Physiology Flashcards
(30 cards)
GIT Regulation
Centrally by neural and endocrine control of the autonomic nervous system. The autonomic nervous system is linked to the central nervous system with the parasympathetic nervous system mostly via the vagus nerve and pre-ganglionic fibres which synapse on intrinsic neurons and sympathetic nervous system via post ganglionic fibres from the pre-vertebral ganglia via sphlanic neurons and arteries into the gut wall and direct synapse to muscles and glands. The gut also contains afferent neurons that relay information to the central nervous system.
Intrinsically by the it’s own neural system, the enteric nervous system. ENS lies within the gut wall as Auerbach’s plexus and Meissner’s plexus. Contains mechanoreceptors which receive information about the distension of the wall, chemoreceptors which recieve information about the chemical conditions and osmoreceptors which detect the osmolarity of the contents. Acetylcholine, peptides and bioactive amines are the ENS neurotransmitters that regulate epithelial and motor function.
Oral Cavity and Oesophagus Motility (Swallowing)
Direct CNS control of prehension by the glossopharyngeal and trigeminal nerve. Mastication breaks up feed, moistens, lubricates by mixing with saliva and is performed by the jaws, tongue and cheek.
In swallowing, food is directed into the gut and away form the lungs. Breathing stops, the soft palate elevates causing closure of the nasopharynx. The tongue then pushes against the hard palate, the hyoid bone and larynx are pulled forward as the glottis moves under the epiglottis. Pharyngeal contractile waves push the bolus into the oesophagus and relaxation of the upper oesophageal sphincter. The brainstem motor neurons control swallowing via efferent neurons such as the facial, vagus, hypoglassal, glossopharyngeal and motor trigeminal. Striated muscle of the oesophagus is under vagal control. Constriction of the oesophagus is caused by contractions of circular muscle and dilation is created by contraction of the longitudinal muscle. This moves food down the oesophagus via peristalsis.
Stomach Motility
Stomach performs mechanical and chemical digestion. In proximal part of the stomach there are weak tonic contractions that shape and gently squeeze. This adaptive relaxation allows the stomach to expand to accommodate ingesta and there is no increase in luminal pressure. In the distal stomach the antrum has strong perstaltic contraction via slow wave to the pylorus. The pylorus constricts as the wave approaches. Liquid and small particles move to the duodenum and pump action crushes and forces ingesta back up antrum for mechanical digestion into smaller parts.
Interdigestive motility complex occurs between meals when the stomach is cleared of material and is achieved by strong antral peristalsis combined with pyloric relaxation and occurs once per hour.
Control of Stomach Emptying
Emptying of the stomach occurs due to dilation of the stomach and increased gastrin and peptide secretion in the stomach. Conditions in the duodenum such as low pH, high fat content, high osmolarity and high pressure send inhibitory messages to prevent the stomach from emptying. The rate of gastric emptying must match the small intestine rate of digestion and absorption.
Vomiting is a coordinated reaction form the brainstem that occurs due to relaxation of the stomach and lower oesophagus and constriction of the pylorus. Abdominal contraction occurs and inspiration against a closed glottis and relaxation of the upper oesophageal sphincter. A chemoreceptor trigger zone in the third ventricle sends blood toxins, drugs and inflammatory disease products. This affects the salivary and respiratory centras as well as the abdominal muscles.
Small Intestine Motility
Mixing segmentation waves in which contraction of the circular muscle layer occludes the lumen and moves the content forward and backwards. Peristaltic movements in which segmental contractions followed by sequential contractions that propels the content forward. Rhythmic segmentation results in mixing of the interstitial contents and enhances absorption. Migrating motility complex pushes material via powerful peristalsis over large areas. It clears undigested bacteria and may even prevent colonic bacterial colonisation. Ileal distension relaxes ileocecal sphincter and stimulates peristalsis to move ingesta into the colon.
Large Intestine Motility
Illeocecal sphincter prevents retrograde movement of colonic contents. It relaxes in response to reflexes form the stomach and small intestine and constricts when the caecum is distended or irritated. Circular contractions contractions have mixing and absorptive functions similar to the small intestine. There are propulsive short range peristalis and there is mass movement about 1-3 times a day forcing faeces into the rectum.
Afferent impulses stimulated by movement of faeces into the rectum. Parasympathetic efferent impulses via the pelvic nerves stimulate peristaltic contraction of rectum and relaxation of internal sphincter. Voluntary signals via the pudendal nerve control the actions of the external anal sphincter.
In carnivores, retrograde peristalsis is initiated by mobile pacemaker regions that generate propulsive contractions in both directions away from the origin.
