Introduction to Actin and Myosin:

Actin and myosin are two essential proteins found in muscle cells and are integral to the process of muscle contraction. They play a fundamental role in enabling various forms of movement, including voluntary actions like walking and running, as well as involuntary processes such as the beating of the heart.

Actin:

• Actin is a thin filamentous protein that is a key component of the cytoskeleton in eukaryotic cells.
• In muscle cells, actin forms long, helical filaments by polymerizing globular actin monomers (G-actin).
• These actin filaments are arranged in a repeating pattern within muscle fibers, called sarcomeres, which are the basic contractile units of muscles.
• Actin filaments have binding sites where myosin can attach during muscle contraction.

Myosin:

• Myosin is a thick filamentous protein that interacts with actin during muscle contraction.
• Myosin molecules consist of elongated tail regions and globular head regions.
• The myosin heads are responsible for binding to actin and generating force during muscle contraction.
• Myosin molecules are arranged within the sarcomere in a way that allows their heads to interact with actin filaments.

Function:

• The primary function of actin and myosin is to enable muscle contraction, which is the process of generating force and movement.
• Muscle contraction occurs when myosin heads bind to actin filaments and undergo a series of conformational changes, known as the sliding filament theory.
• This interaction causes the actin filaments to slide past the myosin filaments, resulting in the shortening of sarcomeres and muscle contraction.
• Actin and myosin also play roles in non-muscle cells, such as in cell motility, cytokinesis during cell division, and intracellular transport.

Structure of Actin and Myosin:

• Actin and myosin are protein filaments found in muscle cells.
• Actin is a thin filament composed of globular actin (G-actin) subunits, which polymerize to form long, helical actin filaments.
• Myosin is a thick filament consisting of myosin molecules with elongated tail and globular head regions.

Proteins Associated with Actin and Myosin:

• Troponin and tropomyosin are regulatory proteins associated with actin filaments in muscle cells.
• Troponin and tropomyosin play a role in regulating the interaction between actin and myosin during muscle contraction.

Sliding Theory of Muscle Contraction:

• The sliding filament theory is a widely accepted model that explains how muscle contraction occurs.
• According to this theory, during muscle contraction, thin actin filaments slide past thick myosin filaments, causing the sarcomeres (the contractile units of muscle fibers) to shorten.
• The sliding is facilitated by the cyclic interaction between myosin heads and actin filaments.
• The steps involved in muscle contraction include:
  1. Calcium ions (Ca²⁺) are released from the sarcoplasmic reticulum in response to a nerve impulse.
  2. Ca²⁺ ions bind to troponin, causing a conformational change in tropomyosin, exposing the binding sites on actin.
  3. Myosin heads (cross-bridges) bind to actin at these exposed sites.
  4. ATP is hydrolyzed to ADP and inorganic phosphate (Pi), providing energy for myosin to undergo a conformational change (the power stroke).
  5. This change in myosin’s shape causes the thin actin filaments to slide along the myosin filaments, shortening the sarcomeres and leading to muscle contraction.
  6. The cycle repeats as long as Ca²⁺ ions are present and ATP is available.

This process of actin and myosin interaction, powered by ATP, allows for muscle contraction and movement. The sliding filament theory provides a detailed explanation of the molecular events that occur during muscle contraction.

Actin:

• Actin is a thin filamentous protein that is a key component of the cytoskeleton in eukaryotic cells.

• In muscle cells, actin forms long, helical filaments by polymerizing globular actin monomers (G-actin).

• These actin filaments are arranged in a repeating pattern within muscle fibers, called sarcomeres, which are the basic contractile units of muscles .

• Actin filaments have binding sites where myosin can attach during muscle contraction.

Myosin:

• Myosin is a thick filamentous protein that interacts with actin during muscle contraction.

• Myosin molecules consist of elongated tail regions and globular head regions.

• The myosin heads are responsible for binding to actin and generating force during muscle contraction.

• Myosin molecules are arranged within the sarcomere in a way that allows their heads to interact with actin filaments.

Function:

• The primary function of actin and myosin is to enable muscle contraction, which is the process of generating force and movement.

• Muscle contraction occurs when myosin heads bind to actin filaments and undergo a series of conformational changes, known as the sliding filament theory.

• This interaction causes the actin filaments to slide past the myosin filaments, resulting in the shortening of sarcomeres and muscle contraction.

• Actin and myosin also play roles in non-muscle cells, such as in cell motility, cytokinesis during cell division, and intracellular transport.

Structure of Actin and Myosin:

• Actin and myosin are protein filaments found in muscle cells.

• Actin is a thin filament composed of globular actin (G-actin) subunits, which polymerize to form long, helical actin filaments.

• Myosin is a thick filament consisting of myosin molecules with elongated tail and globular head regions.

Proteins Associated with Actin and Myosin:

• Troponin and tropomyosin are regulatory proteins associated with actin filaments in muscle cells.

• Troponin and tropomyosin play a role in regulating the interaction between actin and myosin during muscle contraction.

Sliding Theory of Muscle Contraction:

• The sliding filament theory is a widely accepted model that explains how muscle contraction occurs.

• According to this theory, during muscle contraction, thin actin filaments slide past thick myosin filaments, causing the sarcomeres (the contractile units of muscle fibers) to shorten.

• The sliding is facilitated by the cyclic interaction between myosin heads and actin filaments.

• The steps involved in muscle contraction include:

  1. Calcium ions (Ca²⁺) are released from the sarcoplasmic reticulum in response to a nerve impulse.

  2. Ca²⁺ ions bind to troponin, causing a conformational change in tropomyosin, exposing the binding sites on actin.

  3. Myosin heads (cross-bridges) bind to actin at these exposed sites.

  4. ATP is hydrolyzed to ADP and inorganic phosphate (Pi), providing energy for myosin to undergo a conformational change (the power stroke).

  5. This change in myosin’s shape causes the thin actin filaments to slide along the myosin filaments, shortening the sarcomeres and leading to muscle contraction.

  6. The cycle repeats as long as Ca²⁺ ions are present and ATP is available.