Electrical Events of the Cardiac Cycle

Question 1

What is the cardiac cycle controlled by?

Answer 1

Nervous system but can function without:
99% cardiac cells - contractile
1% are capable of spontaneously firing - autorhythmic

Question 2

Where are spontaneously firing cells located?

Answer 2

1. Sinoatrial node - right atrial wall near opening of SVC
2. atrioventricular node - base of right atrium near septum above AV junction
3. Bundle of His (right and left), Purkinje fibres

Question 3

What dictates the rate/rhythm of the heart?

Answer 3

Spontaneous discharge rate of the fastest firing cells - SA node is natural pacemaker

Question 4

Spontaneous discharge rate of SA node

Answer 4

70-80 action potentials/min

Question 5

What dictates the rate of the heart if the SA node is affected?

Answer 5

AV node

40-60 action potentials/min

Question 6

Resting potential of SA node cell

Answer 6

-60mv

Question 7

What happens at the resting potential in a SA node cell

Answer 7

Na+ channels open, increased influx of Na+ (funny current)
Ca2+ channels open, increased Ca2+ influx
K+ channels close, decreased K+ efflux (retain K+ and +ve charge)
Therefore charge becomes less negative, reaching threshold potential

Question 8

Threshold potential of SA node cell

Answer 8

-40mv

Question 9

What happens in a SA node cell once threshold potential is reached?

Answer 9

Increased Ca2+ influx, until reaching 0 mv

Question 10

What happens in SA node cell once 0mv is reached?

Answer 10

Na+ channels close
K+ channels open, K+ efflux
Charge decreases back to resting potential

Question 11

Spread of excitation through the heart

Answer 11

SA node
Rapidly through atria (around 1m/s)
AV node (slowly conducting, 0.05m/s, delay)
Rapidly through bundle of His and down the branches and Purkinje fibres (around 1-4m/s)
Through ventricular muscle cells

Question 12

Why is excitation spread rapidly through the heart?

Answer 12

Allows heart to be an effective pump and can act as a single unit/syncytium
Entire contraction of myocardium

Question 13

What allows the rapid spread of excitation through the heart?

Answer 13

Intercalated discs (gap junctions) between fibres
They provide low resistance pathways

Question 14

What does the coordination of the spread of excitation mean?

Answer 14

Atrial excitation and contraction are complete before ventricular contraction due to AV delay - enables efficient emptying of blood from atria to ventricles

Ventricular excitation occurs synchronously due to rapid spread down septum and through Purkinje fibres - enables ventricles to contract as co-ordinated units and can expel blood effectively

Question 15

Resting membrane potential of ventricular (contractile) cell

Answer 15

-90mv

Question 16

What causes rapid depolarisation of contractile cells?

Answer 16

Opening of Na+ channels causes large and rapid Na+ influx to around 20mv

Question 17

What occurs in contractile cells after reaching 20mv?

Answer 17

K+ channels open resulting in efflux, decreasing charge slightly
Ca2+ influx causes a plateau, the channels begin to close and K+ efflux is dominant, resulting in repolarisation of the cell

Question 18

What does excitation-contraction coupling require?

Answer 18

movement of Ca2+ ions

Question 19

Describe how the movement of Ca2+ causes systole

Answer 19

influx of Ca2+ during action potential
triggers release of further Ca2+ from sarcoplasmic reticulum
free Ca2+ activates contraction of myocardial fibres

Question 20

What does the amount of Ca2+ determine in the excitation-contraction coupling?

Answer 20

cross-bridge cycling and force of contraction

Question 21

Describe how the movement of Ca2+ causes diastole

Answer 21

uptake of Ca2+ by sarcoplasmic reticulum and extrusion of Ca2+ by Na+/Ca2+ exhange and outward Ca2+ pump
lowers free Ca2+ allowing relaxation

Question 22

What is tetanus?

Answer 22

sustained, fused contracton

Question 23

What protects the heart from tetanus occuring?

Answer 23

plateau phase of AP provides long refractory period (without pump action)
additional stimuli will not cause tetanus

Question 24

Direction of ventricular excitation/contraction

Answer 24

endocardium to epicardium (inside to outside)

apex to base (bottom to top)

Question 25

Apart from pacemakers, what can influence the rate of the heart?

Answer 25

nerves

Question 26

How do nerves influence the rate of the heart?

Answer 26

activation of sympathetic nerves increases heart rate by activation of beta 1 adrenoceptors in SA node

Question 27

Consequence of activation of beta 1 adrenoceptors in SA node

Answer 27

increases slope fo pacemaker by increasing funny current (Na+ influx) and influx of Ca2+
therefore decreased time to reach threshold potential and faster reaching -60mv again (resting membrane potential)

Question 28

How can the slope of pacemaker be decreased?

Answer 28

activation of parasympathetic nerves decreases heart rate by activation of M2 muscarinic receptors in SA noce
vagus nerve

Question 29

What nerve dominates at rest?

Answer 29

vagus

cuts off at exercise

Question 30

Effect of activation of vagus nerve on AP

Answer 30

increases K+ permeability to hyperpolarise membrane potential (RMP more negative, -80mv)
decreases slop of pacemaker potential by decreasing funny current and current of Ca2+