GI tract lesson 2

propulsive movement and mixing movements (peristalsis)

moving food forward at an appropriate rate for digestion and absorption

mixing movements by local intermittent contractions

solid organ and biliary system

• solid- does NOT come in contact with food and does secretions
• hollow- comes in contact with food

converting an electrical stimulus to a mechanical response. This process is fundimental to muscle physiology, whereby electrical stimulation is usually an action potential and mechanical response is contraction

• step 1: circular muscles contract behind the food mass
• step 2: longitudinal muscles contract in front of the food mass
• step 3: contraction of circular muscles propels food mass forward

stretching of gut wall is 1st stimulus, that occurs with large amount of food, stimulates contraction 2-3 cm behind stretched wall. This initiates peristalsis that propels food forward. Other stimuli are : parasympathetic signals (positive action) and physical or chemical stimulation of epithelial lining

uniform and coordinated. enteric nervous system: myenteric plexus and submucosal plexus (Auerbach and Meissner's plexus)

• 1. increase in tonic contractions
• 2. increase in intensity of contractions
• 3. slight increase in rhythm of contractions
• 4. increase in velocity of excitatory waves along the gut wall

greatly decreases peristalsis

cells in parasympathetic nucleus from the 10th CN (vagus), located in medulla

• 1. Thick and thin filaments
• 2. Not arranged in sarcomeres
• 3. Appears homogenous and not striated
• *4. Unitary smooth muscle (single unit)
• 5. Spontaneously active (slow waves)
• 6. Exhibit pacemaker activity
• 7. Modulated by neural and hormonal mechanisms
• *8. High degree of coupling b/t cells- leads to coordinated contraction

smooth muscle contractions

coordinates the cells so that they act in unison fashion

syncytium

behave as a mass of cytoplasm having many nuclei, but no internal cell boundaries (cytoplasm that does not seperate into individual cells)

autonomous of electrical activity

• 1. Different from skeletal muscle
• 2. There is no troponin in SM
• 3. Calcium regulates myosin on thick filaments
• 4. Depolarizaiton opens voltage gated Ca2+ channels
• 5. So Ca2+ flows into the cells
• 6. There may be additional release of Ca2+ from SR
• 7. Intracellular Ca2+ increases
• 8. Ca2+ binds to calmodulin
• 9. Ca2+ + calmodulin binds and activates MLCK (myosin light chain kinase)
• 10. MLCK phosphorylates myosin which binds to actin --> contraction
• 11. Dephosphorylation --> relaxation

cells of cajal, Basic Electrical Rhythm

stops propagation of slow wave potential and thus gut motility

before spike potential can propagate

modulate amplitude and frequency of wave

symp. and parasymp

Auerbach and Meissner's plexus

ACh, Substance P

vasoactive interstinal peptide (VIP) and nitric oxide (NO)

osscilating resting membrane potentials

NOT ACTION POTENTIALS

• 1. key for generating contractions from stomach downwards
• 2. Generated by specialized cells (interstitial cells of Cajal, ICC)
• 3. Begin in stomach and travel downward thru GI tract
• 4. Frequency is characteristic of different parts of GI

when spike potentials occur during slow waves (not every slow wave = contraction)

entry of Na+

drive membrane to threshold, which causes spikes to occur.

Spikes cause contraction

influx of primarily Ca2+ and some Na+. The Ca2+ enters the muscle during the spike causes the contraction

occur once resting membrane potential depolarizes to 40 mV. The more depolarized the resting membrane potential becomes during slow waves, the greater the frequency of spikes

throughout GI tract: from esophagus to inner sphincter region of anus

act as pacemaker cell and as an impulse conduction system in the gut musculature in a way analogous to the pacemaker cells in the heart.

at all times in the stomach and SI even when muscle not contracting

fast activity appears on top of slow wave

fast activity, spike potentials, action potentials, response activity spike burst

put into cell and records changes in electrical potental b/t cytoplasm and fluid surrounding cell

record from fluid b/t muscle cells

• 1. slow wave coordinates the adjacent muscle cells so that they are ready to fire fast activity together to give a mechanically effective contraction (preps the muscle for contraction)
• 2. movement and mixing of gut contents are main functions of muscluar coat

• 1. contacts b/t SMCs (physical contact with each together)
• 2. function of membranes of SMCs
• 3. activity of ganglion cells and neural endings

hormones, nervous system, feeding, and temp

giving an increase in frequency in slow waves

decreases frequency of slow waves

duodenum

CNS (symp and parasymp)

long-term alteration in wave form

parameters of electrical stimulation

followed by bursts of coordinated fast activity on gastric and duodenal slow waves

3 hrs

8 or more hours

increases frequency of slow waves

decreases frequency of slow waves

• 1. Gastroesophageal Reflux Disease (GERD)
• 2. Dysphagia --> difficulty in swallowing
• 3. Achalasia --> esophageal dysphagia

• 1. Delayed Gastric Emptying (Gastroparesis) --> food in stomach longer than normal (more than 3-4 hrs)
• 2. Rapid Gastric Emptying (Dumping Syndrome)
• 3. Functional Dyspepsia --> indigestion, bloating
• 4. Cyclic Vomiting Syndrome (CVS) --> usually in kids or elderly
• 5. Idiocyclic Vomiting Syndrome --> don't know cause

• 1. Intestinal Dysmotility
• 2. Intestinal Pseudo-Obstruction
• 3. Small Bowel Bacteral Overgrowth

• 1. Constipation
• 2. Diarrhea
• 3. Hirschsprung's Disease
• 4. Irritable Bowel Syndrome (IBS)

• 1. Fecal Incontinence
• 2. Hirschsprung's Disease
• 3. Outlet Obstructive Type Constipation (Pelvic Floor Dyssynergia)