GI Physiology Part 1 (GI Tract Anatomy/Function/Structure, Innervation, Motility, Defecation)

What is the primary role of the GI system?

digestion and absorption of nutrients

Motility

contraction/relaxation of walls and sphincters to propel ingested food from mouth toward rectum; mixes and reduces size of food

What is the role of secretions from salivary glands, pancreas, and liver?

add fluid, electrolytes, enzymes, and mucus to lumen of GI tract; aid in digestion and absorption

Ingested foods are digested into ________ ________

absorbable molecules

How are nutrients, electrolytes, and water absorbed?

from intestinal lumen to bloodstream

Structural Sequence of the GI Tract

mouth, esophagus, stomach, small intestine, large intesting at the cecum, anus

Structural Sequence of Small Intestine

duodenum, jejunum, ileum

Layers of GI Wall: Mucosa

faces lumen; contains epithelial cells, lamina propria, muscularis mucosae

Mucosa: Epithelial Cells

specialized for absorption and secretion

Mucosa: Lamina Propria

connective tissue, blood vessels, lymph vessels

Mucosa: Muscularis Mucosae

smooth muscle cells that contract to change shape and surface area of epithelial cell layer

Layers of GI Wall: Submucosal Layer

collagen, elastin, glands, blood vessels

What 2 types of muscles make up the submucosal layer?

- circular (thick, densely innervated)

- longitudinal muscle (thin, scarcely innervated)

Function of Submucosal Muscles

propel food down digestive tract

Layers of GI Wall: Serosa

faces blood vessels

4 Other Structures Associated with GI Tract

- salivary glands

- pancreas

- liver

- gallbladder

3 Types of GI Tract Regulation

- extrinsic autonomic innervation

- intrinsic enteric innervation

- regulatory peptides

What 2 pre-ganglionic nerves innervate parasympathetic GI function?

- vagus nerve (CN X)

- pelvic nerve

Parasympathetic Innervation: Vagus Nerve

upper GI tract = upper 1/3 esophagus, wall of stomach, small intestine, ascending, portion of transverse colon

Parasympathetic Innervation: Pelvic Nerve

lower GI tract = walls of transverse, descending, sigmoid colons

Parasympathetic post-ganglionic neurons are either ________, which release _______, or are ________, which release ________

cholinergic; ACh; peptidergic; peptides

Where do the sympathetic pre-ganglionic fibers synapse?

sympathetic ganglia = celiac, superior/inferior mesenteric, hypogastric

What type of post-ganglionic neuron is involved in sympathetic innervation of the GI tract?

adrengergic (release NE)

Parasympathetic NS _____ _____ digestion, whereas the sympathetic NS _____ _____ digestion

speeds up; slows down

Intrinsic Innervation: Enteric Nervous System

- controls local contractile, secretory, and endocrine functions of GI tract

- modulates info from parasympathetic + sympathetic systems

How does the enteric system receive and send information?

directly receive info from chemo/mechanoreceptors in mucosa for direct motor response to target cells

In the enteric NS, information is relayed between ganglia using ______ and _______

interneurons; neurocrines

T or F: the enteric nervous system is self-regulatory

true

4 Regulatory Functions of Peptides

- contraction/relaxation of smooth muscles + sphincters

- secrete enzymes, fluid, electrolytes

- control trophic changes (growth) of tissues

- secretion of other peptides

How are hormones released?

endocrine cells of GI tract

Hormone Pathway within GI Tract

endocrine cell -> portal circulation -> liver -> systemic circulation -> target cell

How are neurocrines synthesized & released in the GI system?

synthesized in neurons of GI, released by action potential

How are paracrines released?

endocrine cells of GI tract

Paracrines act _______ within the same tissue that secretes them

locally

Peristalsis

involuntary waves of muscle contraction push food along unidirectional digestive tract

Almost all of the contractile tissue in the GI tract is ______ ______

smooth muscle

What are the 3 exceptions for contractile tissue in the GI tract?

striated muscle in the pharynx, upper 1/3 of esophagus, external anal sphincter

How are APs propagated in the GI tract?

gap junctions

Motility: Circular Muscle Contraction

decrease diameter of segment

Motility: Longitudinal Muscle Contraction

decreased length of segment

Phasic Contraction

periodic contractions followed by relaxation

What type of contraction occurs in organs for mixing/propulsion?

phasic

Tonic Contraction

constant level of contraction or tone without regular periods of relaxation

In what parts of the GI system does tonic contraction occur?

