LECTURE 2 (Propulsion and mixing of food) Flashcards
Most of the muscles of chewing are innervated by the motor branch of which cranial nerve?
5th cranial nerve
The chewing process is controlled by the ______ _______
Brain stem
What is the rate of digestion dependent on?
The total surface area exposed to the digestive secretions
Explanation: Digestive enzymes act only on the surfaces of food particles
What is the importance of saliva?
- Begins carbohydrate digestion through SALIVARY AMYLASE
- Facilitates swallowing by moistening food particles
- Has antibacterial effects
[first by LYSOZYME - enzyme that destroys bacteria by breaking down their cell walls] - Helps swallowing, speech, taste and oral health
What can salivary secretion be described as?
Continuous and can be reflexly increased
Describe the Salivary simple reflex
1) Pressure receptors and chemoreceptors in mouth within the oral cavity respond to the presence of food
2) Salivary center in medulla via Afferent nerves
3) Autonomic nerves
Describe the Salivary conditioned reflex
1) Thinking of food, seeing food or smelling food initiates the Cerebral cortex
2) Salivary center in medulla
3) Autonomic nerves
What are the different stages of Deglutition (swallowing)?
- Voluntary stage (initiates swallowing process)
- Pharyngeal stage (involuntary, passage of food through pharynx into oesophagus)
- Oesophageal stage (involuntary, transports food from pharyngeal to stomach)
What happens during the Involuntary Pharyngeal Stage of swallowing?
1) Bolus of food stimulates epithelial swallowing receptor areas
2) Trachea closed, Oesophagus opened and a fast PERISTALTIC WAVE initiated by the nervous system of the pharynx forces the bolus of food into UPPER OESOPHAGUS
3) Impulses are transmitted from these areas to the sensory portions of the TRIGEMINAL and GLOSSOPHARYNGEAL NERVES into the MEDULLA OBLONGATA
4) The motor impulses from the swallowing center to the PHARYNX and UPPER OESOPHAGUS are transmitted by the V, IX, X and XII cranial nerves
What happens during the involuntary oesophageal stage?
1) PRIMARY PERISTALSIS -> continuation of the peristaltic wave that begins in the pharynx and spreads into the oesophagus during the pharyngeal stage of swallowing
2) SECONDARY PERISTALTIC WAVES result from distention of the oesophagus itself by the retained food
3) “RECEPTIVE RELAXATION” of the LOWER OESOPHAGEAL SPHINCTER occurs ahead of the peristaltic wave allowing easy propulsion of the swallowed food into the stomach
What does the stomach secrete?
Hydrochloric acid (Hcl) and enzymes that begin protein digestion
Where does Gastric storage, Gastric mixing and Gastric emptying take place?
Gastric storage = body of stomach
Gastric mixing = antrum of stomach
Gastric emptying = duodenum of stomach
What is the main gastric factor that influences the strength of contraction?
The amount of chyme in the stomach
What is the function of the stomach?
- Storage of large quantities of food
- Food mixing with gastric secretions until it forms CHYME
- Slow emptying of chyme from stomach to small intestine at a suitable rate
- Secretes hydrochloric acid (HCl) and enzymes that begin digestion
Where do waves of contraction begin in the stomach?
In the middle of the body of the stomach and move distally along the caudad stomach
What does parasympathetic stimulation and the hormones gastrin and motilin do?
Increase the frequency of action potentials and the force of gastric contractions
What does sympathetic stimulation do?
Decrease the frequency of action potentials and the force of contractions
What are Migrating Myoelectric Complexes?
Periodic gastric contractions during fasting which are mediated by motilin
What are the factors in the duodenum that influence the rate of gastric emptying?
- The neural response
[mediated through INTRINSIC PLEXUSES (short reflex) and AUTONOMIC NERVES (long reflex) -> constitute the ENTEROGASTRIC REFLEX] - The hormonal response
[release of ENTEROGASTRONES (hormones) from small intestine mucosa into blood (secretin and CCK)]
What is the importance of mucous cells?
- Line the gastric pits and entrance of the glands
- Secrete a thin, watery mucus
What is the importance of chief and parietal cells?
- Line the deeper parts of the gastric glands
- Chief cells secrete enzyme precursor PEPSINOGEN
- Parietal cells secrete HCl and intrinsic factor
___________ _____ is secreted by parietal cells and activates pepsinogen
Hydrochloric acid
What is the function of HCl?
- Activates pepsinogen into PEPSIN and provides acid environment optimal for pepsin action
- Aids breakdown of connective tissue and muscle fibers
- Denatures protein
- Along with salivary lysozyme, HCl kills most microorganisms ingested with food
What does the Pyloric Sphincter do?
Prevents passage of food particles until they have become mixed in the chyme to almost fluid consistency
What promotes increased emptying from the stomach?
Increased food volume
What are factors that initiate enterogastric inhibitory reflexes?
- Distention of the duodenum
- Presence of any irritation of the duodenal mucosa
- Acidity of the duodenal chyme
- Osmolality of the chyme
- Presence of certain breakdown products in the chyme
______ can inhibit gastric emptying when excess quantities of chyme enter the duodenum from the stomach
Cholecystokinin (CCK)
Describe the antibacterial effects of saliva
- Lysozymes destroys certain bacteria by destroying the cell wall
- Salivary IgA antibodies
- Salivary lactoferrin which binds to iron that bacteria need to multiply
- Rinsing away material that could be a food source for bacteria
What are the four motility aspects?
- Filling
- Mixing
- Storage
- Emptying
What is Receptive relaxation?
During a meal the folds get smaller and nearly flatten out as the stomach relaxes slightly with each mouthful -> allows stomach to accommodate meal with little change
Describe how Gastric storage takes place in the body of the stomach
1) Group of pacemaker cells (interstitial cells of cajal) located in The Fundus of stomach generate a SLOW-WAVE POTENTIAL that sweeps down the length of the stomach towards th PYLORIC SPHINCTER
2) Once initiated, PERISTALTIC WAVE spreads over the Fundus and Body to the Antrum Sphincter (FUNDUS & BODY WAVES are WEAK but gets stronger at ANTRUM since muscle is thicker)
3) Food delivered to stomach is stored in Body without being mixed then food goes to ANTRUM to get mixed with the gastric fluid and make up the chyme
What does Parasympathetic stimulation and Gastrin & Motilin cause?
- Increased frequency of action potential
- Increased force of gastric contraction
What does Sympathetic stimulation cause?
- Decreased frequency of action potential
- Decreased contraction
Gastric mixing is located in ________ of stomach
Antrum
What is Retropulsion?
The process of astral chyme to go back and forth by the peristaltic waves which works to grind and sheer chyme until particles are small enough for emptying
What is the driving force for gastric emptying?
The astral peristaltic contraction
What does the amount of chyme escaping to the duodenum before pyloric sphincter closes depend on?
The strength of antral peristaltic contraction
What are the factors that regulate gastric motility and emptying?
- Volume of chyme (within stomach)
- Fluidity of chyme (within stomach)
- Presence of fat, acid, hypertonicity or distention (within duodenum)
What are the functions of Mucous cells?
- Line the gastric pits and entrance of glands
- Secrete thin, watery mucous