Oral Secretion (Salivary Glands)
Moistens, lubricates and initiates digestion. Salivary secretions originate in the gland acini and are modified in the collecting ducts. Salivary glands are regulated by the parasympathetic nervous system. Secretions include water, electrolytes, mucous and amylase which is species dependent. Duct cells modify molecules, sodium and chloride are reabsorbed but potassium and bicarbonate is excreted but when secretory rate is high, this process is impaired. Saliva secretion is regulated by high nervous areas which lead to increased secretion from anticipation and chewing. No endocrine control.
Gastric Secretion
Two general types of mucosa including glandular and non-glandular. The cardiac regions of the stomach secrete an alkaline mucous and pyloric glandular region secretes gastrin and pepsinogen.
Glandular pits contain mucous neck cells, parietal cells, chief cells, G cells and enterochromaffinc cells which secrete histamine.
Secretions and Regulation of Parietal and Chief Cells
Parietal cells secrete HCl from the endoplasmic reticulum and H+ from CO2 and carbonic anhydrase. There is an energy requiring canalicular-luminal membrane H+, K+ exchanger ATPase, Cl- canalicular membrane and Na+ pump at luminal membrane and Na+/K+ exchanger at the basal membrane. Intrinsic factor secreted for vitamin B12 absorption in the ileum. Histamine, gastric and acetylcholine receptors influence secretion in response to H+.
Chief cells secrete pepsinogen which is from a family of proteases which is secreted in an inactive proenzyme form. Is not secreted in an active form as it will degrade its own tissue. Is stored as granules in chief cells and exocytosed in stimulus response. Acid pH of gastric juice activates autocatalysis. Chief cells respond to H+ levels and gastrin receptors.
Regulation of Gastric Juice Secretion (3 phases)
Cephalic phase-begins by sight smell and chewing of food. The brain, through the PNS sends messages to many organs and in particular the stomach which results in increased secretion of HCl from parietal cells, increased secretion of gastrin from G cells and increased histamine from enterochromaffin cells. Gastrin and hostamine potentiate the actions of acetylcholine at parietal cells leading to increased in HCl.
Gastric phase-occurs when food enters the stomach and causes gastric distension. Build up of peptides and amino acids stimulates G cells to produce gastrin. Acetylcholine stimulates parietal and G cells which further increase HCl secretion. Chief cells are stimulated to release pepsinogen.
Intestinal phase-stimulatory effects include chyme, stretch, peptides, a pH>3. inhibitory effects include pH fall of duodenal chyme, inhibition of secretion via secretin and enteric reflex of cells and parietal cells, fat in duodenal chyme increases gastric inhibitory peptide and CCK which inhibits parietal cells and hyperosmotic chyme inhibits parietal cells.
Pancreas Secretions
The pancreas is composed of compound acini glands that secrete proteases such as trypsn, chymotrypsn that are activated in the duodenum, lipase, pancreatic amylase, nucleases and other hydrolytic enzymes. Duct and centroacinar cells modify serous acinar secretion by producing an alkaline secretion to increase the pH and add alot of bicarbonate as a buffer. Pancreatic secretion is controlled by hormones secretin and cholecystokinin secreted from S and I cells respectively. Secretin is secreted in response to H+ and CCK is secreted in response to peptides, fatty acids and H+.
Liver and Gall Bladder Secretions
Bile contains phospholipid and cholestrol maintained in aqueous solution by the detergent action of bile acids. The gall bladder stores and concentrates bile in the period between feeding. Secretion is initiated by the presence of digesta in the duodenum and stimulated by the return of the bile acids to the liver. Hepatocytes synthesise bile or recycle from the blood. Bile salts are composed of cholestrol, cholic acid and glycerine or taurine, phosopholipids, bile pigments and mucous. It is an isotonic solution that is secreted into the canaliculi where duct cells secrete HCO3- making it more alkaline which is all in response to secretin. The sphincer of oddi diverts bile t the gall bladder where the gall bladder epithelium absorbs sodium, chloride and bicarbonate from the liver and the bile salts are concentrated x5 and forms micelles.
Enterohepatic circulation is products from the liver become bile, stored in the gallbladder, released into small intestine and reabsorbed by the portal vein to the liver.
Intestine Secretion
Crypts of leiberkuhn are found throughout the small intestine and has goblet cells that produce mucous. They also secrete water and electrolytes. Large intestine secretes mucous due to tactile and enteric nervous system. The mucous gives protection to the epithelial cells and acts as a lubricant.
