Electrical Activity of the Heart Flashcards
What is excitation-contraction coupling?
The physiological process of converting an electrical stimulus to a mechanical response
Transduction between the action potential generated in the sarcolemma and the start of a muscle contraction
What depolarises the myocyte?
Action potential
When and how do calcium ions enter the cells?
During phase 2 of the action potential through L-type calcium channels located on the sarcolemma
What does calcium entry trigger?
Subsequent release of calcium, that is stored in the sarcoplasmic reticulum and released through calcium-release channels
What does calcium released by the sarcoplasmic reticulum increase?
Intracellular calcium concentration
What does free calcium bind to?
Troponin
What does the binding of calcium and troponin cause?
Conformational change in the regulatory complex so troponin exposes the binding sites of the actin molecule
What does exposure of the binding sites of the actin molecule allow?
Myosin to bind
What does actin-myosin binding result in?
ATP-hydrolysis that supplies energy for a conformational change to occur in the actin-myosin complex
What does the conformational change occurring in the actin-myosin complex result in?
Shortening of sarcomere length
How is initial sarcomere length restored?
ATP binds to the myosin head and displaces ADP, initial sarcomere length is restored
Of myosin and actin, which is the thin filament and which is the thick filament?
Myosin is the thick filament, actin is the thin filament
How does actin-myosin binding cause changes in sarcomere length?
Binding forms cross bridges in the sarcoplasmic reticulum to shorten the muscle
Why can the regulation of calcium release be used to vary the strength of contraction?
Calcium release does not saturate the troponin
What is needed in order for the whole heart to shorten on contraction?
Physical connections and electrical connections between the two membranes
What are the electrical and physical connections?
Gap junctions are the electrical connections
Desmosomes form the physical connections
How are the physical and electrical connections arranged?
Intercalated discs;
Desmosome-gap junction-desmosome-gap junction etc. etc.
What is the action potential in skeletal muscle compared to cardiac muscle?
2ms in skeletal muscle vs 250ms in cardiac muscle
What is the reason for the difference in AP between skeletal and cardiac muscle?
Partially due to calcium entering the cell and contributing to excitation contraction coupling
How can the number of gap junctions and the strength of contraction be regulated?
The amount of calcium entering from outside can be regulated by cardiac muscle
What type of muscle can exhibit tetanus?
Skeletal muscle
Cardiac muscle cannot exhibit tetanus
Is the resting membrane potential in the non-pacemaker action potential high or low?
High RMP
What happens to PNa+ in initial depolarisation of the non-pacemaker action potential?
Increases
What happens to PCa2+ and PK+ in the plateau phase of the non-pacemaker action potential?
Increase in PCA2+ through L type channels
Decrease in PK+
What happens to PCa2+ and PK+ during depolarisation of the non-pacemaker action potential?
Decrease in PCa2+
Increase in PK+
How is the plateau phase maintained and K permeability increased while Ca permeability is decreased, in the non-pacemaker action potential?
Voltage gated sodium channels close very quickly but the membrane potential doesn’t go back to 0 as the calcium channels open slowly and for longer
How does the pacemaker action potential affect PCa2+?
Increases PCa2+ through L-type channels
What is the pacemaker action potential a result of?
Gradual decrease in PK+
Early increase in PNa+
Late increase in PCa2+
What does the pacemaker explain?
Auto-rhythmicity, also a basis for understanding modulation of the heart
Why is there a decrease of PK+ in the pacemaker action potential?
Due to leaky potassium channels
What are the modulators of electrical activity?
Sympathetic and parasympathetic systems Drugs Temperature Hyperkalaemia Hypokalaemia Hypercalcaemia Hypocalcaemia
How do calcium channel blockers affect electrical activity?
Decrease force of contraction and cardiac glycosides
How does temperature affect electrical activity?
Increase of around 10 beats per minute per 1 degree Celsius increase in temperature
How does hyperkalaemia affect electrical activity?
High plasma K+
Fibrillation
Heart block
How does hypokalaemia affect electrical activity?
Low plasma K+
Fibrillation
Heart block
How does hypercalcaemia affect electrical activity?
High plasma Ca2+
Increased heart rate and force of contraction
How does hypocalcaemia affect electrical activity?
Low plasma Ca2+
Decreased heart rate and force of contraction
What is the annulus fibrosis?
Non-conducting fibrous ring
Only place in the heart where gap junctions don’t connect cells
Only route for action potentials to travel through the atrioventricular node
What is the effect of the slow travel of APs through the atrioventricular node?
Allows atria time to contract and expel blood before the ventricles work
Where are the Bundle of His located?
One right bundle branch and two left bundle branches due to more muscle on the left side of the heart
What are the Purkinje fibres?
Rapid conduction system
How do action potentials in myocytes cause activity that can be recorded on an ECG?
AP in a single myocyte evokes a very small extracellular electrical potential
Lots of small extracellular electrical potentials evoked by multiple depolarising and repolarising cells can summate to create large extracellular waves
These waves can be recorded at periphery as the electrocardiogram
What does the P wave of the ECG correspond to?
Atrial depolarisation
What does the QRS complex of the ECG correspond to?
Ventricular depolarisation
What does the T wave of the ECG correspond to?
Ventricular repolarisation
