Cardiovascular System: Cardiac Muscle

Comparison: Skeletal Muscle vs Cardiac Muscle

• Skeletal Muscle:

  • Large fibers, often the entire length of the muscle
  • Multinucleated
  • Organized in sarcomeres (striated muscle)

• Cardiac Muscle:

  • Smaller fibers compared to skeletal muscle
  • Branched fibers
  • Single nucleus per fiber

Cardiac Muscle Fiber Connections

• Intercalated Discs:

  • Structures connecting cardiac muscle fibers
  • Composed of two types of junctions:
    • Desmosomes (Anchoring Junctions):
      • Mechanically hold neighboring cells together
    • Gap Junctions:
      • Allow rapid transmission of electrical signals between cells

• Functional Syncytium:

  • The heart functions as a syncytium, a group of tissues working synchronously
  • Enables efficient functioning of the heart as a pump

Cardiac Muscle Contraction

• Calcium-Mediated Calcium Release:
  • Different mechanism from skeletal muscle
  • Involves extracellular and intracellular calcium

Excitation-Contraction Coupling in Skeletal Muscle

• Graded potential at the motor end plate
• Release of acetylcholine binding to nicotinic receptors
• Action potential travels down T-tubule system
• Interaction with DHP receptor and ryanodine receptor
• Release of intracellular calcium
• Calcium binds to troponin
• Crossbridge cycle occurs

Excitation-Contraction Coupling in Cardiac Muscle

• Action potential travels along the cell surface and T-tubules

  • T-tubules less developed than in skeletal muscle

• Voltage-sensitive Calcium Channels (DHP receptors):

  • Open to allow calcium entry into the cell

• Calcium Interaction:

  • Enters cell and interacts with smooth endoplasmic reticulum
  • Opens ryanodine receptor
  • Causes additional calcium release from smooth ER into cytosol (calcium-mediated calcium release)

• Calcium binds to troponin, initiating the crossbridge cycle

• Sarcomeres shorten, creating tension in cardiac muscle cells

This setup allows cardiac muscle cells to contract effectively and contribute to the heart's role as a powerful and efficient pump, maintaining blood circulation throughout the body.