Steps of Digestion
Digestion is the break down of ingesta to absorbable nutrients which involves four steps:
1. Mechanical breakdown
2. Chemical action of HCl in stomach
3. Enzymatic cleavage of complex molecules
4. Microbial fermentation
The stomach converts ingesta to chyme due to low pH and digests feed, especially connective tissue, activates pepsinogen into pepsin and maintains optimum pH for pepsin proteolysis.
The luminal phase of enzymatic digestion results in the breakdown of large polymeric molecules to small polymers by digestive enzymes that are active in the lumen of the gut.
The membranous phase involves the breakdown of small polymers such as polysaccharides to monomeric molecular suitable for absorption. This occurs via digestive enzymes synthesised in the enterocyte and attached to the apical membrane.
Small Intestine Structure
Small intestine mucosa has a large surface area and epithelial cells with leaky junctions between them. A single cell epithelial layer covers vili and crypts with a jelly like layer above this composed of glycoprotein known as glycocalyx, mucous and an unstirred water layer. This creates a diffusion barrier through which nutrients must pass to enter enterocytes.
This epithelial mucosal blanket is a dynamic defence system that is protective, is a nutrient source for microflora, prevents bacterial adhesion to the surface and modulates dietary amino acid entry into circulation. The enterocyte has an apical brush border and a basolateral membrane. The attachments between adjacent enterocytes are called tight junctions. Adjacent basolateral membranes are normally unattached forming the lateral space.
Carbohydrate Digestion
Most fibres cannot be digested and are subjected to microbial fermentation. Sugars are made up of repeating saccharide units and include glucose, galactose and fructose. Complex sugars include lactose and sucrose. Starch comes in two forms, amylase a double helix of maltose units and amylopectin, a branching chain of maltose units. The two major forms of breakdown are amylose by salivary and pancreatic amylase into maltose and maltotriose and amylopectin by isomaltose and limit dextrins. The luminal phase of digestion of carbohydrates applies to starch and some fibres resulting in the production of short chain polysaccharides. Sugars are subsequently digested in the membranous phase at the brush border mainly to glucose. Different transport mechanisms such as facilitated diffusion or Na+ co-transport are then needed to take up the monosaccharide’s into the blood. Sugars are exclusively absorbed as monosaccharides.
Protein Digestion
Proteins are polymers of amino acids joined by peptide bonds. Proteins are digested into peptides and then from peptides into amino acids. Protein is broken down into peptides via proteases such as pepsin, trypsin, chymotrypsin and carboxypeptidase. Peptides are broken down into amino acids by peptidases such as dipeptidases and oligopeptidases at the epithelial intestinal surface. Proteins may be absorbed as di/tri peptides or as free amino acids. These enzymes are secreted as pro-enzymes that are activated by the low pH in the stomach and pre-activated enzymes.
Lipid Assimilation
Involves the process of emulsification, hydrolysis, micelle formation and absorption. Examples of lipids include triglycerides, cholestrol, cholestrol esters, phospholipids, waxes and vitamins. The process of breaking down fat globules into smaller fat droplets is called emulsification and is achieved by bile salts and lecithin and occurs in the stomach and duodenum. Pancreatic, gastric and salivary lipases break triacyl glycerides into diacyl glycerides or monoacyl glyceride in the lumen. These are then resynthesised into triacylglycerides or phospholipids in the intestinal epithelial into chylomicrons. The chylomicrons are then transported into the blood stream.
Methods of Nutrient Absorption
Passive transport-occurs through ion channels in cell membranes or directly through tight junctions. The products of membranous phase digestion are absorbed by sodium co-transport.
Facilitated diffusion-some molecules need a specific carrier to transport them from one side of the cell membrane to the other.
Active transport-some nutrients such as glucose and amino acids must be absorbed actively. These nutrients move against a concentration gradient therefor using ATP. The active transport system of greatest importance is the sodium/potassium ATPase pump which is the mechanism that keeps the interior of the cell electrially positive compared to ECF. Active transport of Na+/K+ ATPase system (primary) establishes the gradient that drives the sodium/hydrogen exchanger (secondary) which establishes the gradient that drives the chloride/bicarbonate exchanger (tertiary).
Membrane bound transport protein-where the transport protein resides in the cell membrane and picks up substances on one side of the membrane and carries them to the other side.
Nutrient absorption includes both absorption into and out of the enterocyte.
Sodium Absorption Methods
Co-transport
Simple diffusion
Coupled with chloride
Chloride Absorption Methods
Couple sodium-chloride absorption
Paracellular chloride absorption
Chloride bicarbonate exchanger
Bicarbonate Absorption Method
Carbon dioxide and water broken down into H+ and HCO3-
Potassium Absorption Method
Simple diffusion
Water Absorption Method
Moving through cell itself or through lateral spaces
