Learning Objectives

• Differentiate between contractile and conductive myocytes
• Delineate the anatomy of the conduction system
• Discuss the events associated with a wave of excitation spreading through the conduction system, explaining why there is a delay the AV node
• Describe ‘automaticity’ and evaluate the underlying ionic currents
• Compare and contrast action potentials in myocytes from different parts of the heart
• Describe the 4 phases of the action potential
• Compare and contrast activation, inactivation, recovery from inactivation and closing of ion channels (deactivation)
• Explain the absolute and relative refractory periods
• Describe the role of intercalated discs and gap junctions in cardiac excitation
• Explain how the parasympathetic and sympathetic nervous systems modulate heart rate, conduction velocity through the AV node, and contractility

Cardiac Action Potential & Pacemaker Cells

• AP phases in cardiomyocytes/ contractile myocytes?
• AP phases in pacemaker cells/ conductive myocytes?
• what is meant by VG $\small\ce{Na+}$ channels being self-limiting? why is this?
• what are the key channels of each AP phase in contractile vs conductive myocytes?
• what is meant by the term ‘automaticity?’
• what are the inherent rates of automaticity, under resting PNS input?

Refractory Periods