Origin and Conduction of Cardiac Impulse

Question 1

What is the heart?

Answer 1

Electrically controlled muscular pump which sucks and pumps blood

Question 2

Where are the electrical signals which control the heart generated?

Answer 2

Within the heart

Question 3

What is autorhythmicity?

Answer 3

Heart is capable of beating rhythmically in the absence of external stimuli

Question 4

Where does excitation of the heart normally begin?

Answer 4

Pacemaker cells in the sinoatrial node

Clusters of these cells initiate the heart beat

Question 5

Where is the SA node situated?

Answer 5

Upper right atrium close to where superior vena cava enters the right atrium

Question 6

What does the SA normally do?

Answer 6

Drives the entire heart rate (sets the pace)

Question 7

What does it mean if a heart is in sinus rhythm?

Answer 7

Heart controlled by sino atrial node

Question 8

Role and characteristics of cells in SA node?

Answer 8

No stable resting membrane potential

They generate regular spontaneous pacemaker potentials

Question 9

What happens when a spontaneous pacemaker potential is generated?

Answer 9

It takes the membrane potential to a threshold and every time a threshold is reached an action potential is generated

Question 10

What does an action potential being produced result in?

Answer 10

Generation of regular spontaneous action potentials in the SA nodal cells

Question 11

Permeability to K+ in pacemaker cells?

Answer 11

Does not remain the same between action potentials

Question 12

Pacemaker potential?

Answer 12

Slow depolarization of membrane potential to a threshold

Question 13

Pacemaker potential?

Answer 13

Slow depolarization of membrane potential to a threshold

Question 14

What does pacemaker potential look like on a diagram?

Answer 14

It is the beginning of a slight incline between two rises (-60mV->-40mV)

Question 15

What does pacemaker potential look like on a diagram?

Answer 15

It is the beginning of a slight incline between two rises (-60mV->-40mV)

Question 16

Ionic basis for a pacemaker action potential?

Answer 16

Once the threshold is reached:

• The rising phase (depolarization) of action potential is caused by activation of long-lasting L type Ca++ channels
• Resulting in Ca++ influx
• The falling phase (re-polarization) is caused by inactivation of L-type Ca++ channels and
• Activation of K+ channels resulting in K+ efflux

Question 17

What is threshold?

Answer 17

-40mV

Question 18

General description of how cardiac excitation spreads through heart?

Answer 18

starts at SA node 
Passes by cell to cell conduction to get to the AV node
Then travels to the Bundle of His 
then down the left and right branches 
and to the purkinje fibers

Question 19

Cell to cell excitation via GAP junctions?

Answer 19

From SA node through both atria
From SA node to AV node
Within ventricles

Question 20

What is the AV node?

Answer 20

Bundle of specialized cardiac cells located at the base of the right atrium just above junction of atria and ventricles

Question 21

What is special about AV node?

Answer 21

ONLY point of electrical contact between atria and ventricles

Question 22

Spread of excitation across atria?

Answer 22

Mainly cell to cell via gap junctions

Question 23

Spread of excitation from SA node to AV node?

Answer 23

Mainly cell to cell conduction via gap junctions but also some inter-nodal pathways

Question 24

What happens to conduction in AV node?

Answer 24

It is delayed - this allows atrial systole (contraction) to precede ventricular systole

Question 25

What do Bundle of His and Purkinje fibres allow?

Answer 25

Rapid spread of action potential to the ventricles

Question 26

Spread of excitation across ventricular muscle?

Answer 26

Cell to cell conduction

Question 27

Action potential in contractile cardiac cells and pacemaker cells?

Answer 27

Differ considerably

Question 28

Resting membrane potential of atrial and ventricular myocytes?

Answer 28

-90mV until excitation

Question 29

Depolarization of myocytes? and it's name

Answer 29

- Caused by fast Na+ influx - Rapidly reverses membrane potential to +20mV - Phase 0 of AP in contractile cardiac muscle cells

Question 30

Phases of ventricular muscle AP?

Answer 30

Phase 0- fast influx of Na+ Phase 1- Closure of Na+ channels and transient K+ efflux Phase 2- Mainly Ca++ influx Phase 3- Closure of Ca++ channels and K+ efflux Phase 4- Resting membrane potential

Question 31

Phase 0=

Answer 31

Fast influx of Na+

Question 32

Phase 1=

Answer 32

Closure of Na+ channels and transient K+ efflux

Question 33

Phase 2=

Answer 33

Mainly Ca++ influx

Question 34

Phase 3=

Answer 34

Closure of Ca++ channels and K+ efflux

Question 35

Phase 4=

Answer 35

Resting membrane potential

Question 36

Plateau phase of action potential?

Answer 36

- Membrane potential maintained near the peak of AP for a few 100 milliseconds - Unique characteristic of contractile cardiac muscle cells - Mainly due to influx of Ca++ through L type Ca++ channels

Question 37

Falling phase of ventricular muscle action potential?

Answer 37

Repolarization is caused by inactivation of Ca++ channels and activation of K+ channels= K+ efflux

Question 38

What is the heart rate mainly influenced by?

Answer 38

Autonomic nervous system

Question 39

What does sympathetic stimulation do to heart rate?

Answer 39

Increases HR

Question 40

What does parasympathetic stimulation do to heart rate?

Answer 40

Decreases HR

Question 41

What do changes of the heart rate usually involve?

Answer 41

Reciprocal action of sympathetics and parasympathetics

Question 42

What does the vagus nerve do?

Answer 42

Parasympathetic supply to the heart | Exerts a continuous influence on the SA node under resting conditions

Question 43

Vagal tone under normal resting conditions?

Answer 43

Dominates

Question 44

What does vagal stimulation do?

Answer 44

Slows the intrinsic heart rate from ~100bpm to produce normal resting HR and increases AV nodal delay

Question 45

Bradycardia

Answer 45

Resting heart rate less than 60bpm

Question 46

Tachycardia

Answer 46

Resting heart rate over 100bpm

Question 47

What does the vagus nerve supply?

Answer 47

AV and SA node

Question 48

Parasympathetic supply of heart

Answer 48

Vagus nerve | Neurotransmitter= Acetylcholine acting through muscarinic M2 receptors

Question 49

Atropine?

Answer 49

Competitive inhibitor of acetyl choline used in extreme bradycardia to speed up the heart

Question 50

What do negative chronotropes do?

Answer 50

Decrease HR

Question 51

What do cardiac sympathetic nerves supply?

Answer 51

AV node, SA node and myocardium

Question 52

What does sympathetic stimulation do?

Answer 52

Increases HR Decrease AV nodal delay Force of contraction

Question 53

Sympathetic Neurotransmitter?

Answer 53

Noraderinaline acting through B1 adrenoceptors

Question 54

Effect of sympathetic nervous system on HR in detail?

Answer 54

Increases= - Slope of pacemaker potential - Heart rate - Pacemaker cell Na+ & Ca++ influx Decreases= - Pacemaker cell K+ influx - AV nodal delay

Question 55

Effect of parasympathetic nervous system on HR in detail?

Answer 55

Increases= - Pacemaker cell K+ influx - AV nodal display Decreases= - Slope of pacemaker potential - Heart rate - Pacemaker cell Na+ & Ca++ influx

Question 56

What is the ECG?

Answer 56

Record of depolarization and repolarisation cycle of cardiac muscle obtained from skin surface

Question 57

How to record ECG?

Answer 57

lead 1: RA to LA Lead 2: RA to LL Lead 3: LA to RL