AN SC 311 Lecture 1
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What are the functions of the GIT
Motility, secretion, digestion, absorption, communication, and immune system
Components of the GI tract
- stomach (4 compartments)
- small intestine
- large intestine
Components of the accessory organs?
- salivary glands
- gall bladder
What are/describe four layers of the GI tract
- Mucosa - absorptive/secretory layer
- epithelium, lamina propria,
- lining epithelial cells, muscle portion, and also goblet cells
- made up of vili and microvilli
- Submucosa - it is thick, highly vascular, supports the mucosa, takes up nutrients
- glands and nerve plexuses
- Muscularis - segmented contraction and peristalsis
- Serosa - membrane that wraps, protects and lubricates
- -has a secretory epithelial layer and underlying connective tissue layer
Key points of the control of the GI function
- GI TRACT HAS an independent nervous system within the gut wall
- -Enteric nervous system - has receptors sensory and motor neurons
- - receives extrinsic innervation from autonomic nervous system that has afferent neurons to signal to the CNS
- GI tract has an intrinsic endocrine system
Explain the nerves involved with the enteric nervous system?
- Myenteric plexus - located between circular and longitudinal smooth muscle
- Submucosal plexus - located in the submucosa; interneurons minor
- Neurons - Sensory and motor
- input from the mechanoreceptors (wall) and chemoreceptors (lumen)
Autonomic effector tissues?
Heart, blood vessels, respiratory airways, GI tract.
What are the sympathetic effects?
- fight or flight responses
- postganglionic neurons release norepinephrine
- decrease GI tract motility
- decrease exocrine secretion
- decrease endocrine secretion
- postganglionic neurons release acetylcholine
- increase GI tract motility
- exocrine secretion
- endocrine secretion
How is the neural control of the GI tract controlled?
- CNS (long reflex)
- ENS (short reflex; Myenteric plexus and submucosal plexus)
How is the gut hormones of the GI orchestrated?
- endocrine cells - produce true hormones
- paracrine hormones - local factors
- columnar cells - broad base narrow tip at lumen
- collectively known as "Neurohumoral regulatory molecules"
What are the neurohumoral regulatory molecules (peptides and nonpeptides)
- gastrin releasing peptide
- ACH (acetetylcholine)
- NO (nitric oxide)
- ATP (adenine triphosphate)
- serotonin (5-hydroxytryptamine)
What does gastrin stimulate?
- gastric secretion
- mucosal growth
what does CCK stimulate?
- pancreatic enzyme secretion
- gallbladder contraction
what does secretin stimulate?
pancreatic HCO3 secretion
what does GIP inhibit?
What are the 3 steps to the endocrine system
- 1. Signaling cell - physiological stimulus, hormone synthesis, storage, and secretion
- 2. transport by the circulatory system
- 3. target cells - receptor, second messenger, and biological effects
Hydrophilic hormones descriptions
- hormones stored in intracellular vesicles
- hormones released by exocytosis
- attach to transmembrane cells
- amplify signals via second messengers (cAMP, Ca, and IP3
- rapid biological effects
- act quickly and go away quickly
description of hydrophobic hormones?
- hormones synthesized on demand
- hormones released by diffusion
- diffuse and attach to intracellular receptors
- hormone receptor complex binds to target gene
- mRNA transcription is altered
- slow but more sustained biological effects
- lasts a longer time
description of endocrine?
- released into general circulation
- specificity is a property of the target tissues
- specific receptors are present
- Ex. gut hormones are gastrin, secretin, CCK, and motilin
description of paracrine?
- released from endocrine cells and diffuse thru extracellular space to the target tissue
- somatostatin (D cells) inhibits gastrin release (G cells)
description of autocrine?
a released substance that regulates the cells own function
description of neurocrine?
sensory cells secrete neurotransmitter
What are the 3 sphincters that divide the GI tract?
- esophageal sphincter - prevents heartburn
- pylorus sphincter - stomach to SI, when closed allows the stomach to have a high acid content
- ileocecal sphincter - bewteen SI and LI, you dont want to lose partially digested food to the LI this prevents that from happening.
What is segmentation reflex and why does it happen?
- mixing of digesta
- contraction of circular muscle
- not propagated in one direction
what is peristalitic reflex and why does it happen?
- programmed by the ENS and moves food down the GI tract
- it is accomplished by the contraction of longtitudinal muscle relaxation of circular muscle and then the opposite.
- moves in one direction
What is the mechanism for contraction throughout the GI tract
- Basic Electrical Rhythm - smooth muscle has an intrinsic rhythm of depolarization and repolarization. Frequency is stomach ever 3 min, duodenum every 12 min, ileum every 9 min
- Contraction occurs when depolarization is high enough to cause an action potential
- Regulation - acetylcholine increases the resting basal membrane potential and stimulates action potentials and contraction, Norepinephrine decreases the resting basal membrane potential and prevents generation of actions potentials and contraction
What is smooth and striated muscle controlled by?
- smooth - myenteric nerve plexus
- striated - somatic nerve
What is the function of the fundus, corpus, and Antrum of the stomach?
- Fundus - store contents by adapting volume
- Corpus - mixing vat for saliva, food and gastric secretions
- Antrum - propulsion of food thru the pyloric sphincter to the duodenum
- controlled by distention, and parasympathetic
Why is there lots of folding on the stomach?
allows the stomach to change size and stretch
Factors that increase gastric emptying?
- neural control - distention of gastric wall, increased parasympathetic
- endocrine control (gastrin)
factors decreasing gastric emptying?
- neural control - chemoreceptor, osmoreceptor mechanoreceptor at the duodenum (hypertonicity, H plus)
- increased sympathetic
- endocrine control (CCK, gastric inhibitory peptide, secretin)
What is the segmentation reflex of the SI?
- major type of motility
- mix luminal content
- regulated by ENS
What is the migrating myoelectric complex?
- peristalic activity
- interrupted during feeding in carnivores but not in herbivores
- has 3 phases - 1. quiescent period, 2. irregular contraction, 3. regular contraction
- motilin stimulate stomach and duodenum
- intrinsic nerves stimulates distal duodenum and jejunum
Describe LI motility (functions)?
- functions - microbial digestion, reabsorption of water and electrolytes
- stationary haustral contractions
- peristaltic contractions
- antiperistaltic movement (fill cecum)
- aboral mass movement (evacuate entire length of colon
Why does acid and pepsin not digest the stomach?
- epithelial cells are protected from acid by mucous layer
- mucous secretion increased by: cholinergic stimulation and mechanical stimulation
- bicarbonate secreted by epithelial cells is trapped in the mucous gel
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