What are the main GI organs? What do they form?
- mouth
- pharynx
- esophagus
- stomach
- small intestine (duodenum, jejunum, ileum
- large intestine (colon)
they all form a long, twisting tube from the mouth to the anus
What are the accessory organs and what do they do?
- liver
- pancreas
- gall bladder
- salivary glands (parotid, sublingual, submandibular)
- they secrete various substances into the GI lumen to aid in digestive processes
Stomach
- muscular organ that vigorously churns food, breaking it down into smaller and smaller particles
- pyloric sphincter (at end of stomach, connected to duodenum) serves as a gate to control the opening from the stomach to the small intestine
*draw the stomach and its divisions
Small intestine
- duodenum, jejunum, ileum
- chyme (semiliquid food bolus) enters duodenum from the stomach
- function of intestine is to finish the digestive process started in the mouth and continued in the stomach
- absorbs most of the nutrients and passes the residues on to the large intestine
Large intestine
- cecum => ascending colon => transverse colon => descending colon => sigmoid colon => rectum
- main function is to absorb water and electrolytes and store the feces until it is expelled from body
Sphincter muscles
- control the passage of food bolus and fluids from one part of the gut to another
- external anal and upper esophageal sphincters are under voluntary control, regulated by CNS
- all other sphincters are under smooth muscle control (involuntary control) => lower esophageal sphincter, pylorus, sphincter of Oddi, ileocecal valve, internal anal sphincter
What is the mucosa in the wall of the GI tract composed of?
- epithelium, lamina propria and muscularis mucosae
Epithelial cells in the mucosa of the GI wall
play a role in nutrient absorption, and the secretion of hormones and mucus (some absorb and some secrete)
Lamina propria
contains blood and lymphatic vessels that absorb the end products of digestion
Submucosa of the wall of the GI tract
a connective tissue layer containing large blood vessels, and an extensive lymphatic system
Muscular mucosa
thin muscle layer that generates local movements
- produces folds to increase the absorptive surface area of the mucosa in the stomach and small intestine
- contains circular and longitudinal muscle layers that work together to coordinate the contractions necessary for moving and mixing the food bolus along the gut
What muscle forms the sphincters?
circular muscle layer
Enteric nervous system (ENS) is comprised of what 2 nerve layers?
- submucosal plexus
- myenteric plexus
What covers the muscle layers?
the serosa
What are the 2 main controls of gastrointestinal function?
- neural control
- chemical control
Neural control (brief overview)
central control via parasympathetic and sympathetic autonomic nervous systems
intrinsic control by the enteric nervous system (ENS)
- capable of complex functions: sensation, info processing, motor output
Chemical control (brief overview)
endocrine control; enteroendocrine cells (EECs) secrete peptides that travel to target organs through bloodstream
paracrine control; EECs secrete peptides that act on nearby cells nearby
What are the two nerve plexi located in the ENS?
- myenteric plexus
- submucosal plexus
Explain the neural control of gastrointestinal function
- involves the ENS, ANS, CNS
- the two plexi of the ENS (intrinsic to the gut) communicate with end organs (smooth muscles, blood vessels, glands) coordinating gut functions
- parasympathetic branch is active during digestion and innervates ENS neurons crucial for digestive functions
- sympathetic branch inhibits gut activity via the ENS => also has a direct inhibitory effect on smooth muscle in the gut
- sensory neurons in gut wall detect stimuli like distension, chemicals, irritation, and pain => send afferent fibers to ENS and ANS, some projecting to DRG of spinal cord
- sensory afferents elicit local reflexes within the gut wall via ENS => also trigger longer reflexes in gut via the ANS and higher centres in CNS
ENS (human enteric nervous system) (intrinsic innervation)
- diversity of neurons, glial cells, and neurotransmitters is like CNS
- myenteric plexus: inhibitory motor neurons release nitric oxide (NO) and ATP, excitatory motor neurons release ACh and substance P => plexus controls mainly motility
- submucosal plexus is responsible for controlling secretion, absorption, and contraction of muscularis mucosae
- both ENS layers receive sensory info
- ENS can operate autonomously, but typical digestive functions require communication between ENS and CNS
Which NTs are released by the excitatory and inhibitory motor neurons in the ENS?
excitatory: ACh or substance P => results in contraction
inhibitory: nitric oxide or ATP => relaxation
Extrinsic innervation
- ANS integrates activity of ENS with that of entire body
- most autonomic fibers do not directly innervate structures in the GI tract, but instead, they modulate activity of enteric neurons
- most extrinsic innervation is provided by afferent sensory neurons
Parasympathetic (cholinergic) => stimulate ENS neurons => increased motility, increased secretion
Sympathetic (adrenergic) => inhibit ENS neurons => reduced secretion, motility and blood flow to the gut
Parasympathetic innervation
- exerts mostly excitatory effect on gut functions
- oral cavity, stomach, small intestine and proximal colon mainly supplied by the vagus nerve (CN X)
- distal colon is innervated by the pelvic nerves => plays crucial role in propulsive colonic motility (defecation)
- both vagal and pelvic preganglionic fibers are cholinergic => synapse on nicotinic ACh receptors on postganglionic neurons in or near the target organs
- postganglionic fibers are also predominantly cholinergic => form synapses on muscarinic ACh receptors on target tissues
Sympathetic innervation
- usually inhibitory (GI muscles and secretion)
- innervates entire GI tract
- regulates GI blood flow through vasoconstriction
- preganglionic neurons start in thoracic and lumbar spinal cord, postganglionic neuron cell bodies located in sympathetic ganglia (SG)
- fibers terminate in ENS or end organs
- preganglionic = cholinergic, synapse on nAChR on SG
- posganglionic = adrenergic (NE), synapse on adrenergic receptors
