Electrical activity of the heart

Question 1

What is the process of the electrical activity of the heart

Answer 1

1. Initiated by SAN
2. Conduction to atria via AVN
3. Atrioventricular ring prevents direct spread of impulse to the ventricle
4. Passage therefore is through the Bundle of His
5. This is redistributed through the Purkinje fibres
6. This is achieved through electrical conduction rather than chemical (use of neurotransmitters)

Question 2

What is an important characteristic of pacemakers resting potentials

Answer 2

They are never fixed - They have an internal trigger in order to produce an action potential

Question 3

How does activity from the right atria spread to the left?

Answer 3

Through Bachman’s bundle

Question 4

Where is the sino-atrial node found

Answer 4

Posterior aspect at the junction of the superior vena cava and right atrium

Question 5

What is the speed of conduction through the SAN

Answer 5

0.05m/s

Question 6

How does electrical activity spread to the AV node

Answer 6

Through atrial contractile cells - these have intercalated disks with gap junctions

Question 7

What is the speed of conduction in contractile cells

Answer 7

1.00m/s

Question 8

Where is the AVN found

Answer 8

Posterior aspect on the right side of the interatrial septum near the ostium of the coronary sinus

Question 9

What is the ostium of the coronary sinus

Answer 9

Opening of the coronary sinus where a collection of small veins drains into one area

Question 10

What are the three sub zones of the AVN

Answer 10

1. A-N transitional zone - cells smaller than that of normal atrial cells
2. N region - Consists of round/P cells - similar to SAN cells
3. N-H region - Transitional area near the bundle of His

Question 11

Why is the AVN delay so important

Answer 11

Allows atrial contraction to finish

Question 12

What is the importance of decremental conduction of the AV node

Answer 12

The more the AV node is stimulated the slower it conducts - important in preventing excess ventricular contraction - If the SAN stops working the AVN takes over the pacemaker role but at a slower rate - problem

Question 13

What is spiral muscle contraction

Answer 13

Structure of the muscles contracting evokes torsion - more effective at expelling blood

Question 14

What is the pacemaker potential

Answer 14

The prepotential - reduced potassium efflux and increased cation influx

Question 15

what is If current

Answer 15

“funny” current - induced by hyperpolarization of the cell - as the membrane potential becomes more negative ion channels begin to open - causing a slow sodium influx

Question 16

How does the pacemaker potential evoke its own action potential

Answer 16

If current causes Na influx slowly causes the cell to reach its threshold (-40,-50mV) At this point VGcalcium channels are opened depolarizing the membrane - this depolarization also stops If
Repolarization caused by potassium efflux

Question 17

How does the parasympathetic nervous system control pacemaker cells

Answer 17

Vagal fibres release Ach onto muscarinic receptors - these decrease cAMP in the pacemaker cells - Specific sodium channels are stimulated by cAMP - dependent protein kinases, without these there is less phosphorylation of the sodium channels - By decreasing the action of this channel Na influx is reduced so heart rate is also reduced

Question 18

How does sympathetic nervous system control pacemaker cells

Answer 18

Noradrenaline acts on adrenergic receptors that increase prepotential slope - as there is an increased sodium influx so an increase in firing rate - increased heart rate

Question 19

What are calcium clock oscillations

Answer 19

Calcium release in a regular pattern

Question 20

Give the steps of a cardiac muscle action potential

Answer 20

Voltage gated sodium channels open

2. Sodium influx depolarises the membrane causing more to open - positive feedback loop causes rapid depolarisation
3. Sodium channels close when the cell depolarises and the voltage peaks at around +30 mV
4. Calcium entry through slow calcium channels prolongs depolarization of the membrane - creates a plateau phase - this falls slightly due to some potassium leakage - But most potassium channels remain closed until the end of the plateau
5. Calcium channels close and intracellular calcium is transported out of the cell. Potassium channels open and repolarize the cell with their outflow