Unit 3 Cardiac 1: Anatomy & Physiology

Question 1

Cardiac myocytes have skeletal muscle properties as well as neural tissue properties:
Like skeletal muscle:
Like Neural tissue:
Just cardiac myocytes:

Answer 1

Like skeletal muscle:
-actin and myosin myofilaments
-capable of contraction
-T-tubule system and SR work to maintain Ca+2 homeostasis for contraction and relaxation
Like Neural tissue:
-generates a RMP
-can initiate an action potential
-can propagate an action potential
Just cardiac myocytes:
-tight junctions serve as low resistance pathways to help spread cardiac potential
-cardiac myocytes contain more mitochondria than skeletal muscle

Question 2

Define automaticity

Answer 2

The ability to spontaneously generate an action potential

Question 3

Define excitability

Answer 3

The ability to respond to an electrical stimulus by depolarizing and firing an action potential

Question 4

What is conductance?

Answer 4

Because of their charge, ions do not freely pass through cell membrane. Instead, an ion requires an open channel to cross from one side of the membrane to the other. An open channel increases the conductance of that ion, while a closed channel reduces conductance of that ion.

Question 5

Define chronotropy

Answer 5

Heart rate

Question 6

Define inotropy

Answer 6

Strength of contraction (contractility)

Question 7

Define dromotropy

Answer 7

Conduction velocity (how fast the action potential travels per time)

Question 8

Define lusitropy

Answer 8

Rate of myocardial relaxation (during diastole)

Question 9

What is equilibrium potential?

Answer 9

Equilibrium is achieved when there is no concentration gradient and therefore no net flow of ion across the cell membrane.

Question 10

What equation can be used to calculate an ion’s equilibrium potential?

Answer 10

Nernst equation

E ion = -61.5 log ([Ion] inside / [ion] outside)

Question 11

What is resting membrane potential? What 3 mechanisms establish RMP?

Answer 11

The difference in electrical potential between the inside and outside of the cell. The inside is negative relative to the outside.

1. Chemical force
2. Electrostatic counterforce
3. Sodium/potassium ATPase

Question 12

What is threshold potential?

Answer 12

The internal voltage at which the cell depolarizes

Question 13

What is depolarization?

Answer 13

Depolarization takes place when there is a reduced polarity across the membrane. In excitable tissue, it results in an action potential

Question 14

What is hyperpolarization?

Answer 14

Hyperpolarization takes place when there is an increased polarity across a membrane.

Question 15

What is repolarization?

Answer 15

The restoration of a membrane potential towards resting membrane potential following depolarization

Question 16

What ion is the myocyte permeable to? What does this mean?

Answer 16

Potassium.
The cell continuously leaks potassium losing positives charges, why inside the cell is negative.
Explains why K+ is the primary determinant of RMP

Question 17

What happens to RMP when K+ decreases? Increases?

Answer 17

Decreases: RMP becomes more negative, and the myocytes become more resistant to depolarization.
Increases: RMP becomes more positive and myocytes depolarize more easily.

Question 18

Explain what happens to Na+ permeability with RMP approaches threshold potential

Answer 18

Voltage-gated sodium channels open and sodium conductance increases, this depolarizes the cell.

Question 20

How much Na and K are exchanged by the Na/K pump?

Answer 20

For every 3 Na+ ions it removes it brings 2 K+ ions into the cell

Question 21

What type of mechanism is the Na/K pump? What does it require?

Answer 21

An active transport mechanism

It requires ATP

Question 22

What makes the action potential for ventricular muscle unique?

Answer 22

A plateau phase where depolarization is prolonged, giving cardiac myocytes more time to contract, so the heart has enough time to eject its SV.

Question 23

What are the 2 purposes of the Na/K pump?

Answer 23

1. Remove the Na+ that enters the cell during depolarization

2. Return the K+ that left the cell during repolarization

Question 24

List the 5 phases of the ventricular action potential and describe the ionic movement during each phase.

Answer 24

Phase 0: depolarization -> Na+ influx 
Phase 1: initial repolarization -> K+ efflux & Cl- influx 
Phase 2: Plateau -> Ca+2 influx 
Phase 3: Repolarization -> K+ efflux 
Phase 4: Na+/K+ pump restores RMP

Question 25

Events in phase 0 of ventricular action potential

Answer 25

Phase 0: Upstroke
Threshold potential at -70 mV -> depolarization
Activation of fast voltage-gated Na+ channels -> Na+ in
Slope indicates conduction velocity

Question 26

Events in phase 1 of ventricular action potential

Answer 26

Phase 1: Initial Repolarization
Inactivation of Na+ channels
Cell becomes slightly less positive 
-K+ channels open -> K+ out
-Cl- channels open -> Cl- in

Question 27

Events of Phase 2 of ventricular action potential

Answer 27

Phase 2: plateau
Activation of slow voltage-gated Ca+ channels counters loss of K+ ions to maintain the depolarized state. -> Ca+ in
Delays repolarization
Maintains fast Na+ channels in inactivated state
Prolongs the absolute refractory period
Sustained contraction is necessary for the heart’s pumping action

Question 28

Events of phase 3 of ventricular action potential

Answer 28

Phase 3: Final Repolarization
K+ channels open -> K+ out
K+ leaves the cell faster than Ca+ enters -> repolarization 
Slow Ca+ channel deactivate 
Restores transmembrane potential to RMP