Gastrointestinal Flashcards
Major Layers of the GI Tract
- Mucosa
- Submucosa
- Muscularis
- Serosa
Major Layers of the GI Tract - Mucosa:
Moist epithelial membrane
Major Layers of the GI Tract - Submucosa
Rich supply of blood & lymphatic vessels & nerve fibres
Major Layers of the GI Tract - Muscularis
Responsible for peristalsis & mixing, contains an inner circular layer and outer longitudinal layer. In areas, thickening of the circular layer forms sphincters.
Major Layers of the GI Tract - Serosa
Outer protective layer
Neuronal Reg - digestive activity is modulated by what two mechanisms?
a) mechanical and chemical sensory receptors
b) Intrinsic/Extrinsic neural & endocrine control
Neuronal Reg - receptors located in the walls of the tract organ induce:
- local reflexes through ENS
- Spinal reflexes
- brain stem (medulla/pons/hypothalamus)
Neuronal Reg - receptors located in the walls of the tract organ respond to:
- stretch
- osmolarity
- pH
- food substrates
Neuronal Reg - receptors located in the walls of the tract organ modulate:
- glandular secretion of digestive juices & hormones
- stimulate smooth muscle contraction (mixing and peristalsis)
Enteric Nervous System: composed of:
Submucosal Nerve Plexus
- occupies the submucosa
- includes sensory & motor neurons
- regulates GI secretions & blood flow
Myenteric Nerve Plexus
- lies between the circular and longitudinal smooth muscle fibres
- regulates motility: mixing and peristalsis
ENS linked to the CNS how?
through afferent visceral fibres and ANS innervation
The ENS is innervated by what fibres?
Both parasympathetic and sympathetic fibres
Parasympathetic - Innervation of the ENS
- Enhances GI activity
- Cranial division: innervates oesophagus, stomach, pancreas and somewhat the intestines
- Sacral division: innervates distal half of large intestine to the anus
- Acetylcholine stimulates further ENS excitability
Sympathetic - Innervation of the ENS
- Inhibits GI activity
- Innervates entire GI tract
- Norepinephrine directly inhibits activity of the ENS
What inhibits and what stimulates ENS?
Inhibits: norepinephrine via sympathetic
Stimulates: acetylcholine via parasympathetic
Local enteric reflexes within the gut wall control:
- GI secretions
- peristalsis
- mixing contractions
- local inhibitory effects
Types of GI reflexes
- local enteric reflexes
- reflexes from pre-vertebral sympathetic ganglia
- reflexes from spinal cord/brain stem
Reflexes from spinal cord/brain stem:
- Reflexes via the vagus nerve control gastric motor and secretory functions
- Pain reflexes cause general inhibition of GI functions
- Defecation reflex - provides the powerful colonic, rectal, & abdominal contractions
Reflexes from pre-vertebral sympathetic ganglia:
- Transmit signals to other regions of the GI tract
- Gastrocolic reflex - signal from stomach to evacuate colon
- Enterogastric reflexes - inhibition of motility and secretions in the stomach by the small intestine/colon
- Colonileal reflex - inhibits emptying of small intestines into the colon
Two basic electrical waves of smooth muscle contraction
Slow waves
Spike potentials
What are slow waves?
Not action potentials but changes in resting membrane potential.
Caused by Cajal (pacemaker) cells.
[ Most GI contractions occur rhythmically and this rhythm is determined mainly by the frequency of so-called “slow waves” of smooth muscle membrane potential]
What are spike potentials?
Action potentials, occur at the top of slow waves.
AP are created by an influx of both Na+ and Ca2+.
[Slow wave: only Na - no contraction, spike: Na and Ca, contraction]
What are the contractile forces of the GI tract?
Peristalsis: propulsive movement
Segmentation: Mixing movement
Mastication:
Voluntary: placing food in mouth, manipulation of chewing motion
Involuntary:
- chewing begins digestion & is controlled by nuclei within the brain stem that innervates the jaw muscles.
- chewing reflex: in response to food in mouth (bolus), rhythmic jaw movements are initiated by stretch receptors in the cheeks, gums, tongue, but can be voluntary as well.
- Important for digestion, especially of fruits & raw vegetables due to indigestible cellulose membranes.
Salivation Allows:
- Tasting/moistening of food
- Formation of bolus
- Chemical breakdown of starches (alpha amylase)
- cleanses mouth
Salivation glands:
Parotid (cheek right in front of ear)
Submandibular
Sublingual
Salivation controlled by:
- parasympathetic system in response to chemo- (taste) and mechano- (tactile) receptors.
- salivary nuclei in medulla/pons (higher centres: smell/appetite - hypothalamus)
- also, reflex arcs from stomach (esp. w/ irritants)
- send signals via the glossopharyngeal (IX) and facial (VII) nerves
Swallowing Phases
- Buccal phase (voluntary)
- Pharyngeal Phase (involuntary stage - reflex arc)
- Oesophageal Phase (involuntary stage)
Buccal Phase
- stage one swallowing
- Placement of the tip of the tongue against the hard palate and then contraction of the tongue forces the bolus into the oropharynx.
Pharyngeal Phase - controlled by
the swallowing centre (medulla/pons)
Pharyngeal Phase
- respiration temporarily inhibited.
- all avenues besides the oesophagus are blocked ( tongue blocks backflow into mouth, soft palate blocks off the nasopharynx, epiglottis blocks of the respiratory, upper oesophageal sphincter reflexes)
- peristalsis contractions move into the oesophagus.
Oesophageal Phase
Two types of Peristalsis contraction waves:
- Primary: continuation of pharyngeal peristaltic wave. Passes from the pharynx to the stomach.
