Cardiovascular L2: Electrical activity of the Heart Flashcards
What is the flow of body from the vena cava to the body?
- Vena cava
- R atria
- R ventricle
- Pulmonary arteries
- Lungs
- Pulmonary veins
- L atrium
- L ventricle
- Body
Since blood flows through the heart in defined pattern, to achieve this pattern of flow, contraction of the heart must occur sequentially. What happens?
first the atria, then the ventricles
- In a specific direction: atria downwards, ventricles upwards (squeeze blood to body)
How is the flow pattern coordinated? List 3 contributors.
- inter-connected muscle cells (talk to each other)
- ‘self-excitation’ (heart beats on its own)
- conduction system (electrical activity passes through the heart)
Heart walls are composed of ______ arranged cardiac muscle fibres
spirally
What are the 3 layers of the heart?
- inner layer, endothelium, lines the heart
- middle layer, myocardium, cardiac muscle
- external layer, epicardium, covers the heart
What is the inner layer of the heart? What is its function?
endothelium- lines the heart
What is the middle layer of the heart? What is its function?
myocardium- cardiac muscle (muscle cells)
What is the external layer of the heart? What is its function?
epicardium- covers the heart (sheath that holds it together)
What are the 5 similarities between cardiac and skeletal muscle cells?
- Striated appearance: same arrangement of thick/thin filaments
- Same contractile mechanism: actin, myosin, crossbridges
- Similar t-tubule system (although cardiac are bigger)
- Similar sarcoplasmic reticulum system
- Action potentials do not summate
What are 8 differences between cardiac and skeletal muscle cells?
- Contraction is involuntary Smaller cells (100 µm long)
- Cells connected via intercalated disks
- Entire heart muscle contracts in a coordinated fashion: “syncytia”
- SR provides 80% of calcium for muscle contraction, remainder from ECF
- The cardiac muscle AP lasts 200-300msecs, compared with 2-3 msecs in skeletal muscle
- AP propagation slower: Cardiac 0.05-0.5 m/sec vs Skeletal 3-5 m/sec
- AP refractory period much longer: 200-300msecs
Contraction is involuntary in _____ muscle cells
cardiac
Cells are bigger in cardiac or skeletal muscle?
skeletal
Cardiac muscle cells connected via ________ disks
intercalated
Entire heart muscle contracts in a coordinated fashion: “______”
syncytia
SR provides 80% of ______ for muscle contraction, remainder from ECF
calcium
What can skeletal muscles do that cardiac muscle cells can never do? List 2 things. This is a good thing or not?
- The cardiac muscle AP lasts 200-300msecs, compared with 2-3 msecs in skeletal muscle
- AP refractory period much longer: 200-300msecs
This means that cardiac muscle cannot summate (i.e can’t have tetanus in cardiac muscle cells) (unlike skeletal- used as a way to increase strength in muscles) due to the long AP and long refractory period This is a good thing = don’t want it in the heart
Cardiac muscle fibres are interconnected by ________ to form ‘functional’ syncytia
intercalated discs
What are 2 things that intercalated discs contain?
- desmesomes
- gap junctions
What are desmesomes?
Desmesomes holds cells together
- Glue that holds discs together
What are gap junctions?
Gap junctions allow action potentials to spread to adjacent cells
- Pass through from one muscle cell to the next AP spreads across all cells
Cardiac muscle cells all act together as one, this is called _________.
functional syncytia
The heart is ‘_________’, initiating its own rhythmic contractions
self-excitable
What are 2 things that the heart contains?
- Contractile cells, 99% of the cardiac muscle cells, who do the mechanical work
- Autorhythmic cells initiate the action potentials which spread across the heart (develop AP spontaneously and quickly = starts the heart beat)
Both their APs differ
Cardiac autorhythmic cells are _________. What does this mean
pacemakers
- Their membrane potential slowly depolarizes between action potentials, drifting to threshold This cyclically initiates APs that spread throughout the heart to trigger rhythmic contractions`
Action potentials in auto-rhythmic cells are self-induced. True or false.
True
What are the 4 steps of actions potentials in auto-rhythmic cells?
- Slow depolarisation Na+ permeability increases, K + permeability decreases = Na+ in
- Slow depolarisation (cont) Ca+ permeability increases, Na+ permeability decreases = Ca+ in
- Fast depolarisation transient Ca+ channels close, long-lasting Ca+ channels open = more Ca+ in
- Repolarisation K+ permeability increases, Ca+ permeability decreases = K+ out
What is step 1 of actions potentials in auto-rhythmic cells?
