Muscle Tissue

Muscle Tissue Overview

• Muscle tissue is responsible for body movement.
• Muscle cells contain protein filaments (actin and myosin) that enable muscle contraction.

Types of Muscle Tissue

• Three types of muscle:
  • Smooth
  • Cardiac
  • Skeletal

Smooth Muscle

• Spindle-shaped, non-striated, uninucleated fibers.
• Located in the walls of internal organs.
• Involuntary (e.g., churning of the stomach, constriction of arteries).

Cardiac Muscle

• Striated, branched, uninucleated fibers.
• Located in the walls of the heart.
• Involuntary.
• Intercalated discs facilitate signal relay and synchronized heart contraction.

Skeletal Muscle

• Striated, tubular, multinucleated fibers.
• Usually attached to the skeleton.
• Voluntary.
• Muscle building in adults increases fiber size but not the number of fibers.

Skeletal Muscle Structure

• Attached to bones by tendons.
• Composed of bundles of long cells (muscle fibers).
• Muscle fibers are formed by the fusion of multiple cells, resulting in multiple nuclei.
• Arrangement of contractile units (sarcomeres) gives a striated appearance.

Sarcomere Structure

• Sarcomeres are contractile units within muscle fibers.
• Defined by Z lines.
• Contains thick (myosin) and thin (actin) filaments.
• M line is in the middle of the sarcomere.

Sliding-Filament Model of Muscle Contraction

• Muscle contraction occurs through the sliding of actin and myosin filaments.

Myosin-Actin Interaction

• Myosin heads bind to actin, forming cross-bridges.
• ATP provides the energy for the myosin head to change configuration and move the thin filament.
• The process involves:
  • Myosin head in a low-energy configuration binding to actin.
  • ATP binding to the myosin head.
  • Hydrolysis of ATP to ADP and Pi, causing the myosin head to shift to a high-energy configuration.
  • The power stroke: release of Pi, causing the myosin head to return to its low-energy configuration, sliding the thin filament.
  • Release of ADP.

Role of Calcium and Regulatory Proteins

• Tropomyosin and the troponin complex regulate muscle contraction.
• At rest, tropomyosin blocks myosin-binding sites on actin.
• Calcium ions $Ca^{2+}$ bind to troponin, causing tropomyosin to shift and expose myosin-binding sites.
• High $Ca^{2+}$ concentration leads to muscle contraction; low concentration stops contraction.

Muscle Fiber Contraction

• Initiated by an action potential in a motor neuron that synapses with the muscle fiber.
• The motor neuron releases acetylcholine, which depolarizes the muscle and generates an action potential.
• Action potentials travel along T tubules, causing the sarcoplasmic reticulum (SR) to release $Ca^{2+}$.
• $Ca^{2+}$ binds to troponin, exposing myosin-binding sites and allowing the cross-bridge cycle to occur.

Steps of Muscle Contraction

1. Action potential arrives at the synaptic terminal
2. Acetylcholine is released into the synaptic cleft
3. Acetylcholine depolarizes the muscle fiber, generating an action potential
4. Action potential travels along T tubules
5. Sarcoplasmic reticulum releases $Ca^{2+}$
6. $Ca^{2+}$ binds to troponin, exposing myosin-binding sites
7. Cross-bridge cycle begins, leading to muscle contraction