Anatomy and Physiology lecture notes on the gastrointestinal tract PDF

Preview

Summary

Anatomy and Physiology lecture notes on the gastrointestinal tract...

Description

GIT (gastrointestinal tract)

Major organs       

Oral cavity (mouth) Esophagus Stomach Small intestine Large intestine Rectum Anal sphincter

Accessory organs      

Teeth Liver Pancreas Salivary glands Spleen Gallbladder

Sphincters      

upper esophageal sphincter lower esophageal sphincter (gastroesophageal sphincter) pyloric sphincter sphincter of oddi iliocecal sphincter anal sphincter

General Function of GIT      

Mixing Movement Absorption o Digestion Secretion Adequate neuromuscular control

Mouth (oral cavity)    



Provides lubrication Contains salivary glands Bolus is broken down by salivary amylase produced by salivary glands Movement on tongue facilitates; o mixing of food o production of voice anatomy of the oral cavity o Roof is a tough hard palate in the front half and soft in the back half o Floor is formed by the tongue

Salivary glands 



There are 3 major salivary glands o Submandibular o Sublingual o Parotid (responsible for (60-70% saliva production) Saliva is an antibacterial and anti-inflammatory agent

Pharynx  

Site of Air and bolus entry Division between esophagus and trachea

Esophagus  

Also known as food pipe Move food down by a process known as peristalsis

Spleen 

Role is to detoxify along with liver

Pancreas  

Produces insulin and other digestive enzymes Part of the endocrine system

Stomach   

Storage organ Mixing organ (churning) Provides acidic environment

Small intestine  

Site of maximum nutrient absorption Divided into 3 parts o Duodenum

o o

Jejunum Ilium

Large intestine  

Main function is excretion Secondary function is absorption

Four layers of the Gastointestinal Tract The GI tract contains four layers: the innermost layer is the mucosa, underneath this is the submucosa, followed by the muscularis and finally, the outermost layer - the serosa The structure of these layers varies, in different regions of the digestive system, depending on their function.

Serosa 

Protects and lubricates (reduces friction) the outer environment

Muscularis  

muscularis is responsible for the segmental contractions and peristaltic movements in the gastrointestinal (GI) tract. Contains two layers of muscle o Circular smooth muscle layer (2-10µm) o Longitudinal smooth muscle layer (200-500µm) The two layers of muscle are arranged in a branching lattice work (criss-cross arrangement) the fibers and arranged in the form of bundles and work as a unit in syncytium. The bundles are both co-joined (to allow working as a unit) and separated by lose connective fibers (to be able to function even after damage of one fiber)

Layer of both circular and muscular are covered by interstitial cells of Cajal (ICC), serving as a pacemaker which creates the bioelectrical slow wave potential that leads to contraction of the smooth muscle Submucosa    

Is relatively thick compared to mucosa highly vascular has glands and nerve plexuses (Meissner’s plexus) contains the lymphatics

Functions of the lymphatic system   

Help rid the body of toxins, waste and other unwanted materials. Transport of lymph absorbs and transports fatty acids and fats

Mucosa  

function is absorption and secretion contains specialized goblet cells that secrete sticky mucus throughout the GI tract

Membrane potential in intestinal smooth muscles   

Are of two main types; slow wave and spike potentials Resting membrane potential: ( −50 to −60mv) True potential: (−40mv or more positive)

Slow wave: initiated continuously by the interstitial cells of cajal (also known as electric pacemaker of the GIT) Spike potentials: last for 10-20ms 



are true action potentials (depolarization) which are initiated by the o parasympathetic system (80-90% excitatory) o stretch (distention) o acetylcholine (released by cholinergic fibers) Hyper polarization cased by; o Norepinephrine (in abundance) o Epinephrine (sparsely) o Sympathetic system o Na-K pump normalizes RMP

Tonic contractions:  

Are continuous partial contractions caused by o repetitive spike potentials o hormonal factors o continues entry of calcium ions into the interior of the cell o other factor which bring about partial depolarization

Channels involved

 

Sodium channels: fast channels Calcium-sodium channels: slow to open slow to close, accounts for long duration of action potentials

Enteric nervous system   

Number of neurons in the enteric nervous system is approximately 100 million (equal to number in spinal cord) Involved in controlling GIT secretion and movements Mainly composed of two plexuses o Auerbach’s plexuses/ myenteric plexuses (between longitudinal and circular smooth muscles)  Controls mainly gastrointestinal movements  Mostly excitatory (parasympathetic- preganglionic nerve fibers)  Has overall effect o Submucosal plexuses/ meissner’s plexuses (lies in the submucosa)  Controls mainly hormonal secretion  Mostly inhibitory (sympathetic- postganglionic)  Has local effect

Myenteric Mainly involved in muscle activity (increase tonic contractions, increased rhythm of contractions, increased velocity of conduction of excitatory waves) some neurons are inhibitory, secreting VIP (vasoactive intestinal polypeptide) inhibits function of some intestinal sphincter

Submucosal Mainly concerned with minute segments of intestine controlling (local intestinal secretion, local absorption, local contraction of submucosal muscle)

Neurotransmitters secreted by enteric neurons         

Acetylcholine Norepinephrine ADP Serotonin Dopamine Cholecytokinin Substance P Vasoactive intestinal polypeptide Somatostatin (inhibitory function endocrine pancreatic secretions)

