GI tract

Functions of the GI tract

Digestion, absorption, excretion

4 processes of the GI tract

Motility, glandular secretion, hydrolysis and absorption into the blood stream

What is the mucosal layer of the GI tract composed of?

Epithelial cells
Lamina propria
Muscularis mucosae

Lamina propria

Areolar connective tissue between the epithelia and muscularis mucosae of the GI tract

Muscularis mucosae

Smooth muscle cells that contract to change the surface area of epithelial layer in the GI tract

Where is the muscularis mucosae?

Between the lamina propria and the submucosa

What is the submucosal layer composed of?

Collagen, elastin, glands and blood vessels

Function of blood vessels in the submucosal layer

Remove waste and metabolites

Function of collagen in the submucosal layer

Structureal protein

Function of elastin in the submucosal layer of the GI tract

Facilitates recoiling

Where is the submucosal layer?

Between the muscularis mucosa and the muscularis externa

What shape is the inner layer of muscle in the muscularis externa?

Circular

What shape is the outer layer of muscle in the muscularis externa?

Longitudinal

Function of muscle in the GI tract

Motility

Serosa

Serous membrane

What makes up the mucosa?

Epithelium, lamina propria, muscularis mucosae

Serous membrane

Membrane that lines a cavity without an opening to the outside of the body

Submucosal and myenteric plexuses

Neuronal networks in the gut wall that house most of the enteric nervous system neurons

Enteric nervous system

The nervous system of the digestive tract

What are the 2 main plexuses of the ENS?

Myenteric and submucosal

Where is the submucosal plexus?

Submucosa

Where is the myenteric plexus?

Inbetween inner circular and outer longitudinal layer

What are the 3 divisions of the ANS?

Sympathetic, parasympathetic, enteric

Why is the ENS called the intrinsic nervous system?

Because it can operate independently without the CNS and is sometimes called the second brain

Extrinsic nervous system

Parasympathetic and sympathetic

Where does the extrinsic nervous system stem from?

Vagus and pelvic nerve

What is the intrinsic nervous system?

ENS

Function of parasympathetic ganglia within plexuses?

Coordinate information from the PNS and relay to smooth muscle, endocrine and secretory cells

PNS

Peripheral nervous system

Peripheral nervous system

The sensory and motor neurons that connect the central nervous system to the rest of the body

Cholinergic

Release ACh

Peptidergic

Release peptides such as P, VIP

VIP

Vasoactive intestinal peptides

Sympathetic postganglionic neurons

Adrenergic

Parasympathetic postganglionic neurons

Cholinergic or peptidergic

Somatostatin

Hormone that inhibits release of growth hormone and insulin
Acts in a paracrine way

How are action potentials spread so rapidly in the smooth muscle?

Muscle cells are electrically coupled via gap junctions

How is coordinated contraction achieved in the smooth muscle?

Through electric coupling via gap junctions

Phasic contraction

Periodic contraction and relaxation
3-12 contractions per minute

Tonic contraction

Constant level of contraction

Interstitial cells of cajal

Pacemaker cells that generate electrical slow waves in gastrointestinal (GI) smooth muscles

Sphincters

Specialised circular muscles with positive resting pressure that encircle and, by contracting, constrict a duct, tube, or opening

What are the 7 sphincters of the GI tract?

Upper esophageal
Lower esophageal
Pyloric
Ilececal
Internal anal
External anal
Sphincter of Oddi

Function of sphincters

Regulate antegrade and retrograde movement

Antegrade peristalsis

Moving food towards the anal direction

Retrograde peristalsis

Moving food towards oral direction

Upper esophageal sphincter

The muscular ring located at the top of the esophagus

Lower esophageal sphincter

Ring of muscles between the esophagus and the stomach

Pyloric sphincter

Ring of muscle that guards the opening between the stomach and the duodenum

Ileocecal sphincter

The ring-like muscle that controls the flow from the ileum of the small intestine into the cecum of the large intestine

Internal anal sphincter

Formed by smooth muscle and involuntarily controlled

External anal sphincter

Skeletal muscle, voluntary

Sphincter of Oddi

Controls the amount of bile into the small intestine

Mastication

Chewing

Function of the mouth in digestion

Teeth and tongue mechanically breakdown food ready for swallowing
Salvia lubricates the bolus
Alpha-amylase hydrolyses starch

Function of alpha amylase

Breaks down starch

What is the mastication muscle innervated by?

