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Flashcards in Electrical activity of the heart Deck (20)
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What is the process of the electrical activity of the heart

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)


What is an important characteristic of pacemakers resting potentials

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


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

Through Bachman's bundle


Where is the sino-atrial node found

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


What is the speed of conduction through the SAN



How does electrical activity spread to the AV node

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


What is the speed of conduction in contractile cells



Where is the AVN found

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


What is the ostium of the coronary sinus

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


What are the three sub zones of the AVN

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


Why is the AVN delay so important

Allows atrial contraction to finish


What is the importance of decremental conduction of the AV node

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


What is spiral muscle contraction

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


What is the pacemaker potential

The prepotential - reduced potassium efflux and increased cation influx


what is If current

"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


How does the pacemaker potential evoke its own action potential

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


How does the parasympathetic nervous system control pacemaker cells

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


How does sympathetic nervous system control pacemaker cells

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


What are calcium clock oscillations

Calcium release in a regular pattern


Give the steps of a cardiac muscle action potential

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