Autorhythmicity- heart’s ability to trigger its own contractions
Phase O- a lot like depolarization in neuronal action potentials. Sodium channels open, increase of sodium INTO cell.
Phase 1- sodium channels close, potassium channels close, decrease in potassium and sodium. Calcium channels open, increase of calcium into cell.
Phase 2- Plateau phase, membrane still depolarized (contract). Potassium channels closed, L-type calcium channels stay open. Lasts 0.2 seconds/200 milliseconds.
Phase 3- second set of potassium channels open, potassium decrease. Failing membrane potential cause calcium channels to close, calcium decrease to cell. Membrane repolarizes to resting potential.
Phase 4- resting membrane potential is reached until next depolarization from neighboring cardiac pacemaker cells. Total cardiac AP last 0.25-0.3 seconds or 250-300 milliseconds
Wave Summation- occurs when a skeletal muscle is stimulated with such frequency that muscle twitches overlap and result in a stronger contraction than a single muscle twitch.
When enough of these twitches occur at a frequent rate, muscle reaches fused tetanus, or smooth movement. Individual twitches cannot be distinguished. Tetanus occurs in skeletal muscle because skeletal muscle has a relatively short absolute refractory period(a period during which APs cannot be generated no matter how strong the stimulus).
Cardiac muscles has a relatively long refractory perios and is thus incapable of wave summation. Cardiac muscle is incapable of reacting to any stimulus before middle of phase 3 and will not respond to a normal cardiac stimulus before phase 4.
Absolute refractory period- time between the beginning of the cardiac AP and middle of phase 3.
Relative refractory period- time between absolute refractory period and phase 4.
Total refractory period = 200-250 milliseconds. Almost as long as the contraction of the cardiac muscle.
Vagus Nerve Stimulation
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