Electrical Properties of The Heart Flashcards
What is the calcium store in muscle?
Sarcoplasmic reticulum
What forms the electrical connection in the heart?
Gap junctions
What are the intercalating disks formed by?
Desmosome followed by gap junction.
Desmosome ensures cells contract at same time.
Why does cardia muscle have a long action potential?
Long refractory period = no tetanic contraction
Means Ca2+ entry from outside cell can regulate contraction
The reason why cardiac muscle has a longer action potential is to extend the absolute refractory period to prevent another action potential. If too many action potentials stimulate the cardiac muscle it can get into tetanus which keeps the heart continuously contracted without relaxation.
How does Ca2+ entry from outside the cell regulate contraction?
Ca2+ release does not saturate the troponin in cardiac muscle
Regulation of Ca2+ release used to vary strength of contraction
What are the cells called with an unstable resting membrane potential?
Pacemakers
What causes the wave of electrical activity in the non-pacemaker action potential?
Resting membrane potential?
Initial depolarisation?
Plateau?
Repolarisation?
Resting membrane potential - High resting permeability for potassium
Initial depolarisation - Increase in permeability for sodium
Plateau - Increase in permeability for calcium (L - type channels) and decrease in permeability for potassium.
Repolarisation - Decrease in permeability for calcium and increase in permeability for potassium
What causes the wave of electrical activity in the pacemaker action potential?
Action potential?
Pacemaker potential (or pre-potential)?
Action potential - increase in permeability for calcium (L-type receptors)
Pacemaker potential (pacemaker potential):
- Gradual decrease in permeability for potassium
- Early increase for permeability for sodium (Pf channels)
- Late increase in calcium in (T-type channels)
When do sodium pF channels open?
During repolarisation of previous action potential
How can drugs affect electrical activity of the heart?
Ca2+ channel blockers
Decrease force of contraction
Target L type calcium channels
Fewer cross bridges formed
Smaller strength of contraction
-Cardiac glycocides
How does temperature affect the electrical activity of the heart?
–increases ~10 beats/min/ºC
How does hyperkalemia affect the electrical activity of the heart?
Fibrillation and heart block
Reduces concentration gradient for potassium
Cell starts to depolarise
Spontaneous firing of action potentials
What is the affect of hypokalaemia on electrical activity of the heart?
fibrillation & heart block (anomalous)
Cells start to hyperpolarise
Loads of channels open
Cell depolarises.
What is the affect of hypercalcemia on electrical activity of the heart?
Increased HR & force of contraction
More calcium comes in when the channels open
Increase force of contraction.
What is the effect of hypocalcaemia on electrical activity of the heart?
Decreased HR
Force of contraction
What is the function of the sinoatrial node?
Pacemaker - 0.5 metres per second
Describe the annulus fibrosis
Non- conducting insulator between atria & ventricles
Action potential pass through atrioventricular node
Describe the stages shown
What is the arterioventricular node?
A delay box - 0.05 m/s
What is the function of the bundle of his?
Bundle of his ensures all the ventricle contracts at the same time
What is the name given to the fibres responsible for innervating the ventricular cardiac muscle?
•Purkinje fibres
–rapid conduction system
–~ 5 m/sec
What is the extracellular effect of an action potential in a single myocyte?
Evokes a very small extracellular electrical potential
How are large extra-cellular electrical waves created?
Lots of small extracellular electrical potentials evoked by many cells depolarising
Repolarise at same time to summate to create large extracellular electrical waves
What does each wave correspond to?
P wave corresponds to atrial depolarisation
QRS complex corresponds to ventricular depolarisation
T wave corresponds to ventricular repolarisation
What are blocks the result of?
Issues in conduction
What are flutters and fibrilations due to?
Errors in rhythm
What is characteristic of atrial fibrillation on an ECG?
No distinguishable P wave
How does a defibrillator work?
Defibrillator puts all the cells into their refractory state – they are now ready to receive the next action potential
what does ventricular fibrillation look like