Slow depolarisation Na+ permeability increases, K + permeability decreases = Na+ in
What is step 2 of actions potentials in auto-rhythmic cells? (after Na+ comes in)
Slow depolarisation (cont) Ca+ permeability increases, Na+ permeability decreases = Ca+ in
What is step 3 of actions potentials in auto-rhythmic cells? (after Ca+ comes in)
Fast depolarisation transient Ca+ channels close, long-lasting Ca+ channels open = more Ca+ in
What is step 4 of actions potentials in auto-rhythmic cells? (after more Ca+ comes in)
Repolarisation K+ permeability increases, Ca+ permeability decreases = K+ out
What happens to action potentials in contractile cells?
- The action potential of cardiac contractile cells shows a plateau phase (allows ventricles time to contract)
- Due primarily to activation of slow L-type Ca2+ channels
- Ensures adequate ejection time
Why do action potentials of cardiac contractile cells shows a plateau phase? How is done?
- Ensures adequate ejection time (the delay allows contraction to take place with refractory = never summate (allow time for blood to go out = no tetanus)
- Due primarily to activation of slow L-type Ca2+ channels
Which 2 things prevent summation or tetanus in cardiac muscle cells?
- Plateau phase
- long refractory period
What is the purpose of the cardiac conduction system?
To ensure that the propagation of action potentials (and therefore contraction of cardiac muscle) happens in the coordinated sequence and direction required for optimum ejection of blood (blood must leave atria before ventricles start to contract)
What are the 3 reasons why conduction of action potentials through the heart must be coordinated?
- Atrial excitation and contraction are complete before ventricular contraction begins
- Excitation of cardiac muscle fibres is coordinated so each atrium and each ventricle contracts as a unit
- Each pair of atria and pair of ventricles needs to be coordinated so that each pair contract simultaneously
What is the pathway of the cardiac conduction system. List the 6 structures. (the pattern of AP spread)
- Sinoatrial node (SA node)
- Interatrial pathyway; internodal pathway
- Atrioventricular node (AV node)
- Bundle of His (atrioventricular bundle)
- Purkinje fibres
- Spread through muscle fibre
What is the Sinoatrial node (SA node)?
in right atria near opening of superior vena cava pacemaker of the heart - *pacemaker*
What is the Atrioventricular node (AV node)
small bundle of cells located at base of right atrium near septum
What is the Bundle of His (atrioventricular bundle)?
cells originate at AV node and enters interventricular septum
What are the Purkinke fibres?
terminal fibers that extend from bundle of His and spread throughout ventricular myocardium
What is the pathway of the cardiac conduction system. List the 5 steps (the pattern of AP spread). Be specific
- Cardiac action potential originates at the SA node
- AP spreads throughout right and left atria
- AP can only pass from atria into ventricles via AV node, does so after a brief delay (which allows atrial contraction to complete ventricular filling before ventricular contraction begins)
- AP travels rapidly down interventricular septum via Bundle of His, then rapidly throughout myocardium through Purkinje fibres (ventricle begins contracting from base upward)
- Remainder of ventricular cells activated by APs moving through gap junctions (completes ventricular contraction and ejection)
How do you measure the electrical activity of the heart?
Electrocardiogram (ECG)
How is the ECG measured?
- Surface of skin
- Safe
- Non-invasive
- A lot of information given
What is the ECG?
The ECG is sum of the all the electrical activity as it spreads through the heart
What does the P wave?
P wave represents atrial depolarization
What is the QRS complex? What is Q? What is R? What is S?
QRS complex represents ventricular depolarization
Q- crossing of septum
R- coming down septum to bundle of His
S- coming up towards muscle wall
What is T wave?
T wave represents ventricular repolarization
What is the PR segment?
PR segment represents the AV node delay
What does the ECG and electrical activity look like over time? What ECG trace?
The sum of all electrical activity.
A ______ AP will cause a contraction.
Depolarising
How does a contraction occur (mechanically and electrically)?
How do we measure an ECG using limb leads?
Each ‘lead’ gives a different viewpoint, but with the same pattern: P,QRS,T…
- L and R wrist and L leg
Why is 12-lead ECG trace normal? How is this clinically relevant?
This is the standard
Angles = slight differences because size of waves are different.
This allows to see if there is a part of the heart that is damaged, not working properly..etc
Clinically, area of damage conduction working properly?
ECG allows us to identify _____.
abnormalities
What is a heart abnormalities in rate?
Tachycardia
What are 3 heart abnormalities in rhythm?
- Extrasystole (premature ventricle contraction = extra contraction)
- Ventricular fibrillation (first sign which results in death)
- Complete heart block ( VA nodes not working)
What is a cardiac myopathy?
Myocardial infarction (heart attack)
