Heart muscle function - PP

Question 1

Does cardiac muscle does require

innervation to be activated?

Answer 1

No. Unlike skeletal muscle, cardiac muscle does not require innervation to be activated.

Question 2

What makes heart transplant possible?

Answer 2

Cardiac muscle does not require action potentials from nerves to activate its own electrical activity (a fact that makes heart transplantation operations possible).

Question 3

What innervates the heart?

Answer 3

Both branches of the autonomic nervous system (ANS) innervate the heart, the ANS modulates cardiac function.

Question 4

Nexi:

Answer 4

All myocardial cells are coupled electrically through gap junctions at points called nexi.

Question 5

When are all cardiac cells activated?

Answer 5

During systole, all cardiac cells are activated

Question 6

Automaticity:

Answer 6

Automaticity - unique ability to generate own action potentials

Question 7

Rhythmicity

Answer 7

Rhythmicity - ability to generate these potentials in a regular, repetitive manner

Question 8

Where does automaticity occur?

Answer 8

automaticity can occur in atrial and ventricular myocytes as well as in the Purkinje fibers.

Question 9

What initiates cardiac electrical activity?

Answer 9

Normally, prior to each contraction of the heart, cardiac electrical activity is initiated by a modified set of muscle cells of the right atrium called the sinoatrial (SA) node

Question 10

Pulse generators:

Answer 10

• SA : approx. 80 /min
• AV : approx. 50/min
• Purkinje fib.: approx. 20/min

Question 11

Where are fast response action potentials observed?

Answer 11

Fast response action potentials – observed in atrial and ventricular muscle and Purkinje fibers

Question 12

Where are slow response action potentials observed?

Answer 12

Slow response action potentials – observed in sinoatrial (SA) node and atrioventricular(AV) node

Question 13

What are the phases of fast response action potential?

Answer 13

• Phase 0
• Phase 1
• Phase 2
• Phase 3
• Phase 4

Question 14

What happens in Phase 0 (in fast response action potential)?

Answer 14

Phase 0: membrane depolarisation from -85mV up to at least -55mV opens voltage-sensitive Na+ channels, allowing a rapid influx of Na+ to a cell – self reinforcing and self limiting depolarisation - few milliseconds later 99% of Na channels are inactivated.
- They can’t be open again unless the cell membrane becomes repolarized below -50mV

Question 15

What happens in Phase 1 (in fast response action potential)?

Answer 15

Phase 1: opening transient outward K+ channels allows small temporary repolaryzation

Question 16

What happens in Phase 2 (in fast response action potential)?

Answer 16

Phase 2: Opening of a calcium L channels » influx of a Ca 2+ to a cardiac cell. In the same time there is opening of a K+ channel - efflux of a K+. Both currents approximately matches to each other » potential remains relatively constant at a positive value - plateu

Question 17

What happens in Phase 3 (in fast response action potential)?

Answer 17

Phase 3: rapid repolarization by efflux of a K+

Question 18

What happens in Phase 4 (in fast response action potential)?

Answer 18

Phase 4.: resting membrane potential

Question 19

Excitability:

Answer 19

• Is the ability of cardiac cells to initiate action potentials in response to depolarizing current.
• Reflects the recovery of channels that carry the inward currents for the upstroke of the action potential.
• Changes over the course of the action potential are described by refractory periods

Question 20

Absolute refractory period (ARP):

Answer 20

• Begins with the upstroke of the action potential and ends after the plateau.
• Reflects the time during which no action potential can be initiated, regardless of how much inward current is supplied.

Question 21

Relative refractory period (RRP):

Answer 21

• Is the period immediately after the ARP when repolarization is almost complete
• Is the period during which an action potential can be elicited, but more than the usual inward current is required

Question 22

Supranormal excitability (SNP):

Answer 22

A period during which the treshold for producing a second action potential is less than it is in the steady state

Question 23

What are the phases in slow response action potential?

Answer 23

• Phase 0
• Phase 3
• Phase 4

Question 24

What happens in Phase 0 (in slow response action potential)?

Answer 24

The action potential is slower (than in myocardium and Purkinje fibers), carried entirely by slow L-type voltage-gated Ca2+ channels (inward current)

Question 25

What happens in Phase 3 (in slow response action potential)?

Answer 25

Outward K+ current

Question 26

What happens in Phase 4 (in slow response action potential)?

Answer 26

Unique for SA and AV spontaneous, progresive, depolarization caused by progresive reduction in K+ outward current, and Na + inward current (If)

Question 27

Chronotropic positive:

Answer 27

activation of If current
               ↓
spontaneus depolarisation is faster
               ↓
Heart Rate (HR)↑

Question 28

Dromotropic positive:

Answer 28

activation of CaL channel
                   ↓
amplitude of action potential ↑
                   ↓
Conduction velocity ↑

Question 29

Chronotropic &

dromotropic negative:

Answer 29

activation K+ outward channel
↓
slowing down spontaneus depolarisation & decreasing amplitude of action potential

Question 30

How are myocardial contraction is modified?

Answer 30

Myocardial contraction is modified by altering the gating of the L /DHP Ca2+ channels in the cell membrane that are activated during phase 2 of the action potential.

Question 31

Inotropic positive – increasing intracellular Ca:

Answer 31

• β receptor agonists (norepinefrine, epinefrine);
• Digitalis (inhibition of a Na/K ATP-ase ⇛ intracellular Ca↑)
• Inhibitors of cAMP phosphodiesterase (↑ cAMP): caffeine, theophyline

Question 32

Inotropic negative – decreasing intracellular Ca:

Answer 32

• Β - blockers
• Ca channel blockers,
• Acetylocholine (weak inotropic negative in atria),
• Ishemia (lack of ATP)
• Acidosis ( ↑H+)

Question 33

End diastolic volume (EDV):

Answer 33

End diastolic volume (EDV) - volume of blood in ventricle at the end of diastole (maximal volume during cardiac cycle)

Question 34

End systolic volume (ESV):

Answer 34

End systolic volume (ESV) - volume of blood in ventricle at the end of systole (minimal volume during cardiac cycle)

Question 35

Stroke volume (SV):

Answer 35

Stroke volume (SV) - volume of blood ejected from a vetricle during 1 cycle.
SV = EDV – ESV

Question 36

Cardiac output (CO):

Answer 36

Cardiac output (CO) - volume of blood ejected from the heart per unit time, usual resting values for adults are 5 to 6 L/min, or approximately 8% of body weight per min. CO = SV × HR

Question 37

Cardiac index:

Answer 37

CO divided by body surface area - cardiac index.

Question 38

Ejection fraction (EF):

Answer 38

Ejection fraction (EF) = (SV/EDV) x 100%

Question 39

STUDY PAGE 21 AND UP ON YOUR OWN

Answer 39

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