Unit 3: Muscular Tissues
Table of Contents
1. Types and Functions of Muscles
Muscle tissue is specialized for contractility, excitability, extensibility, and elasticity.
Classification of Muscle Tissues
| Type | Striations | Control | Location |
|---|---|---|---|
| Skeletal | Present | Voluntary | Attached to bones |
| Cardiac | Present | Involuntary | Wall of the heart |
| Smooth | Absent | Involuntary | Walls of hollow organs (e.g., gut) |
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
- An action potential reaches the neuromuscular junction, triggering the release of Acetylcholine.
- Calcium is released from the sarcoplasmic reticulum into the sarcoplasm.
- Calcium binds to Troponin, causing Tropomyosin to move and expose actin binding sites.
- 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.
Exam Tips & Common Pitfalls
- Z-Disc to Z-Disc: Always define a Sarcomere this way; it is the most common 1-mark question.
- Ca²⁺ Role: Remember that contraction cannot happen without Calcium, as it is the "switch" that moves Troponin.
- ATP: ATP is required for both contraction (power stroke) and relaxation (detaching the myosin head).