Electrical activity of the heart Flashcards
What is the process of the electrical activity of the heart
- Initiated by SAN
- Conduction to atria via AVN
- Atrioventricular ring prevents direct spread of impulse to the ventricle
- Passage therefore is through the Bundle of His
- This is redistributed through the Purkinje fibres
- 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
0.05m/s
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
1.00m/s
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
- A-N transitional zone - cells smaller than that of normal atrial cells
- N region - Consists of round/P cells - similar to SAN cells
- 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
- Sodium influx depolarises the membrane causing more to open - positive feedback loop causes rapid depolarisation
- Sodium channels close when the cell depolarises and the voltage peaks at around +30 mV
- 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
- Calcium channels close and intracellular calcium is transported out of the cell. Potassium channels open and repolarize the cell with their outflow
