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*