- Secondary: activated by distension of the oesophagus. Controlled by the ENS and higher centres. These waves continue until the oesophagus is clear.
What muscle - pharyngeal wall and upper third of the oesophagus?
striated
striated muscle pharyngeal wall and what part of oesophagus
upper third
Innervation of pharyngeal wall/oesophagus upper third?
vagus & glossopharyngeal nerve from higher centre
what part of oesophagus is smooth muscle?
lower two thirds
describe muscle in oesophagus
upper third: striated
lower two thirds: smooth
How long from swallowing to stomach?
~8 seconds: food
~2 seconds: fluid
The stomach allows:
- a temporary storage tank
- mixing of foods with gastric juices to convert it to a creamy paste called chyme
- physical and chemical breakdown of food (esp. protein - pepsin)
- slow emptying of chyme into the small intestine
Gastric Juice secretion - which glands
- Oxyntic gland
- Pyloric gland
Oxyntic gland secretions:
HCl: pH 0.8
Pepsinogen: Protein digestion, activated by acidic conditions (pepsin, optimal pH 1.8-3.5)
Intrinsic Factor: absorption of vitamin B12
Mucus: Protection/Lubricant
Pyloric gland secretions:
Mucus (mostly): mucosal protection
Gastrin: (gastric acid secretion, stomach motility)
Major stimulants of gastric juice secretion:
- ENS/Parasympathertic (ACh)
- gastrin
- histamine
Mucus - thick secretion composed of:
water, electrolytes & mixture of glycoproteins
Mucus properties:
- Adheres to food
- Coats the GI wall, preventing contact with food
- Low resistance, allows slipping
- Allows faecal particles to adhere to each other
- Strong resistance to digestion
- Can buffer to some extent both acidic and alkaline solutions
Mixing of food with gastric juices occurs through:
peristaltic constrictor waves (mixing waves)
Stomach emptying occurs through:
Intense peristaltic contractions of the antrum and opening of the pyloric sphincter.
The rate of stomach emptying is regulated by:
signals from both the stomach and duodenum (more potent)
Gastric factors - stomach emptying
Stretch receptors in the wall of the stomach stimulate the ENS (myenteric) reflex. The release of the hormone GASTRIN promotes stomach emptying.
Duodenal Factors - stomach emptying
THE ENTEROGASTRIC REFLEX: strong reflexes that slow or inhibit further stomach emptying.
Stimulated by: Distension of the duodenum, presence of irritants, degree of acidity, osmolarity, presence of fats and proteins.
HORMONAL FEEDBACK: release of CCK in response to fatty acids and to a lesser extent secretin and gastric inhibitory peptide
STOMACH - EXPOSURE
Gastric juice is highly corrosive because it is acidic.
Protein digesting enzymes also digest the stomach.
STOMACH - PROTECTION
- Wall is coated in bicarbonate-rich mucus.
- Epithelial cells have tight junctions, prevent gastric juices contacting underlying tissues.
- Epithelial cells that are damaged are replaced rapidly.
Small Intestine - Subdivisions
Duodenum, Jejunum, Ileum
Small Intestine - Extends from X to X
Pyloric sphincter in stomach to ileocecal valve of the large intestine.
Digestion in small intestine occurs due to secretions from:
Liver: Bile
Pancreas: Digestive Enzymes
Digestive Enzymes for all 3 major food types:
Protein: Trypsin, Chymotrypsin, Carboxypolypeptidase
Carbs: pancreatic amylase
Fats: pancreatic lipase, cholesterolesterase, phsopholipase
Pancreatic secretions stimulated by:
CCK (Duodenum)
Chyme
ACh (parasympathetic/ENS)
What neutralises the acidic nature of stomach contents entering the small intestine?
Bicarbonate ions
What is the role of bicarbonate ions in small intestine digestion?
Neutralises the acidic nature of stomach contents entering the small intestine
Bicarbonate ions in small intestine stimulated by:
chyme and secretin (duodenum)
3 phases pancreatic (bicarb?) secretion:
cephalic
gastric
intestinal
Liver secretions to small intestine
- Secretion of bile directly into the duodenum or gall bladder (storage).
- Important in:
- Fat digestion and absorption: bile salts emulsify through the mucosa
- Excretion of waste products (bilirubin[haemoglobin destruction], excess cholesterol)
Liver secretions stimulated by:
- CCK (Duodenum)
- ACh (less extent - ENS/Parasympathetic)
Mixing/Segmentation in the Small Intestine:
- When small intestine becomes distended from chyme entering, local concentric contractions occur at intervals.
- This chops up the chyme to allow greater digestion, absorption.
- Rate is determined by electrical slow wave, but muscle contraction induced by myenteric ENS division.
Propulsive Contractions in Small Intestine:
- Forward movement is very slow. 1cm/min. 3-5 hours to reach large intestine.
- REGULATION:
- Distenstion: ENS local reflex
- Hormonal: gastrin, CCK, insulin, motilin, seratonin
- Peristaltic rush: irritation of the intestines induces powerful rapid peristalsis (involves brain stem & ENS)
Large Intestine: Primary Functions
Proximal Half: Absorption of water and electrolytes from chyme to form solid faeces.
Distal Half: storage of faeces before it can be discharged.
Defecation Reflex:
- ENS REFLEX: distension of the rectum causes peristaltic contractions in the colon & rectum (weak)
- PARASYMPATHETIC REFLEX: Stretch receptors signal parasympathetic motor fibres in the spinal cord, stimulates contraction of the colon, rectum and anus through the pelvic nerve (strong)
- Leads to relaxation of the internal anal sphincter. You then voluntarily relax the external anal sphincter and defecation occurs.
