Muscle Cell Contraction and Calcium's Role

Overview

• Neurons stimulate muscle cells, leading to muscle contraction.
• Action potentials play a crucial role in this process.

Action Potential and Calcium Release

• An action potential sweeps over the plasma membrane of a muscle cell.
• This potential releases internal stores of calcium in the cell.

Muscle Cell Architecture

• Muscle cells have an elaborate structure to distribute calcium ions efficiently.
• T Tubules:
  • Deep tubular invaginations of the plasma membrane.
  • Crisscross the cell and facilitate the spread of action potentials.

Mechanism of Calcium Release

• Action potential spreads from the synapse over the plasma membrane and through T tubules.
• A voltage-sensitive protein in the T tubules opens a calcium release channel in the adjacent cytoplasmic reticulum (sarcoplasmic reticulum).
• The sarcoplasmic reticulum is the major calcium store in muscle cells.
• Releases a burst of calcium ions throughout the cytosol.

Interaction with Protein Filaments

• Within a contractile bundle called a myofibril, calcium triggers contraction.
• Contracting unit (Sarcomere):
  • Thin actin and thick myosin filaments are aligned but do not interact without calcium.

Role of Tropomyosin and Troponin

• Tropomyosin:
  • Rod-shaped protein covering myosin binding sites on actin filaments.
• Troponin:
  • Calcium-sensitive complex attached to tropomyosin.
  • Binds to calcium, moving tropomyosin off myosin binding sites.

Muscle Contraction Process

1. Calcium floods the cell, troponin binds to it, and tropomyosin is moved.
2. Myosin binding sites on actin filaments are exposed.
3. Myosin motors crawl along actin filaments, resulting in muscle fiber contraction.

Calcium Reuptake

• Calcium is quickly returned to the sarcoplasmic reticulum by a calcium pump.
• Without calcium, myosin releases actin, and filaments slide back to their original positions.