cardiac action potential and ECG

Question 1

what are the 2 types of cardiac cells

Answer 1

1. contractile cells (99% normally do not initiate APs)
2. autorhythmic cells (do not contract: initiate or conduct APs)

Question 2

What is autorhythmicity

Answer 2

the property of the heart where it contracts rhythmically as a result of action potentials it generates itself

Question 3

what are features of cardiac autorhythmic cells

Answer 3

they do not have a resting membrane potential but they have a pacemaker activity

Question 4

what occurs when autorhythmic cells cyclically initiate APs

Answer 4

it spreads through the heart to trigger contraction without any nervous stimulations

Question 4

where are the specialised non-contractile cells that demonstrate
autorhythmicity located

Answer 4

1) The sinoatrial node (SA node)
2) The atrioventricular node (AV node)
3) The bundle of His (atrioventricular bundle)
4) Purkinje fibres

Question 4

what is a pacemaker potential

Answer 4

autorhythmic cell membranes slow drift to threshold

Question 5

describe the spread of cardiac excitation

Question 6

what are the functions of the atrioventricular node

Answer 6

forms only conducting pathway between atrial muscle and bundle of his, hence ventricles

introduces considerable delay to spread of excitation allowing time for blood to move from atria to ventricle

AV node cells have well developed latent powers of rhythmicity and can take over pace making if impulses from SA node fail to reach them

Question 7

describe the pace maker activity of cardiac autorhythmic cells graph

Question 8

what are the 5 phases of the cycle of voltage change across cardiac myocytes

Answer 8

1. Depolarization
2. Early repolarization
3. Plateau phase
4. Late repolarization
5. Resting potential

Question 9

Describe an ECG graph

Answer 9

P wave - Atrial depolarisation
PR segment - AV nodal delay
QRS complex - ventricular depolarisation (atria repolarizing simultaneously)
ST segment - Time during which ventricles are contracting and emptying
T wave - Ventricular repolarisation
TP interval - Time during which ventricles are relaxing and filling

Question 10

what does the ECG record

Answer 10

electrical activity

Question 11

what does not generate a ECG and why

Answer 11

SA node, Bundle of His, Purkinje fibres

they do not generate a potential difference large enough to be recorded at the body surface

Question 12

key points

Answer 12

Points to note:
1. ECG is much smaller than intracellular action potentials (approximately 1 mV compared to 100 mV).
2. Waves only recorded when the potential is changing across cell
membranes - ECG flat during plateau phase of the action potential (between the QRS and T waves) and diastole.