Mandibular nerve of trigeminal

Trigeminal

V

Is chewing voluntary or involuntary?

Both

What are the 3 phases of swallowing?

Oral
Pharyngeal
Oesophageal

What type of epithelia is the oesophagus lined with?

Stratified squamous epithelia

Swallowing reflex

Closes the upper oesophageal sphincter
Initiates primary peristaltic wave (vagus reflex)

Primary peristaltic wave

Sweeps from the beginning to end of the esophagus, forcing the bolus ahead of it toward the stomach

What type of reflex is the primary peristaltic wave?

Vagus

What type of reflex is the secondary peristaltic wave?

ENS

What is the opening of the lower oesophageal sphincter mediated by?

Vagus nerve releasing VIP neurotransmitter

Function of buccal cavity

Mechanical breakdown of food and mixes with saliva to form soft bolus

Function of lower oesophageal sphincter

Controls food entry into the stomach

Function of oesophagus

Conveys food from mouth to stomach

Function of duodenum

Digestive enzymes from the pancreas and liver enter via pancreatic duct
Bulk of digestion occurs here
Digestive enzymes and intestinal juice is secreted

Function of jejunum

Absorption of sugar, fatty acids and amino acids

Function of ileum

Absorption of B12, bile salts and remaining digestive products

Function of caecum

Absorbed fluid and remaining salts
Mix with mucous

Function of rectum

Absorption of electrolytes and water
Faeces is thickened and mucous is added

Anal canal

Contience organ
Regulates defaecation
Lubricates and transmits faeces from the rectum

Function of the mouth in the GI tract

Mastication
Saliva lubricates bolus
Swallowing

Function of oesophagus in GI tract

Swallowing reflex
Closes and opens sphincter
Coordinates sequential contractions

Function of the stomach

Motility
Receives food
Contraction and mixing of food
Gastric juice secretion

Gastric juice

A digestive fluid secreted by the stomach.

What makes up the gastric juice?

HCl and pepsin

Function of the small intestine

Digestion, absorption and secretion
Has villi

Function of large intestine

Absorption of water, vitamins and electrolytes
Forms and propels faeces

Describe the motility in the GI tract

Subthreshold slow waves that produce weak contractions

Function of sphincters in the GI tract

Separate low pressure organs to prevent backflow

Accessory organs of the GI tract

Pancreas and liver

Function of the liver as an accessory organ to the GI tract

Secretes bile, gall bladder stores, concentrates and ejects the bile
Formation of bile salts
Emulsification

Function of pancreas

Produces digestive enzymes

What makes up the mucosal layer of the GI wall?

Epithelial cells
Lamina propria
Muscularis mucosae

What makes up the submucosal layer of the GI wall?

Collagen, elastin, glands and blood vessels

2 main plexus of GI tract

Myenteric and submucosal

Function of myenteric plexus

To control movement of food through the GI tract

Function of submucosal plexus

Control intestinal secretions, digestion & absorption

What is the contractile tissue of the GI tract?

Unitary smooth muscle
Cells are electrically coupled via gap junctions for rapid spread of action potentials leading to coordinated contraction

Phasic contraction

Periodic contraction and relaxation
3-12 mins

Tonic contraction of smooth muscle

Constant level of contraction

GI pacemaker

Interstitial cells of cajal (ICC)

Slow waves

Slow, rhythmic oscillation of smooth muscle membrane potentials toward and away from threshold, due to regular fluctuations in ionic permeability

Where are the ganglia of the ENS located?

Submucosal and myenteric plexuses

What type of neurons are the parasympathetic postganglionic neurons of the ENS?

Cholinergic or peptidergic

What type of neurons are the sympathetic postganglionic neurons of the ENS?

Adrenergic