electrical activity of the heart

Question 1

why is cardiac action potential long?

Answer 1

due to L-type Ca2+ depolarisation

Question 1

describe atrial flutter, atrial fibrillation and ventricular fibrillation

Answer 1

[1] Atrial flutter - quicker depolarisation in atria

[2] Atrial fibrillation - no coordinated P waves - uncoordinated depolarisation and contraction in atria

[3] Ventricular fibrillation - uncoordinated depolarisation and contraction in ventricles

Question 2

state what P wave, QRS complex and T wave correspond to:

Answer 2

P wave - atrial repolarisation

QRS complex - ventricular depolarisation

T wave - ventricular repolarisation

Question 3

what is the atrioventricular node?

Answer 3

delay box

the only pathway through which action potentials can reach the contractile fibres of the ventricles.

Question 4

describe the special conducting system:

Answer 4

SA NODE → AV NODE → BUNDLE OF HIS → PURKINJE FIBRES → APEX

Question 5

what is the function of the bundle of his and Purkinje’s fibres? state the conduction rate:

Answer 5

they are both specialized conducting cells of the ventricles, a rapid conduction system.

conduction rate ~ 5 m/sec

Question 6

define the AV node and state the conduction rate:

Answer 6

they are myocardial cells involved in the conduction of action potential, connecting the atria and ventricles.

conduction rate ~ 0.05 m/sec (delay box - gives time for atria to contract)

Question 7

what is the only non conducting part of the heart that separates the atrium from the ventricles?

Answer 7

annulus fibrosus

Question 8

where does depolarisation begin? state the condution rate:

Answer 8

SA node, which is the main pacemaker of the heart

depolarisation speed determines heart rate

conduction rate ~ 0.5 m/sec

Question 9

why does the depolarisation phase occur in a pacemaker cell?

Answer 9

due to an increase in PCa2+ (L-type)

Question 10

describe the pacemaker potential:

Answer 10

• unstable membrane potential
• starts at -60mV

[0] hyperpolarisation

[1] slow depolarisation - PNa+ influx

[2] rapid depolarisation - PCa2+ (Pf) influx

[3] repolarisation - PK+ efflux

Question 11

describe the non pacemaker action potential

Answer 11

[0] rapid depolarisation - increase in PNa+, opening of sodium channels

[1] initial depolarisation - closing of sodium channels

[2] plateau - increase in PCa2+ (L-type) and a decrease in PK+

[3] repolarisation - decrease in PCa2+ and an increase in PK+

[4] Na/K pump, sodium out and potassium in.

Question 12

state the reason why the resting membrane potential is -90mV in a non-pacemaker cell:

Answer 12

due to high resting Pk+

Question 13

what is the main difference between the action potential of the myocardial contractile cell and those of skeletal muscle fibres and neurons?

Answer 13

The myocardial cell has a longer action potential due to Ca2+ entry

Question 14

what does summation in skeletal muscle lead to?

Answer 14

tetanus

fatal if it happened in the heart.

Question 15

state the significance of cardiac muscle having a long action potential:

Answer 15

longer than the skeletal action potential

~2 msec compared to ~250 msec

[1] long refractory period (the time during which the muscle cannot contract again after an action potential)
- prevents tetanic contraction

[2] Ca2+ entry from outside the cell can regulate contraction

Question 16

why do the actin-myosin binding sites not become fully saturated in cardiac muscle, compared to skeletal muscle?

Answer 16

Allows regulation by controlling Ca2+ levels.

Question 17

state the functions of desmosomes and gap junctions in the heart:

Answer 17

desmosomes (physical connection) and gap junctions (electrical connection) cause the heart cells to contract as one big muscle
-> forms intercalated discs.

Together these form a functional syncytium