroendocrine cells in the gastric glands.
- Gastrin. This is a hormone produced by enteroendocrine G cells in the pyloric glands.
ACh and gastrin increase Ca into cells which increases proton pumps (H/K pump) on lumen layer, while histamine activates cAMP cascade to promote proton pump action.
Below pH of 2, stomach acid inhibits the parietal cells and G cells; this is a negative feedback loop that winds down the gastric phase as the need for pepsin and HCl declines!
