Unit 3: Muscular Tissues

1. Types and Functions of Muscles

Muscle tissue is specialized for contractility, excitability, extensibility, and elasticity.

Classification of Muscle Tissues

2. Ultrastructure of Skeletal Muscle

Skeletal muscle is composed of long, cylindrical cells called muscle fibers.

• Sarcolemma: The plasma membrane surrounding the muscle fiber.
• Sarcoplasm: The cytoplasm of the muscle fiber, rich in myoglobin and glycogen.
• Myofibrils: Rod-like contractile elements that occupy most of the sarcoplasm.
• Sarcomere: The functional unit of contraction, defined as the region between two Z-discs.

3. Molecular and Chemical Basis of Contraction

Muscle contraction follows the Sliding Filament Theory, where thin filaments slide over thick filaments without changing length.

Contractile Proteins

• Myosin (Thick Filament): Composed of a tail and a "head" that forms cross-bridges.
• Actin (Thin Filament): Contains binding sites for myosin heads.
• Regulatory Proteins: Tropomyosin and Troponin block or reveal actin binding sites depending on Calcium (Ca²⁺) levels.

Steps in Contraction

1. An action potential reaches the neuromuscular junction, triggering the release of Acetylcholine.
2. Calcium is released from the sarcoplasmic reticulum into the sarcoplasm.
3. Calcium binds to Troponin, causing Tropomyosin to move and expose actin binding sites.
4. Myosin heads bind to actin (cross-bridge formation), hydrolysis of ATP provides energy for the "power stroke".

4. Isotonic vs Isometric Contraction

Muscles can contract in different ways depending on whether the length or tension changes.

• Isotonic Contraction: The muscle changes length while maintaining constant tension (e.g., lifting a weight).
• Isometric Contraction: The muscle develops tension but does not change length (e.g., pushing against a wall).

5. Characteristics of Muscle Twitch and Electromyography

Muscle Twitch Phenomena

A muscle twitch is a single contraction-relaxation cycle in response to a single stimulus.

• Treppe: The "staircase effect" where subsequent stimuli produce stronger contractions as the muscle "warms up".
• Summation: Adding together individual twitches when stimuli arrive in rapid succession.
• Tetanus: A smooth, sustained contraction resulting from high-frequency stimulation.

Electromyography (EMG)

Definition: Electromyography is a diagnostic procedure to assess the health of muscles and the nerve cells that control them (motor neurons).

It records the electrical activity (Action Potentials) of muscle fibers during contraction.