- upper region of stomach

- lower esophageal, ileocecal, internal anal sphincters

Sphincters

specialized regions of circular muscle that separate 2 adjacent regions of GI system

Upper Esophageal Sphincter

separates pharynx and upper esophagus

Role of Upper Esophageal Sphincter

prevents air from entering esophagus

Lower Esophageal (Cardiac) Sphincter

separates esophagus and stomach

Role of Lower Esophageal Sphincter

prevents gastric content from going back up

Pyloric Sphincter

separates stomach and duodenum

Role of Pyloric Sphincter

prevents backflow of intestinal contents

Ileocecal Sphincter

separates ileum and cecum

Role of Ileocecal Sphincter

prevent the contents of the large intestine from reentering the small intestine

Internal & External Anal Sphincters

maintain fecal continence

Role of Internal & External Anal Sphincters

stops fecal movement/expelling

How are changes in sphincter pressure and contraction of smooth muscles coordinated?

reflexes (ex: swallowing reflex)

Movement of Food Through GI Tract

chewing, swallowing, esophageal motility, gastric motility, small intestine motility, large intestin motility

3 Phases of Swallowing

oral, pharyngeal, esophageal

3 Functions of Chewing

- lubricate food with saliva

- decrease size of food particles for swallowing

- mix ingested carbs with salivary amylase to being carb digestion

What type of digestion is chewing?

mechanical; combination of voluntary + involuntary mechanisms

Mechanism of Swallowing

initiated voluntarily in mouth but becomes involuntary (reflex)

What controls the reflex portion of swallowing?

swallowing center in the medulla

Reflex Portion of Swallowing: Step 1

sensory info detected by somatosensory receptors near pharynx

Reflex Portion of Swallowing: Step 2

sensory info carried to medullary swallowing center via vagus + glosspharyngeal nerves

Reflex Portion of Swallowing: Step 3

medulla coordinates sensory info and directs motor output to striated muscle of pharynx + upper esophagus

Phases of Swallowing: Oral

tongue force bolus of food back toward pharynx -> activation of somatosensory receptors -> triggers involuntary swallowing reflex

What is the purpose of the pharyngeal phase of swallowing?

propel food bolus from pahrynx to esophagus

Phases of Swallowing: Pharyngeal

soft palate lifts -> epiglottis covers larynx -> upper esophageal sphincter relaxes -> peristalsis initiated (breathing inhibited)

What is the purpose of the esophageal phase of swallowing?

propel food from esophagus to stomach

Phases of Swallowing: Esophageal

primary and secondary peristaltic wave to clear esophagus of food

What is the function of the UES during swallowing?

prevent air from entering upper esophagus

What is the function of the LES during swallowing?

prevent acidic gastric contents from entering lower esophagus

What are the only times that the UES and LES are open?

when food passes from pharynx into esophagus or esophagus into stomach

Both upper and lower esophageal sphincters are _______ initially and at rest

closed

GERD

stomach content flows back up to the esophagus, causing irritation and discomfort

4 Causes of GERD

- LES dysfunction/weakness

- esophageal motility disorders

- delayed gastric emptying

- increased intraabdominal pressure

3 Anatomic Sections of the Stomach

fundus, body, antrum

2 Regions of the Stomach

orad, caudad

Orad Region of Stomach

proximal fundus and body; thin-walled to receive fluid bolus

Why is the orad region of the stomach thin-walled?

needs to be able to distend to account for food bolus

Caudad Region of Stomach

distal body and antrum; thicker-walled (muscle) for strong contractions to mix and propel food

3 Layers of the Stomach

- outer longitudinal

- middle circular

- inner oblique

The thickness of the stomach muscle wall increases ______ to ______

proximal; distal

3 Components of Gastric Motility

relaxation, contractions, gastric emptying

Gastric Motility: Step 1 - Receptive Relaxation

distention of lower esophagus by food -> mechanoreceptor detection of stretch -> CNS -> simultaneous smooth muscle relaxation of LES + orad

What neurotransmitter is involved in receptive relaxation?

vasoactive intestinal peptide (VIP)

In a relaxed state, the stomach can accommodate as much as ______ of food

1.5 L

What type of vasovagal reflex is receptive relaxation of the stomach?

afferent and efferent information carried by vagus nerve

Gastric Motility: Step 2 - Mixing & Digestion

caudad region (thick muscle) produces contractions for mixing and digesting food

Waves of contraction begin in the ______ of the _______ of the stomach and slowly move _______ along the _______ stomach

middle; body; distally; caudad

T or F: contractions decrease in strength as they approach the pylorus

false

Retropulsion

wave of contraction closes pylorus, propelling gastric contents back into the stomach for further mixing/breakdown

Gastric Motility: Step 3 - Gastric Emptying

takes about 3 hours to empty gastric contents into duodenum of small intestine; need to relax pyloric sphincter

What is close regulation of the rate of gastric emptying is necessary for?

neutralizing gastric H+ and digestion/absorption of nutrients

Gastric emptying is fastest with ______ and slowest with ______

liquids; solids

Isotonic solutions go through gastric emptying ______ than hypo/hypertonic solutions

faster

2 Major Factors that Slow/Inhibit Gastric Emptying

- presence of fat

- presence of H+ ions in duodenum

Primary Function of the Small Intestine

digestion and absorption of nutrients

How does small intestine motility assist digestion?

mix chyme with digestive enzymes and pancreatic secretions

How does small intestine motility support absorption?

expose nutrients to intestinal mucosa