Cardiac AP and ECG

Question 1

what property means the heart contracts as a result of APs it generates itself

Answer 1

autorythmia

Question 2

what are the two specialised cell types of the heart

Answer 2

contractile cells -
autorythmic cells - do not contract

Question 3

explain pacemaker activity

Answer 3

the membrane slowly depolarises to threshold between action potentials with no nervous stimulation

Question 4

what are the four locations of autorythmic cells in the heart

Answer 4

sinoatrial node
atrioventricular node
the bundle of His
Purkinje fibres

Question 5

explain the route of an AP through the heart (specialised conduction system)

Answer 5

SA node generates AP in the RA wall
passes through the internodal pathway to the AV node near the septum
the bundle of His pass into the septum and divides into right and left branches
branches into Purkinje fibres - small terminal fibres

Question 6

what conduction pathway connects the atria

Answer 6

interatrial pathway

Question 7

which heart node has the fastest possible rate of AP generation

Answer 7

sinoatrial

Question 8

what are the functions of the AV node

Answer 8

only conducting pathway between atrial muscle and the Bundle of His
introduces a delay (~100ms) allowing emptying of atria
can take over pacemaking if SA node fails

Question 9

summarise the ionic mechanism of pacemaker automaticity and rhythmicity

Answer 9

early phase:
- specific voltage-gated sodium channels called funny channels are open allowing Na+ entry down their concentration gradient
- close after voltage change
late phase:
- cause opening of voltage-gated transient calcium channels
- depolarise the cell to threshold causing opening of L-type calcium channel

Question 10

what is the advantage of the long plateau and refractory of the cardiac AP

Answer 10

ensures that contraction and most of relaxation are complete before another action potential can be initiated
prevents heart rate going so fast it cannot pump more blood

Question 11

where are the electrode positions for recording standard limb leads I, II and III
what is formed

Answer 11

lead I - right arm to left arm
lead II - right arm to left leg
lead III - left arm to left leg
forms Einthoven’s triangle

Question 12

what causes the P wave

Answer 12

atrial depolarisation

Question 13

what causes the QRS complex

Answer 13

ventricular depolarisation (hides atrial repolarisation)

Question 14

what causes the T wave

Answer 14

ventricular repolarisation

Question 15

what are the 5 phases of depolarisation in ventricular muscle

Answer 15

depolarisation
early repolarisation
plateau phase
late repolarisation
resting potential

Question 16

what causes depolarisation of cardiac contractile cells

Answer 16

Na+ entry

Question 17

what causes early repolarisation of cardiac contractile cells

Answer 17

transient K+ channels allowing efflux

Question 18

what causes the plateau phase of cardiac contractile cells

Answer 18

opening of L-type calcium channels
slow inwards diffusion maintains plateau

Question 19

what causes repolarisation of cardiac contractile cells

Answer 19

activation of ordinary K+ channels

Question 20

what links neighbouring cardiomyocytes allowing rapid spread of APs

Answer 20

gap junctions

Question 21

what determines whether an upward or downward deflection is recorded on an ECG

Answer 21

if the current is flowing towards or away from the electrodes

Question 22

what causes waves to show up on an ECG

Answer 22

when potential is actively changing across a membrane

Question 23

the depolarisation of which structures does not generate potentials great enough to be detected by an ECG

Answer 23

SA node
Bundle of His
Purkinje fibres

Question 24

what causes the PR segment, ST segment and TP interval

Answer 24

PR - AV node delay
ST - plateau phase of ventricular myocytes
TP - heart at rest

Question 25

why does the T wave deflect upwards when it is causes by repolarisation

Answer 25

the repolarisation of the heart occurs in the opposite direction to the depolarisation

Question 26

SA node normal firing rate

Answer 26

70-80 APs per min

Question 27

AV node normal firing rate

Answer 27

40-60 APs per min

Question 28

Bundle of His and Purkinje fibres firing rate

Answer 28

20-40 APs per min