Cardiac Muscle Cell

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How do cardiac AP differ from skeletal muscle AP

Cardiac AP: Automaticity, conductivity (from cell to cell), excitability (long refractory duration); NO TETANY

What are HCN/funny channels

Pacemaker current; activated by HYPERpolarization enhanced by sympathetic and adrenaline suppressed by vagus

What are the phases of cardiac AP generation

Phase 0: Depolarization → Phase 1: Initial repolarization → Phase 2: Plateau → Phase 3: Rapid repolarization → Phase 4: Resting potential 

What occurs during each phase of cardiac AP generation

Phase 0: Depolarization → Na enter cell; Phase 1: Na channel close, K channel open and K leaves; Phase 2 → L type Ca channel allow Ca in, K goes out (both are balance; Phase 3 → Ca channel close, another K channel open; Phase 4 → Na/K pump and Na/Ca exchange

What causes tetany

Hypocalcemia → More Na enters cell than normal → More depolarization → Tetany

What happens in myocardial CONTRACTIOn

Phase 2 (plateau) → Ca enters → Activates RyR2 in SR → Ca release → Cross bridge

What is the RyR2

Ca gated Ca channel found in SR → Supplies Ca to cardiomyocytes → Rhythmic Depolarization

What happens in myocardial RELAXATION

Ca reabsorb into SR and Ca channel close → Thin filament deactivate → No more cross bridge → SERCA transports Ca back into SR

Why is a long refractory period important for heart cells

PREVENTS tetany from happening

What is the supranormal period

Period where the membrane is very sensitive to AP potential activation → Occurs at end phase of refractory period

What is Frank Starling curve

Force/tension in a muscle fiber depends on the extent the fiber is stretched (Tension = F/unit length)

What is the consequence of overactive or underactive muscle

Overactive → Actin myosin overlap and shorten muscle → Tension and force decrease; underactive → muscle is stretched too much and tension decrease

What is inotropic agent

An agent that can increase (positive) or decrease (negative) contractability of the heart

What substances are positive and negative inotropic

Positive: NorE; negative: Beta blockers

How can inotropic agent be used for heart failure

Heart failure from poor contractability → Positive inotropic increase contractability of heart 

How can inotropic agent be used for ischemia

Ischemia = O2 demand > supply of O2 → Treatment is to DECREASE O2 demand → NEGATIVE inotropic agent decrease heart workload and energy

How can inotropic agent by used for low BP

Positive inotropic increase contractability of heart = increases heart rate

How can inotropic agent be used for tachycardia

Negative inotropic reduce HR