Electrical Conduction in the Heart
|previous topic | next topic | syllabus | home page | BME Home Page | search the web | e-mail Doug|
As usual, all of this is in outline format with hypertext, so if you want to review the specifics, or if you have any questions on a specific topic, click the hypertext for that topic.
Many of these topics are incomplete.
They should be completed when the extra credit projects are turned in.
These are Dr. Schechtman's very own notes for this lecture.
I hope you find them helpful.
4 Electrical Conduction in the Heart Differences between myocardial action potentials and pacemaker potentials: Resting Potential of myocardial cells is more negative Very rapid rise in myocardial cell caused by fast Na+ channels Plateau (particularly in ventricular cell) Resting Potential of Pacemaker cell is unstable Resting Potential approx -90 mV Concentration Gradients Sodium/Potassium Pump Calcium Currents Resting cell membrane relatively permeable to K+, but not to Na+ and Ca++, so K+ tends to leave the cells following its concentration gradient and meanwhile electrostatic gradient brings K+ in but electrostatic force is slightly weaker than the diffusional force, so K+ tends to leave cells Na+ tends to enter cells down its concentration gradient, so both chemical and electrostatic forces pull Na+ into cells; Na+ leakage is slow because of low permeability, but would gradually depolarize cell, if not for Sodium-Potassium Pump Uses enzyme Na+ K+-ATPase, which is located in the cell membrane Increased [Na+]i or [K+]o accelerate pump activity Na+ extrusion exceeds K+ introduction by 3 to 2 ratio, so it creates a potential difference Digitalis partially inhibits this pump, making the resting membrane potential less negative than normal Action Potential Rapid Upstroke of Action Potential--caused by influx of Na+ Peak amplitude of action potential is correlated with [Na+]o Partial Repolarization Plateau Repolarization Peak muscle force occurs with repolarization Rapid Upstroke: When threshold voltage (approx -65 mV) is reached, Na+ channels in cell membrane open, allowing influx of Na+ Any process that reduces the membrane potential activates the Na+ channels The influx of Na+ further depolarizes the membrane opening more Na+ channels The influx of Na also initiates closing of channel, however inactivation takes some time When membrane voltage reaches zero, the electrostatic pull on Na+ is neutralized, but Na+ keeps leaking in down its concentration gradient (the quantity of Na+ entering cell is not enough to cause a measurable change in [Na+]i, so chemical force remains virtually constant at +20 mV Na+ keeps leaking in but slowly, as Na+ channels are beginning to close When membrane potential reaches +30 mV, the gates are all closed and Na+ influx ceases The channels remain closed during first half of depolarization, therefore this is an absolute refractory period (protects the heart from tetanus) Early limited repolarization between upstroke and plateau phases: Represents an efflux of K+ from the cell because of concentration gradient and positive charge of cell Plateau The influx of positively charged Ca++ is balanced by the efflux of an equal amount of K+ The Ca++ channels open as a result of the positive charge of the cell interior These Ca++ channels are blocked by calcium channel blockers, such as Verapamil Final Repolarization The efflux of K+ from the cell begins to exceed the influx of Ca++. Restoration of Ionic Concentrations Excess Na+ in the cell is removed by Na+/K+ pump Excess Ca++ that entered the cell is eliminated by an Na+/Ca++ exchanger, which exchanges 3 Na+ for 1 Ca++. Peak muscle contraction coincides with repolarization Refractoriness Effect of change in HR on action potential duration Effect of calcium channel blockers
|previous topic | next topic | syllabus | home | BME | search the web | e-mail Doug|
|1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | Exam #1|
|17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26 | Exam #2|
|28 | 29 | 30 | 31 | 32 | 33 | 34 | 35 | 36 | 37 | 38 | 39 | 40 | 41 | 42 | Exam #3|