Introduces the 3 different phases of secretion within the GI Tract. Then starts to go into more depth of each phase, starting with the cephalic phase. Goes into detail about what triggers the phase, as well as stating the importance of it, and the receptors, cell types and enzymes present. It expla...
Gastrointestinal Tract Secretions and its Regulation
1. CEPHALIC PHASE 2. GASTRIC 3. INTESTINAL PHASE
Originate from smell, PHASE Begins when chyme first
sight, thought or taste Begins with the enters the small intestine
of food (BEFORE arrival of food in (CHYME ENTERS SMALL
FOOD ENTERS the stomach INTESTINE - DUODENUM)
STOMACH) (FOOD ENTERS
STOMACH)
• Initiated when RECEPTORS in the head (CEPHALIC) are stimulated by thought, sight, smell and taste of food
• It's a CONDITIONAL REFLEX; prepare stomach for arrival of food
• CONDITIONAL REFLEX = a response to a stimulus, that has been acquired by operant conditioning
• Involves the CENTRAL NERVOUS SYSTEM --> via EFFERENT PARASYMPATHETIC nervous system (vagus nerve)
WHY IS IT IMPORTANT?
• FEED-FORWARD MECHANISM - allows efficient use of food e.g. Digestion, movement and absorption
• Lubrication and mastication of food
• On swallowing, enzymes and emulsifiers are already present with the GI tract, at the correct pH
WHAT IS STIMULATED FOR SECRETION DURING CEPHALIC PHASE?
• SALIVA PRODUCTION (100% OF TOTAL), WATERY, ENZYME-RICH
• GASTRIC ACID (~30% OF TOTAL)
• Gall bladder contraction; releasing bile into the small intestine
• Exocrine pancreatic secretions: releasing bicarbonate and digestive enzymes into small intestine
Integration of Physiological Systems Page 1
, • SALIVA COMPOSITION
i. Mucous
ii. Enzymes e.g. Α-amylase and lipase
iii. Electrolytes (bicarbonate and potassium)
• The cephalic phase of gastric secretion is mediated by the vagus nerve - LONG VAGAL
REFLEX
1. OUTPUT FROM CNS:
i. Neurogenic signals originates from the CEREBRAL CORTEX and in appetite centres
ii. Transmitted through DORSAL MOTOR NUCLEI of the vagi
2. STIMULATE PARASYMPATHETIC VAGUS NERVE FIBRES
3. SYNAPSE/STIMULATE NEURONS IN THE ENTERIC SUBMUCOSAL PLEXUS OF THE
STOMACH
4. STIMULATE SECRETIONS FROM STOMACH
• Parasympathetic fibres in vagus nerve synapse in submucosal plexus
• INNERVATE THE FOLLOWING CELLS IN STOMACH TO STIMULATE THEIR SECRETIONS:
i. PARIETAL (oxyntic) cells --> HCL
ii. MUCUS cells --> mucus
iii. CHIEF cells --> pepsinogen
iv. ENDOCRINE cells: G cells --> the hormone GASTRINE
• Hydrochloric acid (HCL) is the major component of gastric secretions
• CONTROL of ACID SECRETION by the PARIETAL CELL (1/3 total acid production)
• The H+ and Cl- are secreted SEPARATELY
i. H+ is secreted into the LUMEN by the activity of the H+/K+ ATPase - called PROTON
PUMP
• GASTRIC ACID (HCL) PRODUCTION AND SECRETION BY PARIETAL CELLS IS
STIMULATED BY:
1. Acetylcholine (vagus linked in with enteric nervous system) binds to M3 receptors
2. Local production of paracrine hormone HISTAMINE mainly from ENTEROCHROMAFFIN-
LIKE CELLs- ECLs binds to H2 RECEPTORS
3. Circulating gastrin (G-cells) binds to gastrin receptors
• Amount of acid produces and secreted in the stomach lumen is proportional to number of ligands binding
Integration of Physiological Systems Page 2
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