Lecture 4

Question 1

Cardiac Conduction System

Answer 1

1- SA Node
2- Internodal Tracts      2a- Interatrial Tracts
3-AV Node
4-AV Bundle
5-R/L Bundle Branches
6- Purkinje Fibers

Question 2

Excitable cells

Answer 2

1- neurons
2- muscle cells (skeletal, cardiac & Smooth)
3- Cells comprising the cardia conduction system

Question 3

Cardiac Action Potentials (Two Types)

Answer 3

1- Non-pacemaker or Fast response Action Potential

2-Pacemaker or Slow Response Action Potential

Question 4

Non-pacemaker or fast response AP

Answer 4

1- Occur in the atria, ventricles and Purkinje fibers

2- Undergo “rapid” deporlarization

Question 5

Pacemaker or slow response AP

Answer 5

1- Occur in the SA node and AV node

2- UNdergo “slow: depolarization”

Question 6

Fast Response Cardiac AP Phases

Answer 6

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 active

Question 7

Ca influx

Answer 7

Ca influx is required for cardiac contraction

Question 8

ECF Ca Influx

Answer 8

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)

Question 9

Neurotransmitters and drugs can alter Ca conductance

Answer 9

1- Norepinephrine increases release

2- Acetlycholine, Beta blockers and Ca channel blockers decrease release

Question 10

Main Ca Extrusion Mechanism

Answer 10

1- SR Ca pumps (SERCA)
2- Sarcolemmal Ca pump
3- 3Ma/1Ca exchangers (NCX)

Question 11

Two refractory periods

Answer 11

1- Effective (or absolute) refractory period

2- Relative refractory period

Question 12

Effective (or absolute) refractory period

Answer 12

A 2nd AP absolutely cannot be initiated, no matter how large a stimulus is applied

Question 13

Relative refractory period

Answer 13

A 2nd action potential may be evoked only when the stimulus is sufficiently strong (supra threshold)

Question 14

Sinoatrial Node

Answer 14

1- Normally the Pacemaker of the heart
2- Unstable RMP
3- Has Phase 4 depolarization or automaticity

Question 15

Slow Response Cardiac AP

Answer 15

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)

Question 16

I-f

Answer 16

is turned on by repolarization of the membrane potential during the preceding AP

Question 17

Heart Rate

Answer 17

the number of times the SA node “discharges” per minute

Question 18

Conduction Velocity

Answer 18

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

Question 19

Chronotropic effects

Answer 19

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

Question 20

Negative Chronotropic Effect

Answer 20

• 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

Question 21

Positive Chronotropic Effect

Answer 21

-Increases heart rate by increasing the firing rate of the SA node (phase 4)

Question 22

Dromotrophic effect

Answer 22

1- Change in conduction velocity, primarily in the AV node
2- Negative dromotropic effect
3- Positive dromotropic effect

Question 23

Negative Dromotropic effect

Answer 23

decreases conduction velocity through the AV node, slowing conduction of action potentials from he atria to the ventricles and increasing the PR intervals

Question 24

Positive Dromotropic effect

Answer 24

increases conduction velocity through the AV node, speeding the conduction of AP from the atria to the ventricles and decreasing the PR interval

Question 25

review the last 3 slides

Answer 25

review the slide dog.