Parasympathetic nervous system Has cranial and sacral division. Excitatory effect on enteric nervous system Cranial parasympathetic nerve consist of Vagus nerve (X) Sacral parasympathetic nerve include S2,S3,S4

Sympathetic nervous system Sympathetic nerve fiber originate in the spinal cord between T5-L2, inhibitory effect on the whole GIT Thoracic nerve fiber Lumbar nerve fiber

Postganglionic parasympathetic nerve fibers mostly supply the myenteric and submucosal plexuses Supply the oral cavity, intestinal and anal region extensively Secret mainly acetylcholine

Postganglionic sympathetic nerve fibers supply the whole GIT Supply the whole GIT Secret mainly norepinepherine and to a slight extent epinephrine

Afferent (towards) sensory nerve fibers (GIT)  

 

many afferent nerve fibers cell bodies are found in the enteric nervous system & some in dorsal root of ganglia of spinal cord Afferent sensory gut fibers can be stimulated by o Irritation of the gut mucosa o Excessive distention of gut o Presence of specific chemical substance in the gut Signals can be either excitatory or inhibitory 80% of vagus nerve signals are afferent

Reflexes of GIT 





Are of 3 types 1. Enteric reflexes: control mostly peristalsis, mixing contractions, local inhibitory effects 2. Short reflexes: include gastrocolic reflex, enterogastric reflex, colonoileal reflex 3. Long reflexes: pain reflex, defecation reflex, reflexes from stomach and duodenum Gut → Spinal cord/ brain stem → GIT 1. Pain reflexes: cause general inhibition of GIT 2. Defecation reflexes: travel from the colon and rectum to the spinal cord and back to again to produce a powerful colonic, rectal and abdominal contraction required for defecation Gut → prevertebral sympathetic ganglia → GIT 1. Enterogastic reflex: signals from colon and small intestine to inhibit stomach motility and stomach secretion

Hormones of GIT Hormone Gastrin

Cells & site of secretion G cells of antrum of stomach, duodenum and jejunum

Stimulus   

Distention of stomach Protein products nerve

Action Stimulates  Gastric acid secretion (HCl)  Mucosal growth

CCK (cholecystokinin )

I cells of the duodenum, jejunum, ileum

Stimulates  pancreatic enzymes secretion  pancreatic bicarbonate secretion  gallbladder contraction (emulsifying fats)  growth of exocrine pancreas Inhibits  gastric emptying (↓motility) Secretin S cells of duodenum,  acid Stimulates jejunum, ileum  fats  pepsin secretion  pancreatic bicarbonate secretion (↑alkalinity)  biliary bicarbonate secretion  growth of exocrine pancreas Inhibits  gastic acid secretion (HCl) Gastric K cells of the duodenum and  protein Stimulates inhibitory jejunum  fat  insulin release peptide  carbohydrate Inhibits  gastric secretion (↓motility) Motilin* M cells of the duodenum  fats Stimulates and jejunum  acid  Gastric motility  nerve  Intestinal motility * During fasting motilin is released by the stomach and duodenum to increase GIT motility. Motilin is released cyclically and stimulates waves of gastrointestinal motility called interdigestive myoelectric complexes   

Protein Fat acid

Movements in the gastrointestinal tract  Propulsive movement: causes forward movement of food along the tract at an appropriate rate to aid digestion and absorption. 1. Propulsive movement is also known as peristalsis or myenteric reflex 2. Is the inherent property of all syncytial smooth muscles 3. Mechanism: Food collects in gut ↓ Stimuli provided in form of distention (slight relaxation) ↓ Contractile ring appears around gut by stimulated enteric nervous system, gut wall contracts 2-3cm ↓

Peristalsis initiated 4. Law of gut: the movement of peristaltic wave towards the anus Peristalsis theoretically occurs in either direction but dies out rapidly towards the oral cavity (mouth) and continues on towards anus 5. Why peristaltic movement is also called myenteric reflex? Without the presence of the myenteric reflex the complex pattern of rhythmic contraction and relaxation of the gut would not occur.



Mixing movement: keeps intestinal contents thoroughly mixed. 1. is different in different parts of alimentary tracts 2. in some places peristaltic contractions cause mixing (when forward progression of wave is block by sphincter)

Gastrointestinal blood flow

    

GIT blood flow is an extensive system known as splanchnic circulation Includes blood flow through the gut, spleen, pancreas and liver Water soluble material (carbohydrates and proteins) are transported in the portal vein Fats absorbed in the intestine tract are not transported in the portal vein but instead absorbed into the intestinal lymphatic Mechanism All the blood from gut, spleen and pancreas is directed to the liver via portal vein ↓

Blood passes in the liver sinusoids and leaves via hepatic vein [Allows reticuloendothelial cells of liver sinusoids to remove bacteria and other particulate matter thus preventing transport of harmful agents into the body] ↓ Blood from hepatic vein drains into the vena cava

Hypovolemic shock and autoregulator escape 

Hypovolemic shock is a life-threatening condition that results when you lose more than 20 percent (one-fifth) of your body's blood or fluid supply. o To compensate for this loss the sympathetic nervous system reduces the gut supply for a few minutes (causing vasoconstriction) to preserve blood flow to the brain and heart.



Auto-regulatory escape is the phenomena in which after prolonged tissue hypoxia (oxygen deficient state) of shock; sympathetic-mediated vasoconstriction gives way to vasodilatation in GIT....