Generating The Cardiac Rhythm

Question 1

Heart muscles depolarise and contract rhythmically without nerves how is this described?

Answer 1

Myogenic rhythmicity or autorhythmicty. It requires no stimulus to contract

Question 2

Cardiac action potentials have three broad types what are they and how do they differ?

Answer 2

1. SA and AV nodes
2. Atrial muscle
3. Purkinje fibre and ventricular muscle
   Differ to each other by duration, shape and ionic basis

Question 3

Why are SA nodes good pacemakers?

Answer 3

They don’t have a resting membrane potential, it sits at -60mV which is closer to the threshold than neurones at -70mV. Therefore, it doesn’t take a lot to produce an action potential. It generates signals faster than any of the others so it sets the pace for the heart beat.

Question 4

What is an SA node?

Answer 4

A group of cardiac muscles that don’t contract. They generate impulses and signals can spread throughout the heart causing it to beat. It acts as a pacemakers because it generates signals faster than any of the others so it sets the pace for the heart beat.

Question 5

How does the SA node cells produce and action potential?

Answer 5

The cells are permeable to Na+ so there is a Na+ influx this causes depolarisation which then reaches a threshold. When this happens the Na+ is rushing in and there is decreased conductance of K+, L-type voltage gated Ca+ open and Ca+ rushes into the cell causing rapid depolarisation and an action potential. The K+ channels open and K+ rushes out of the cell and repolarises the membrane.

Question 6

SA nodes are under what control?

Answer 6

The autonomic control (antagonisitic)

Question 7

Neurones making up the sympathetic innervation occurs at?

Answer 7

T1-L2

Question 8

If the intrinsic rate is 100 action potentials/min what is the resting HR?

Answer 8

60-70 beats/min

Question 9

Increase in sympathetic activity causes?

Answer 9

Increase in HR. Sympathetic stimulation stimulates NorAd acts on B1 receptors. Increases the slope of the pacemaker potential (increases Na+ and Ca+ conductance).

Question 10

Increase in parasympathetic activity causes?

Answer 10

Decrease in HR. ACh acts on M2 receptors. It hyperpolarises the cell (opens K+ channels) and reduces the slope of the pacemaker potentials. Negative chronotropic effect.

Question 11

Gap junction purpose

Answer 11

Current flows to adjoining cells though jap junctions to depolarise adjacent cells.

Question 12

Where does the AP from the SA node spread to?

Answer 12

From the SA node through the atrial muscle.

Question 13

Order of depolarisation through the heart

Answer 13

SA node, atrial muscle, AV node, Bundle of His, Bundle of branches, Purkinje fibres and ventricular muscle then ventricles from endocardium to epicardium.

Question 14

Conduction through atrial node

Answer 14

0.5m/s. Rapid depolarisation

Question 15

What is the AV delay?

Answer 15

Ensures atrial depolarisation, contraction and ejection before ventricles depolarise.

Question 16

Why isn’t the AV node the pacemaker?

Answer 16

Because it produces action potentials much slower than SA nodes. Conductance rate falls around 0.05m/s

Question 17

Time taken for depolarisation of bundle of His, bundle of branches, Purkinje fibres and ventricular muscle and ventricles from endocardium to epicardium?

Answer 17

Bundle of His = 2m/s
Bundle of branches = 2m/s
Purkinje fibres and ventricular muscle = 4m/s
Ventricles from endocardium to epicardium = 0.5m/s

Question 18

Eptopic beats are?

Answer 18

Where parts of the ventricle contract out of rhythm. This can be very dangerous in an empty ventricle. These faulty spots can be burnt off.

Question 19

Phases of ventricular myocyte action potential

Answer 19

Rapid depolarisation, partial repolarisation, plateau, repolarisation.

Question 20

What ions are responsible for depolarisation of ventricular myocytes?

Answer 20

Inward movement of Na+ (fast voltage gated channels)

Question 21

What ions are responsible for the partial repolarisation of ventricular myocytes?

Answer 21

Inactivation of Na+ channels

Question 22

What ions are responsible for the plateau of ventricular myocytes?

Answer 22

Inward Ca2+ movement and outward K+ movement (the number of ions coming in is equal to the number of ions going out, therefore it plateaus.)

Question 23

What ions are responsible for repolarisation of ventricular myocytes?

Answer 23

Outward movement of K+.

Question 24

Ventricular absolute refectory period

Answer 24

Absolute refractory periods lasts around 200ms, here you cannot generate another action potential because Na_ channels are inactivated and unresponsive.

Question 25

Ventricular relative refractory period

Answer 25

Relative refractory period lasts around 50ms. You may get an action potential but requires a stronger stimulus.

Question 26

What is the temporal summation process?

Answer 26

Determines whether or not and action potential will be triggered.

Question 27

What is incomplete tetanus?

Answer 27

Each stimulus causes a contraction to be initiated when the muscle has only partly relaxed from the previous contraction.

Question 28

What is complete tetanus

Answer 28

Maintained contraction of the muscle

Question 29

What is a twitch?

Answer 29

A single stimulus causing a muscle to contract and relax

Question 30

What is the difference between complete and incomplete tetanus?

Answer 30

Incomplete tetanus has a relaxation period during contraction and complete tetanus has no relaxation period during contractions