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Content: Cardiac muscle cells are joined together by intercalated discs (desmosomes and gap junctions)
Define the mechanical and functional role of the intercalated disc
Desmosomes mechanically adhere cells together
Gap junctions electrically connect cells and allow action potentials of one cell to spread to another. This allows the heart to contract as a unit. Compare and contrast the excitation-contraction coupling in cardiac and skeletal muscle
The organization of the membrane system involved in excitation-contraction coupling is similar to that in skeletal muscle. A few diffferences include sparser sarcoplasmic reticulum, t-tubules positioned near Z lines instead of A-I band junctions, t-tubules are larger in diameter, SR makes junctions with t-tubules, dyads in cardiac muscle vs triads in skeletal muscle. Explain the mechanisms of Ca2+ entry and extrusion from cardiac muscle cells
Action potential initiated by pacemaker cells propagates to the sarcolemma and spreads into the T-tubular membrane. Voltage-dependent L-type Ca2+ channels open. This causes Ca2+ induced Ca2+ release from the SR through the ryanodine receptors. This release of Ca2+ from the SR causes cytoplasmic Ca2+ levels to rise. After contraction, Ca2+ is pumped back into the SR via Ca2+ pumps. Ca2+ is pumped back into the extracellular space by the Na+/Ca2+ exchanger
Explain why cardiac muscle cannot be tetanized
Cardiac muscle cannot produce a second action potential until its contraction is almost complete.