CR1.5 Electrical control of the cardiac pump

Question 1

Briefly describe the features of a normal ECG.

Answer 1

The ECG is a measure of the sume of all electrical activity in the heart. Characteristics of a normal ECG include:

• P wave represents atrial depolarisation (i.e. atrial systole)
• PR interval is the time between atrial depolarisation and the beginning of ventricular depolarisation
• QRS complex represents the depolarisation of ventricles (i.e. ventricular ejection)
• ST segment is the time between ventricular depolarisation and repolarisation (i.e. ventricles are still contracted)
• T wave represents the repolarisation of the ventricles (i.e. ventricular diastole)

Question 2

Explain the function of interclated discs.

Answer 2

Interclated disks connect adjacent cardiac muscle cells and provide a electrochemical and mechanical link to form a functional syncytia. Interclated disks contain:

• gap junctions for ion movement
• adherens for mechanical coupling
• desmosomes for mechanical coupling and cell signalling

Question 3

Pacemaker cells have ______________ membrane potential and generate ______ action potentials.

Cardiomyocytes have ___________ membrane potential and generate ______ action potentials.

Answer 3

Pacemaker cells have no resting membrane potential and generate slow action potentials.

Cardiomyocytes have a resting membrane potential and generate fast action potentials.

Question 4

Describe in detail the generation of slow cardiac pacemaker potentials.

Answer 4

Cardiac pacemaker cells in the sinoatrial (SA) and atrioventricular (AV) node have no resting membrane potential and generate slow action potentials. These cells have unique channels that generate a ‘funny’ current. These ‘funny’ channels open when membrane polarity is ~-60mV and stimulate spontaneous depolarisation. The ‘funny’ channels are mixed Na+ and K+; simultaneous reduction in permeability of K+ (i.e. prevent efflux of K+ and depolarise cell) and increase in permeability of Na+ (i.e. increase influx of Na+ and depolarise cell). When membrane potential ~50mV open transient Ca2+ channels and close ‘funny’ channels. Further depolarisation (~40mV) closes the transient channels and opens L-type Ca2+ channels causing rapid action potential. Termination of the action potential is achieved with closure of L-type channels and opening of voltage gated K+ channels.

Question 5

Explain the difference between action potential in cardiomyocytes to pacemaker cells.

Answer 5

Cardiomyocytes have a resting membrane potential of ~ -90mV maintained by K+ channels causing efflux of K+. Depolarisation to threshold (~-70mV) opens Na+ channels and causes rapid depolarisation. This Na+ current is not present in cardiac pacemaker cells. This current is short lived due to opening of transient outward K+ channel. This also inactivates Na+ channels resulting in an effective refractory period (i.e. cannot be opened again until fully repolarised). Opening of L-type Ca2+ channels results in a plateau of the membrane potential and prolongation of the action potential. Opening of K+ channels and inactivation of L-type channels returns the cell to resting potential.

Question 6

Define excitation-contration coupling.

Answer 6

Excitation contraction coupling is the process by which an electrical stimulus triggers the release of calcium by the sarcoplasmic reticulum (SR) initating the mechanism of muscle contraction by sarcomere shortening.

Question 7

Describe the role of troponin in excitation-contraction coupling.

Answer 7

Ca2+ binds to troponin C. Troponin C is attached to tropomyosin. Tropomyosin normally prevents binding of myosin heads to actin. When Ca2+ binds troponin C the tropomyosin moves to expose the actin binding sites.

Question 8

List the five important cardiomyocyte channels involved in excitation-contraction coupling.

Answer 8

• Voltage gated Ca2+ channels on T-tubules allow influx of Ca2+
• Ryanodine receptor-channel on sarcoplasmic reticulum bind Ca2+ and release more intracellular Ca2+ (i.e. Ca2+ dependent Ca2+ release)
• Sodium / Calcium exchanger on the cell membrane pumps out one Ca2+ and pumps in three Na+
• ATPase pump exchanges three Na+ for every 2 K+ and maintains Na+ gradient
• SERCA pump restores Ca2+ back into sarcoplasmic reticulum and is ATP driven

Question 9

List three ways that PNS stimulation can decrease heart rate at the SA node.

Answer 9

Acetylcholine release from the vagus nerve to the SA node results in:

1. Slower depolarisation (i.e. reduce steepness by altering current through ‘funny’ channels
2. Reduce maximum diastolic potential (i.e. reduce starting point for membrane potential by increasing K+ conductance)
3. Increase threshold for action potential (i.e. reduce Ca2+ influx through transient channels)

Question 10

Describe the two methods that SNS stimulation increases HR in the SA node.

Answer 10

SNS activation stimulates ß1 and ß2 adrenoreceptors in the SA node. This increases cAMP production and causes:

1. Increased Na+ and K+ influx through funny channels
2. Increased Ca2+ influx through T-type Ca2+ channels