Electrical Activity of the Heart

Question 1

Control of cardiac myocyte contraction

Answer 1

Action potentials initiated, regulated and conducted by pacemaker cells

Question 2

Primary pacemaker of the heart

Answer 2

Sino-atrial node

Question 3

Sino-atrial node function and conduction speed

Answer 3

Pacemaker

0.5m/sec

Question 4

Annulus fibrosus function

Answer 4

Non-conducting cells that separate the atrium and ventricles.
Only cells not connected by gap junctions

Question 5

Atrioventricular node function and conduction speed

Answer 5

Delay box

0.05m/sec

Question 6

Bundle of His function

Answer 6

Travels inferiorly between ventricles and transmits AV node impulses to the ventricles.
Located at the inferior end of the interatrial septum

Question 7

Purkinje fibre function and conduction speed

Answer 7

Rapid conduction system: 5m/sec.
Most release GABA neurotransmitter which exerts inhibitory actions on certain neurons and thereby reduces transmission of nerve impulses

Question 8

Excitation-contraction coupling

Answer 8

AP triggers myocyte contraction

Question 9

Functional syncytium

Answer 9

Single cells act as one cell as they are connected:

• physically by desmosomes
• electrically by gap junctions

Question 10

Resting membrane potential of myocyte

Answer 10

Negative (inside)
- Potassium predominant inside
- Sodium and calcium predominant outside
Ion gradients maintained by ion pumps

Question 11

Pacemaker cells resting membrane potential

Answer 11

Unstable: around -60 mV

Question 12

Pacemaker cells - reach pacemaker potential

Answer 12

1. Spontaneously -60 mV -> -40mV

2. Sodium ions enter cell

Question 13

Pacemaker cells - depolarisation

Answer 13

-40 mV -> +10 mV

Calcium ions into cell

Question 14

Pacemaker cells - repolarisation

Answer 14

+10 mV -> -60 mV

Potassium out of cell

Question 15

Contractile cells resting membrane potential

Answer 15

Stable: around -90 mV

High resting intracellular potassium

Question 16

Contractile cells - reach pacemaker potential

Answer 16

-90 mV -> -70 mV

Positive ions leak through gap junctions (stimulated by adjacent cell)

Question 17

Contractile cells - depolarisation

Answer 17

-70 mV -> +30 mV
Rapid influx of sodium

At -40 mV: calcium into cell

Question 18

Contractile cells - early repolarisation

Answer 18

Small outflux of potassium

Question 19

Contractile cells - plateau

Answer 19

200 ms
Calcium in, potassium out
- calcium induced calcium release

Question 20

Contractile cells - repolarisation

Answer 20

Potassium outflux

Question 21

Cardiac muscle vs Skeletal muscle (2)

Answer 21

1. Contraction time
   - cardiac: 250 ms (long plateau)
   - skeletal: 2 ms
2. Absolute refractory period
   - cardiac: 50 ms
   - skeletal: 1 ms
   - prevent summation and tetanus in cardiac muscle

Question 22

Calcium channel blockers

Answer 22

Decrease force of contraction

Question 23

Cardiac glycosides

Answer 23

Increase force of contraction

Question 24

Temperature

Answer 24

Increases roughly 10 beats/min/degrees Celsius

Question 25

Hyperkalaemia and hypokalaemia

Answer 25

Fibrillation and heart block

Question 26

Hypercalcaemia

Answer 26

Increased HR and force of contraction

Question 27

Hypocalcaemia

Answer 27

Decreased HR and force of contraction