GI tract lesson 2
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What are the 2 types of movement of the GI tract?
propulsive movement and mixing movements (peristalsis)
What is propulsive movement?
moving food forward at an appropriate rate for digestion and absorption
What is peristalsis?
mixing movements by local intermittent contractions
What kind of organ is the gall bladder and what system is it apart of?
solid organ and biliary system
What is the difference b/t a solid and hollow organ?
- solid- does NOT come in contact with food and does secretions
- hollow- comes in contact with food
What is excitation of GI smooth muscle?
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
What are the steps of of the longitudinal and circular muscles to push food throught the GI tract?
- 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
*What stimulates peristalsis?
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
*What is important about the electrical wiring of the GI?
uniform and coordinated. enteric nervous system: myenteric plexus and submucosal plexus (Auerbach and Meissner's plexus)
*Stimulation of myenteric plexus causes what?
- 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
*Blocking myenteric plexus does what?
greatly decreases peristalsis
*The myenteric plexus arises from what?
cells in parasympathetic nucleus from the 10th CN (vagus), located in medulla
*What are characteristics of smooth muscle?
- 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
What is the force behind peristalsis?
smooth muscle contractions
Electrical activity of smooth muscles in gut does what?
coordinates the cells so that they act in unison fashion
Smooth muscle behave as what?
What is syncytium?
behave as a mass of cytoplasm having many nuclei, but no internal cell boundaries (cytoplasm that does not seperate into individual cells)
Electrical activity can be recorded after blockage of central impulses because of _________.
autonomous of electrical activity
***What are the steps in excitation (contraction coupling in GI smooth muscle)?
- 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
*In GI SM a slow wave originates where? as what?
cells of cajal, Basic Electrical Rhythm
*Loss of slow wave cells (cells of Cajal) stops what?
stops propagation of slow wave potential and thus gut motility
**Slow wave threshold must be reached before what can happen?
before spike potential can propagate
***What do intrinsic (Auerbach and Messner's plexus), extrinsic, and hormones do?
modulate amplitude and frequency of wave
*What is extrinsic?
symp. and parasymp
*What is intrinsic?
Auerbach and Meissner's plexus
*What do excitatory compounds include?
ACh, Substance P
*What do inhibitory compounds include?
vasoactive interstinal peptide (VIP) and nitric oxide (NO)
*****What are slow waves?
osscilating resting membrane potentials
NOT ACTION POTENTIALS
*What are some characteristics of slow waves?
- 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
*Contractions = what?
when spike potentials occur during slow waves (not every slow wave = contraction)
*Slow waves are caused by what?
entry of Na+
*What do slow waves do?
drive membrane to threshold, which causes spikes to occur.
*Spikes cause what?
Spikes cause contraction
*What are spikes caused by?
influx of primarily Ca2+ and some Na+. The Ca2+ enters the muscle during the spike causes the contraction
*When do spikes occur?
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
*Where can Interstitial cells of Cajal (ICC) be found?
throughout GI tract: from esophagus to inner sphincter region of anus
*What do ICC do?
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.
*When are slow waves present?
at all times in the stomach and SI even when muscle not contracting
*When muscle begins to contract what happens
fast activity appears on top of slow wave
*Types of electrical activity have several alternateve names. Match the following with their other name: Slow wave, pacesetter potential, control activity, basic electrical rhythm
fast activity, spike potentials, action potentials, response activity spike burst
What does a intracellular electrode do?
put into cell and records changes in electrical potental b/t cytoplasm and fluid surrounding cell
What does extracellular electrodes do?
record from fluid b/t muscle cells
What is the importance of electrical activity of GI SM?
- 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
*Generation and transmission of electrical activity inclusds what?
- 1. contacts b/t SMCs (physical contact with each together)
- 2. function of membranes of SMCs
- 3. activity of ganglion cells and neural endings
*What are factors that influence electrical activity?
hormones, nervous system, feeding, and temp
What do gastrin or pentagastrin do?
giving an increase in frequency in slow waves
What does secretin do?
decreases frequency of slow waves
Where are slow waves not significantly affected by hormones?
*Electrical activity of GI SMCs can be influneced by what?
CNS (symp and parasymp)
*Gastric vagal denervations do what?
long-term alteration in wave form
*Nervous system can give either stimulation or inhibition of fast activity depending on what?
parameters of electrical stimulation
Ingesting of food within a few minutes does what?
followed by bursts of coordinated fast activity on gastric and duodenal slow waves
How long does fast activity happen?
How long do fasting patterns take?
8 or more hours
Increase in body temp does what?
increases frequency of slow waves
cooling of body does what?
decreases frequency of slow waves
What are some motility disorders of the esophagus?
- 1. Gastroesophageal Reflux Disease (GERD)
- 2. Dysphagia --> difficulty in swallowing
- 3. Achalasia --> esophageal dysphagia
What are some motility disorders of the stomach?
- 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
What are some motility disorders of the SI?
- 1. Intestinal Dysmotility
- 2. Intestinal Pseudo-Obstruction
- 3. Small Bowel Bacteral Overgrowth
What are some motility disorders of the Colon?
- 1. Constipation
- 2. Diarrhea
- 3. Hirschsprung's Disease
- 4. Irritable Bowel Syndrome (IBS)
What are some motility disorders of the Anorectal region and pelvic floor?
- 1. Fecal Incontinence
- 2. Hirschsprung's Disease
- 3. Outlet Obstructive Type Constipation (Pelvic Floor Dyssynergia)
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