Cardiac Muscle Anatomy and Function

Cardiac Muscle

• Cardiac muscle shares characteristics of skeletal and smooth muscle.
• It is striated, with myosin and actin filaments forming sarcomeres.
• Contraction occurs via sliding thin filaments.
• Unlike skeletal muscle fibers, cardiac muscle fibers are short, branched, and connected by intercalated discs (gap junctions).

Cardiac Muscle Cells Contract Without Innervation

• Autorhythmic cells (pacemakers) generate signals for contraction, smaller than and with fewer contractile fibers compared to contractile cells.
• They lack organized sarcomeres.
• Contractile cells are striated and organized into sarcomeres.
• Cardiac muscle cells are smaller than skeletal muscle cells, with a single nucleus per fiber.
• They branch and connect to neighboring cells via intercalated discs.
• Gap junctions allow electrical signal transmission between cells.
• T-tubules are larger and branch more extensively than in skeletal muscle.
• The sarcoplasmic reticulum is smaller in cardiac muscle than in skeletal muscle.
• Mitochondria occupy one-third of the cell volume.

Cellular Structure of Cardiac Muscle

• Cardiac muscle displays striations under a microscope.
• Intercalated discs are visible between adjacent cardiac muscle cells.
• Mitochondria are present, contributing significantly to the cell's volume.
• Gap junctions and desmosomes are visible structural features of intercalated discs.

Myocardium

• Myocardium is a mass of cardiac muscle cells interlinked by gap junctions.
• Action potentials in one cardiac muscle cell stimulate all cells in the myocardium, creating a functional syncytium similar to single-unit smooth muscle.
• The atria and ventricles of the heart have separate myocardia.

Pacemaker Potential

• Cardiac muscle can generate action potentials automatically without nerve stimulation, originating in the pacemaker region.
• The heart rate is influenced by autonomic nervous system input and hormones.

Excitation-Contraction Coupling in Cardiac Muscle

• Action potentials from adjacent cells trigger voltage-gated Ca²⁺ channels (DHPR) opening.
• This initiates calcium release from the sarcoplasmic reticulum (SR) through ryanodine receptors (RyR).
• Local Ca²⁺ release ("Ca²⁺ spark") leads to a larger rise in intracellular Ca²⁺.
• Ca²⁺ binds to troponin, initiating contraction.
• Relaxation occurs when Ca²⁺ is pumped back into the SR.
• Calcium is exchanged with sodium using the Na⁺/Ca²⁺ exchanger (NCX).

Calcium Entry in Cardiac EC Coupling

• Action potential begins in pacemaker cells.
• Ca²⁺-induced Ca²⁺ release happens (different from skeletal muscle)
• Voltage-gated L-type Ca²⁺ channels (DHPR) open in the cell membrane.
• Ryandanine receptors open in the SR in response to Ca²⁺ inflow.
• The release of Ca²⁺ from the SR occurs as "Ca²⁺ sparks."
• Summed sparks form a Ca²⁺ signal leading to contraction via Ca²⁺ binding to troponin.
• Relaxation removes Ca²⁺ from the cytoplasm.
• Ca²⁺ is reabsorbed into the SR using Ca²⁺-ATPase pumps.
• Excess Ca²⁺ is removed through the Na⁺-Ca²⁺ exchanger (NCX).

Action Potentials: Skeletal vs Cardiac Muscle

• Cardiac muscle APs have a long absolute refractory period (~250ms) preventing tetanus, crucial for maintaining efficient heart pumping.
• Skeletal muscle APs have a much shorter refractory period, allowing for tetanus.

Duration of Muscle Contraction

• Contraction durations differ across muscle types (skeletal, cardiac, smooth). Cardiac muscle contraction duration is intermediate between skeletal and smooth.

Comparison of Muscle Types

• Skeletal Muscle, Cardiac Muscle, and Smooth Muscle differ greatly in structure, function, control and initiation.
• Cardiac Muscle is Striated, has intercalated discs that interconnect cells, uses Ca²⁺ and calmodulin, has intrinsic pacemaker activity, and is influenced by hormones.

Explore the unique features of cardiac muscle, which shares characteristics with both skeletal and smooth muscle. This quiz covers aspects such as the structure of cardiac muscle cells, contraction mechanisms, and the role of autorhythmic cells in heart function.