Electrical Properties of the Heart Flashcards
What is excitation contraction coupling?
Electrical signals causing physical contraction
What are the main differences between skeletal and cardiac muscle?
- Skeletal muscle is a syncytium (one large fused cell) - Cardiac muscle acts as a syncytium (known as a functional syncytium
What is the function of gap signals in the myocardium?
Allow a signal to be propagated from cell to cell
What is the definition of an intercalated disc?
Desmosome followed by gap junction followed by desmosome and so on
Why is the AP of cardiac muscle 10 times longer that skeletal?
- Requires calcium from outside the cell as the calcium released from the sarcoplasmic reticulum isn’t enough to saturate enough troponin
What is calcium dependent calcium release?
Calcium from outside the cell causes the sarcoplasmic reticulum to release more calcium
What is the strength of heart contraction directly proportional to?
How much calcium enters the cell
Why can cardiac muscle not display tetanus?
Has a long refractory period and has to relax before it can contract again
What is the resting potential for pacemaker cells?
- Cells sit at an unstable RP - Roughly -60mV
What is the RP of non pacemaker cells?
- About -90mV
What is the permeability of the non pacemaker cells membrane to potassium at RP?
High K moves out
What ion causes the rapid depolarisation of non pacemaker cells
- Increase in Na+ permeabilty
What ions permeability changes result in the plateau of repolarisation that allows the refractory period?
- Permeability to Ca2+ which moves in - Permeability to K reduced so more K stays in the cell
What type of calcium channels are responsible for the plateau?
L type
How much calcium do L type channels let in?
A lot
What allows the actual repolarisation of non pacemaker cells?
- Decrease in Ca2+ permeability - Increase in K+ permeability
STUDY THE DIAGRAM
Why is the depolarisation of pacemaker cells slower than non pacemaker cells?
Only affected by L type calcium channels and not sodium
What is the pacemaker potential?
Pre AP potential
What causes the pacemaker potential?
- Gradual decrease in PK+
- Early increase in PNa+ (=PF on diagram)
- Late increase in PCa2+ (t type calcium channels, small amount of Ca2+)
What are the endogenous modulators of electrical activity?
- Autonomic nervous system
- Temperature (an increase in 1 degree increases the HR by 10bpm)
What drugs modulate electrical activity?
- Ca2+ channel blockers that target L type channels
- Cardiac glycosides
How do cardiac glycosides work and what is the most infamous cardiac glycoside?
- Increase force of contraction
- Digoxin
What will be caused by hyperkalemia & hypokalemia?
- Fibrillation and heart block (kalemia refers to serum potassium)
What will hypercalcemia cause?
- Increase HR and force of contraction
What will hypocalcemia cause?
Decreased HR and force of contraction
Where are the fastest pacemakers in the heart?
Sinoatrial node
What is the annulus fibrosis?
Non conducting insulator between atrium and ventricle
What is the function of the AV node?
Delay the AP potential from the SA node to let the left atrium inject it’s blood into the left ventricle
What is the first structure that the AP potential travels through in the ventricular wall?
Bundle of HIS
What is the name of the fibres after the bundle of HIS that the AP travels through to reach bilaterally to both ventricles?
Purkinje fibres
STUDY THE DIAGRAM
What is shown by the P wave?
Atrial depolarisation
What is shown by the QRS complex?
Ventricular depolarisation
What is shown by the T wave?
Ventricular repolarisation
What are the 2 types disorders shown by an ECG?
- Disorders of rhythm
- Disorders of conduction
What are the 3 different traces shown by disorders of conduction?
- 1st degree block
- 2nd degree block
- 3rd degree block
What effect does 1st degree block have on an ECG?
Time between P wave and QRS complex much longer
What effect does 2nd degree block have on an ECG?
- Sometimes no conduction
- Some P waves followed by no QRS complex
What effect does 3rd degree block have on an ECG?
- No QRS complex generated directly by P wave
- QRS complex strange shape as it is innervated by a different pacemaker
What are the 3 disorders of rhythm?
- Atrial flutter
- Atrial fibrillation
- Ventricular fibrillation
What is shown by an atrial flutter
150 bpm HR
What is shown by atrial fibrillation?
- No coordinated P waves
- Random atrial depolarisations
What is shown by ventricular fibrillation
No coordinated QRS complex
