Lecture 4 Flashcards
Cardiac Conduction System
1- SA Node 2- Internodal Tracts 2a- Interatrial Tracts 3-AV Node 4-AV Bundle 5-R/L Bundle Branches 6- Purkinje Fibers
Excitable cells
1- neurons
2- muscle cells (skeletal, cardiac & Smooth)
3- Cells comprising the cardia conduction system
Cardiac Action Potentials (Two Types)
1- Non-pacemaker or Fast response Action Potential
2-Pacemaker or Slow Response Action Potential
Non-pacemaker or fast response AP
1- Occur in the atria, ventricles and Purkinje fibers
2- Undergo “rapid” deporlarization
Pacemaker or slow response AP
1- Occur in the SA node and AV node
2- UNdergo “slow: depolarization”
Fast Response Cardiac AP Phases
0 - Rapid Depolarization
1-Upstroke of AP
2- Fast Na channels open, K channels close
1 - Early/initial repolarization
1- K channels open
2- Fast Na channels close
2- Plateau Phase
1- Ca channels open cause inward Ca
2- K rushes in
3- Late or final repolarization
1- K continues to rush in
2- L-type Ca channels close
4- RMP
1- K channels remain open
2- Ca extrusion mechanism become highly activeCa influx
Ca influx is required for cardiac contraction
ECF Ca Influx
1- During AP amount of Ca is small
2- Does not promote actin-myosin interaction
3- Induces Ca release from sarcoplasmic reticulum (Ca induced Ca release = CICR)
4- Promotes actin-myosin interaction and contraction; occurs via ryanodine receptors (RyR2)
Neurotransmitters and drugs can alter Ca conductance
1- Norepinephrine increases release
2- Acetlycholine, Beta blockers and Ca channel blockers decrease release
Main Ca Extrusion Mechanism
1- SR Ca pumps (SERCA)
2- Sarcolemmal Ca pump
3- 3Ma/1Ca exchangers (NCX)
Two refractory periods
1- Effective (or absolute) refractory period
2- Relative refractory period
Effective (or absolute) refractory period
A 2nd AP absolutely cannot be initiated, no matter how large a stimulus is applied
Relative refractory period
A 2nd action potential may be evoked only when the stimulus is sufficiently strong (supra threshold)
Sinoatrial Node
1- Normally the Pacemaker of the heart
2- Unstable RMP
3- Has Phase 4 depolarization or automaticity
Slow Response Cardiac AP
0 - Upstroke of AP
1- Increase in Ca conductance
2- Inward Na
3- Repolarization
1- Increase in K conductance, results in an outward K current
4- Slow depolarization
1- Accounts for the pacemaker activity of the SA node
2- inward Na (called I-f)I-f
is turned on by repolarization of the membrane potential during the preceding AP
Heart Rate
the number of times the SA node “discharges” per minute
Conduction Velocity
1- Reflects the time required for excitation to spread throughout cardiac tissue
2- Fastest Purkinje system
3- slowest in the AV node allowing for ventricular filling before ventricular contraction
Chronotropic effects
1- changes in heart rate
2- negative chronotropic effect
3- Positive chronotropic effect
4- changes in HR are reflected on the ECG by changes in the R-R intervals
Negative Chronotropic Effect
- Decrease heart rate by decreasing the firing rate of the SA node
- fewer action potentials occur per unit time due to threshold potential reached more slowly and less frequently
- increases the PR interval
Positive Chronotropic Effect
-Increases heart rate by increasing the firing rate of the SA node (phase 4)
Dromotrophic effect
1- Change in conduction velocity, primarily in the AV node
2- Negative dromotropic effect
3- Positive dromotropic effect
Negative Dromotropic effect
decreases conduction velocity through the AV node, slowing conduction of action potentials from he atria to the ventricles and increasing the PR intervals
Positive Dromotropic effect
increases conduction velocity through the AV node, speeding the conduction of AP from the atria to the ventricles and decreasing the PR interval
review the last 3 slides
review the slide dog.
