Human Physiology - Lecture 3

Human Physiology - Lecture 3

1. GI Tract control

   • Principle of control = maximise absorption of nutrients
     • Achieved by: regulating mobility - rate at which food moves through body especially through stomach and S.I.
       • Controlling secretion of digestive juices
   • Function of gut is controlled by content of the gut and not by status of the body
   • Senses the lumen contents:
     • Distension = mechanoreceptors
     • Osmolality -> osmoreceptors
     • Acidity -> chemoreceptors
     • Digestive products -> chemoreceptors

2. Receptor activation triggers 3 pathways

   • Neuronal reflexes - short and long
     • Long = include nerves of autonomic nervous system and expand out to the peripheral nervous system
       • Extrinsic nerve = long reflex interact with ENS
         • Para = increases motility and secretion of substances
         • Sympa = reduces motility
       • In gut wall = vagovagal reflexes (long), where the vagus nerve carries fibres up to brain stem = stimulate efferent nerves
     • Short = gastrointestinal tract has its own nervous system within the wall of the gut and intestine = the enteric nervous system (Enteric meaning intestine, gastric means within stomach) - It is made of 100 million nerve cells present in 2 plexuses in the body
       • Enteric nervous system (ENS) = short reflex
         • Signals from afferent nerves from sensory cells in the gut wall synapse onto interneuron/linking neurone
           • Efferent neurone = produces an action potential
       • Important in motility control and coordination such as peristalsis
     • Muscosa = epithelial cells
       • Sub mucosa and layers of smooth muscle are linked to motility lectures
       • Plexuses are collection of nerve cell bodies
       • Between the 2 layers of smooth muscle is myenteric plexus
   • Hormones
     • Peptides released from wall of gut
     • Sent via blood to parts of body
       • Slow/ long term
     • Main hormones = secretion, gastrin and CCK
       • CCK
         • Secreted by the intestine
         • Affects pancreas, gall bladder, stomach
           • Paracrine transmission = control of acid secretion in stomach
             • Short hormones
               • Don't enter the blood - diffuse through ISF
         • CCK is a neurotransmitter which is also a hormone secreted when chime arrives in the duodenum
       • CNA = satiety
       • Gastric -> histamine
         • Prostaglandins, bradykinin which are released in response to irritation of gut wall
     • Secreted by the enteroendocrine cells in the mucosa into blood

3. Structure of small intestine

   • Large surface area – villi and microvilli
   • In between = crypts of lieberkuhn
   • Sites of stem cells which divide = all cells cover crypt and villus
   • Villi = absorptive cells
   • Enteroendocrine cells are less common
   • Paneth cells = immune system
   • Goblet cells secrete mucus and cover lining of Small Intestine
   • Undifferentiated crypt cells are daughters of the divided crypt cells

4. Structure of large intestine

   • Have crypts but no villus
   • Cell migration creates problem for innervation
     • Nerves can’t follow sensory cell from crypt to villus
       • Solution = enterochromaffin cells
   • 90% of enteroendocrine cells are enterochromaffin cells
   • Mucosal cells are enterochromaffin cells, which secrete adrenaline
     • Enterochromaffin cells are the main chemo and mechanoreceptors of the gut which have a microvillus membrane based in the lumen
       • Stimulation = release of serotonin (5-HT)
   • Vesicles packed with neurotransmitters called 5-HT
     • 5-HT diffuses through i.s.f. and stimulates sensory nerves via 5-HT3 receptor-operated channels

5. Salivary secretion

   • Composition: 99% - H2O, 1% proteins, mucins, α-amylase, anti-bacterial
   • Ions: Na+, Cl-, HCO3-
   • pH = 7.0
   • Functions
     • Lubrication
     • Solvent
     • Protects teeth and gums
   • 3 pairs of salivary glands
     • Paratid - cheeks
     • Sublingual - in tongue
     • Submandibular - under jaw bone
   • Acinar cells produce all the fluid - secret Na Cl which produces and osmotic gradient
     • Ductal cells - reabsorb NaCl and conserve sodium
   • Control is almost entirely nervous and hormones are too slow
   • Basal rate of secretion: Secreting certain amount per min – 0.5mlmin-1

6. Acinar secrection

   • Anions are important: Cl- movement is central to salivary secretion
   • Basolateral membrane transporter uses sodium gradient to move Cl- into cell as well as K+
     • Na+ ions moves out via Na/K+ pump, K+ ion channels open when transporter is pumping ions in
       • 3 ions move in: Then Na+ moves out and K+ leaves cell
     • Cl- is negative so as Cl- moves into the lumen = lumen side more negative → provides a gradient for Na+ ions to move paracellularly from blood into lumen
   • Aquaporin proteins in salivary glands allow H2O to move across due to osmotic effect
   • When not secreting saliva; no parasympathetic drive so these channels tend to be closed
     • When parasympathetic nerves stimulate glands, they release ACh; on basolateral membrane are ACh receptors
       • When activated = release of intracellular Ca2+ ions inside the cell = binds to these channels = open