Lecture 02: Basic Structural and Electrical Chars. of Myocardial Cells (Hayward)

Question 0

diastole. where is pressure greatest during this time?

Answer 0

period of ventricle relaxation as blood from atria fill ventricles. Pressure outside ventricle is greater than inside ventricle

Question 1

Pacemaker cells

Answer 1

specialized myocardial cells in SA node that undergo spontaneous depolarization (produces SELF-GENERATED action potentials) to instigate sequential depolarization of heart muscle, followed by synchronized rhythmic contractions of the heart chambers (systole)

Question 2

systole. where is pressure greatest during this time?

Answer 2

period of ventricular contraction as ventricle pump blood out to pulmonary and systemic circulation. Pressure is greater inside ventricle than outside

Question 3

How is rapid impulse conduction/depolarization achieved across myocardium?

Answer 3

Direct ELECTRICAL COUPLING between cells via intercalated disks with gap junctions

Question 4

How do action potentials of myocardial cells compare to those generated by skeletal muscle cells?

Answer 4

much broader (almost 300x longer)

Question 5

Why do pacemaker cells not have input from external control system?

Answer 5

they spontaneously depolarize and produce their own APs

Question 6

sarcolemma =

Answer 6

myocardial cell membrane

Question 7

2 most important physiological characteristics of the myocardial cell membrane

Answer 7

1) ability to maintain appropriate ion concentration gradients b/w intracellular and extracellular environments
2) ability to respond to electrical depolarization

Question 8

Resting membrane potential of cardiac myocytes is fx of:

Answer 8

1) high permeability of membrane to K+
2) low permeability of membrane to other ions (i.e. Na, Ca)
3) high concentration of K+ in myocardial cells at rest

Question 9

Why is K+ concentration in myocardial cells higher than extracellular environment at rest? (3 main reasons)

Answer 9

1) Myocardial cells have high permeability to K+
2) negatively charged proteins in cells draw K+ in
3) Na+/K+ contributes to negative balance inside cell, which also draws more K+ in

Question 10

Nernst Equation

Answer 10

used to determine the equilibrium potential for different ions across a semi-permeable membrane at rest

Question 11

Changes in extracellular concentrations of Na+ and Ca++ –> myocardial RMP

Answer 11

little effect

Question 12

increased extracellular K+ —> myocardial cells

Answer 12

depolarize the RMP of cardiac muscle fibers

Question 13

What does Na/K pump pump in/out of cell?

Answer 13

3 Na+ out, 2 K+ in. Driven by ATP

Question 14

2 types of action potentials in myocardial cells

Answer 14

fast type and slow type

Question 15

Why is AP of myocardial cells longer than normal cells?

Answer 15

due to large influx of Ca++ during depolarization IN ADDITION to normal Na+ influx

Question 16

Most common type of AP in myocardial cells

Answer 16

fast response AP

Question 17

Where do fast response APs occur?

Answer 17

atrial and ventricular myocytes + Purkinje fibers

Question 18

Where do slow response AP occur?

Answer 18

pacemaker cells of SA and AV nodes

Question 20

What do myocardial cells have a high concentration of?

Answer 20

glycogen

Question 21

Phase 0: Rapid Depolarization (precursor + 2 main events)

Answer 21

Before Phase 0, pacemaker cells fire in wave of membrane depolarization. Causes:

1) opening of abundant fast-type voltage sensitive Na+ channels of myocardial cells
2) rapid influx of Na+ down Na+ concentration and charge gradient

RAPID Na+ INFLUX

Question 22

Where are “backup” pacemakers located?

Answer 22

AV node

Question 23

4 phases of typical fast response cardiac AP

Answer 23

0) Rapid depolarization
1) Initial repolarization
2) Plateau phase
3) Rapid repolarization
4) Resting membrane potential

RAPID Na+ INFLUX –> Ca++ INFLUX –> K+ EFFLUX –> return to RMP

Question 24

decrease in extracellular K+ –> myocardial cells

Answer 24

hyperpolarizes

Question 25

Phase 1: Initial repolarization (2 main events)

Answer 25

1) Na+ channels rapidly close

2) Transient K+ outward current activated by the depolarization

Question 26

Phase 2: Plateau phase (3 main events)

Answer 26

1) Ca++ channels SLOWLY open
2) Ca++ influx according to its conc. gradient
3) Decreased K+ efflux

Question 27

Phase 3: Rapid Repolarization (3 main events)

Answer 27

1) Ca++ channels slowly close/Ca++ conductance decreases
2) K+ efflux increases via delayed rectifier channel that was activated in plateau phase
3) Cell is repolarized/hyperpolarized as membrane potential returns to original resting negative potential

Question 28

Why is plateau phase a plateau?

Answer 28

Low conductance of K+ during this time.

Also, slow Ca++ channel opening and closing(<–main regulator)

Question 29

Why is slow nature of Ca++ channel opening/closing important?

Answer 29

Allows for prolonged AP in cardiac cells. Length of AP is directly related to the rate of intracellular sequestering of Ca++

Question 30

When do delayed rectifier channels become activated?

Answer 30

During plateau phase

Question 31

Phase 4: Resting membrane potential (3 main events)

Answer 31

1) High K+ conductance, low Na+ and Ca++ conductance
2) Na+/K+ pumps correct Na+/K+ concentrations from preceding AP
3) Na/Ca++ channels and sarcolemma restore internal Ca++ concentration

Question 32

Long AP is associated with long:

Answer 32

refractory period

Question 33

refractory period fx

Answer 33

allows for sufficient time between heart beats for adequate cardiac filling time and Ca++ reuptake into intracellular stores

Question 34

When does absolute/effective refractory period occur?

Answer 34

Phase 1-3

Question 35

When does relative refractory period occur?

Answer 35

phase 3

Question 36

When is cell completely unexcitable to new input?

Answer 36

During phase 1-2 in absolute/effective refractory period

Question 37

Stimulation of myocardial cell in beginning of phase 3 will result in:

Answer 37

local AP only with no propagation

Question 38

Stimulation of myocardial cell during middle/end of phase 3 during relative refractory period will result in:

Answer 38

AP conducted, but with slower velocity. AP may not depolarize the rest of the heart and generate a heart beat.

Question 39

What is amplitude of AP directly relate to?

Answer 39

Driving force to open Na+ channels

Question 40

Normal RMP of myocardial cell

Answer 40

-90mV