Electricity in the Body
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What is an action potential?
A brief electrical charge that travels down the axon of a neuron
What is the resting membrane potential?
The difference in charge between the relatively negative intra-cellular environment and the extra cellular environment (the cell is polarised)
Usually, what are the relative concentrations of sodium and potassium inside and outside cells?
High sodium concentration outside cells and low inside cells Low potassium concentration outside cells and high inside cells
What are the stages of a non-cardiac action potential?
Resting state (normal, polarised state), depolarisation and repolarisation
What happens during depolarisation (non-cardiac)?
Voltage gated sodium channels allow sodium to enter the cell, equalising the charge on either side of the cell membrane and depolarising it
What happens during repolarisation (non-cardiac)?
Potassium moves out of the cell via voltage gated potassium channels, restoring the resting membrane potential and repolarising the cell membrane
After repolarisation, how are the correct intracellular and extracellular concentrations of sodium and potassium restored?
The sodium potassium antiporter transfers 3 sodium ions out of the cell and 2 potassium ions into the cell with every phosphorylation by ATP
What are the components (in order) of the conduction system of the heart?
Sinoatrial node, Atrioventricular node, Bundle of HIS, Left and Right Bundle Branches, Purkinje Fibres
Which cells in the heart generate action potentials?
Pacemaker cells
How is synchrony maintained between cardiac myocytes?
Adjacent cardiac myocytes are connected by gap junctions
What are the stages of cardiac action potential?
Depolarisation (Phase 0), Slow Repolarisation (Phase 1), Plateau Phase (Phase 2), Repolarisation (Phase 3)
What causes depolarisation phase (cardiac)?
The rapid entry of sodium into the cardiac myocyte
What causes the slow repolarisation phase (cardiac)?
The slow exit of potassium ions from the myocyte
What causes the plateau phase (cardiac)?
The entry of calcium into the myocyte
What causes the repolarisation phase (cardiac)?
The rapid exit of potassium from the myocyte
What are the waves in an ECG?
P, Q, R, S and T waves
What does the P wave represent in an ECG?
Atrial depolarisation
What does the Q wave represent in an ECG?
Excitation of the interventricular septum (The beginning of ventricular depolarisation)
What does the R wave represent in an ECG?
The spreading of excitation through right and left ventricular myocytes
What does the S wave represent in an ECG?
Completion of ventricular depolarisation (Basal areas of interventricular septum)
What does the T wave represent in an ECG?
Rapid late repolarisation of ventricular myocardium
What can electric shocks cause?
Muscle contractions and pain (may result in patient being unable to let go of the source) - Inability to breathe if respiratory muscles are force to continuously contract Cardiac arrest - Patient goes from normal sinus rhythm into arrhythmia (most commonly ventricular fibrillation) Burns (at entry and exit site of current)
What is the course of action for electrocution first aid?
• Check for safety - You cannot assist if you are also getting electrocuted • Shout for help • Turn of electricity at the mains • Commence CPR if required - Use defibrillator if required to reset heart rhythm • Attend to burns once patient is able to breathe on their own and heartbeat is restored
How does a defibrillator work?
Arrests fibrillation of heart muscle by applying electric shock across the chest, depolarising cardiac myocytes and allowing normal rhythm to return