1394/08/13Wednesday (8-10) Action Potentials in Cardiac Muscle, Cardiac Cycleat: Anatomy Hall
Action Potentials in Cardiac Muscle, Cardiac Cycle
References: Guyton: Textbook of Medical Physiology Ganong: Review of Medical Physiology
Mechanical and ElectricalCritical to remember:There is mechanical aspects to heart function=myofibrils.There is electrical aspects to heart function=Pacemaker cells.Resulting AP from these cells to working muscle cells causes depolarization and repolarization
How does these cells automatically fire??
Important Electrolytes Sodium major extracellular ion
Potassium - major intracellular ion
Chloride - negative ion
Calcium - very important for muscle contraction
Overview of Ion Movement
Normal Resting Membrane
Sodium/Potassium PumpMaintains concentration~ 80% of metabolic energy consumed by pumpLeaky membrane Sodium leaks inPotassium leaks out
The Condensed VersionMaintenance of membranes resting potential External force (stimulus)Slow rise brings it to threshold at - 70 mVIon exchange begins Na+, then Ca+ into cellK+ out of cell
Sodium & Potassium Channels Responds to concentration gradientsdepolarization = movement away from resting potential towards or above 0 mVrepolarization = return to resting potential -90 mVNecessity of channels
RepolarizationBringing the membrane back to its resting stateActive pump moves ions against the gradientsSodium/Potassium Pump
This picture shows a neuron The principle is the same in a cardiac cell
Protein ChannelsSpecial channels allow cations to go through membraneWill cause membrane charge to rise towards 0mV
Action Potential OriginChannel proteinsVoltage gated ion channelspotassiumsodium
DepolarizationAction potential movement = ion exchangeSodium moves into cellPotassium moves out of cellMembrane potential rises-700
Depolarization
Fast and Slow ChannelsSodium and Potassium ions utilize both fast and slow channelsSodium enters cells via slow channels until threshold is reachedMass of sodium then enters cell via the fast channels during contraction
Phases of the Action PotentialSpecific sequence of events
Phases 0 through 4
Important to know what happens in each phase Comprehension of pathology and pharmacotherapy
Phases of Action PotentialPhase 0 = Na+ channels open slow then fast
Phase 1 = K+ channels open slow
Phases of Action PotentialPhase 2 = Ca+ channels open
Phase 3 = K+ channels open
Phases of Action PotentialPhase 4 - Na/K pumpreturns membrane to restNa+ leaks =Slow phase 4 rise =automaticity
Depolarization & RepolarizationDepolarizationCardiac cell resting membrane potential is -90mvexcitation spreads through gap junctionsfast Na+ channels open for rapid depolarizationPlateau phase 250 msec (only 1msec in neuron) slow Ca+2 channels open, let Ca +2 enter from outside cell and from storage in sarcoplasmic reticulum, while K+ channels closeCa +2 binds to troponin to allow for actin-myosin cross-bridge formation & tension developmentRepolarization Ca+2 channels close and K+ channels open & -90mv is restored as potassium entering the cellRefractory period very long so heart can fill with blood
Changes in cell membrane permeability.Action Potential in Cardiac Muscle
Electrocardiogram---ECG or EKGEKGAction potentials of all active cells can be detected and recorded P wave atrial depolarization P to R interval conduction time from atrial to ventricular excitation QRS complex ventricular depolarizationT wave ventricular repolarization
**Slow Na leakFrom 90 mv up until thresholdThen firing of cells
*Leaky membrane = sodium leaks into cell, potassium leaks out of cellPump maintains concentration~80% of our bodys metabolic energy goes into running the pump *Channel proteinsVoltage gated ion channels for sodium & potassium*Na+ leaks = slow phase 4 rise = automaticity*
Top Related