PHYL3002 Lecture Notes - Lecture 6: Resting Potential, Sarcomere, Rectifier
LECTURE SIX: Cardiac Ion Channels and the Heart Beat
Normal Electrical Conduction in the Heart:
• Impulse occurs in SA node
• Where slow response action potentials occur
• Keeps heart beating, even when removed from body
• No hormonal or neuronal influence → maintains its own rhythm
• Propagates across right atrium to AV node
• AV node propagates down each side of heart via left and right bundle
branches to Purkinje fibers
Morphological and Functional Classification of Cardiac Cells:
• Pacemaker cells
o Small, round or oval cells
o 3-9um in diameter
o Reduced number of myofibrils → not contractile
o Found in SA and AV node
• Conducting cells
o Large cylindrical cells
o 50um in diameter
o Reduced number of myofibrils → not contractile
o Many intercellular connections
o Include Bundle of His, bundle branches and Purkinje fibers
• Contractile cells
o Medium sized cylindrical cells
o 10-15um in diameter
o Abundant myofibrils
o Extensive T-tubule system
o Many intercellular connections
o Predominant cell type in atria and ventricles
Ionic Basis of Electrical Properties:
• Nernst equation → predicts membrane potential assuming permeability
to a single ion
• Goldman equation → predicts membrane potential taking into account
membrane permeability to more than one ion
• Ohm’s law → conductance (G) is the inverse of resistance and therefore a
measure of the membrane’s permeability to one or more ions
Equilibrium Potentials for Ions in Cardiac Cell Membranes:
• ENa → +60mV
• ECa → +130mV
• EK → -93mV
• ECl → -80mV
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