Animal Physiology - Skeletal Muscle Review

Skeletal muscles are composed of bundles of muscle fibers arranged longitudinally. They are under voluntary control, meaning that their contractions can be consciously controlled by the nervous system and are essential for locomotion and various physical activities.

Components of Muscle Fibers:

• Muscle Fiber (Cell): Basic building block of muscle; contains myofibrils that facilitate contraction.

• Myofibrils: Composed of thin (actin) and thick (myosin) filaments, they are responsible for muscle contraction through the sliding filament theory.

• Cross Striations: Visible under the microscope, these striations indicate the organized structure of muscle fibers, resulting from the arrangement of myofibrils.

Muscle Length Characteristics

• Longest Muscle: Sartorius, which runs from the hip to the knee, aiding in the flexion of the leg.

• Largest Muscle: Gluteus Maximus, responsible for the movement of the hip and thigh, playing a significant role in maintaining an upright posture during locomotion.

• Smallest Muscle: Musculus stapedius, located in the ear, contributes to the modulation of sound vibrations.

Structure of Muscle Cells

Components:

• Sarcolemma: The cell membrane of the muscle fiber; essential for maintaining the electrochemical gradient.

• Sarcoplasmic Reticulum (SR): A specialized endoplasmic reticulum that stores calcium ions necessary for muscle contraction, releasing them in response to stimuli.

• Transverse Tubules (T-Tubules): Extensions of the sarcolemma that penetrate into the muscle fiber, facilitating the transmission of action potentials to ensure rapid and uniform contraction.

Myofibrils Features:

• Sarcomere: The functional unit of striated muscle; defined by Z discs and M lines, essential for the contraction cycle.

• A Band: Contains both thick and thin filaments and appears darker under a microscope; crucial for the contraction process.

• I Band: Contains only thin filaments and appears lighter; reduces in size during muscle contraction.

• H Zone: The area at the center of the A band where no thin filaments are present; this zone disappears during contraction.

Protein Structure in Sarcomeres

Key Proteins:

• Titin: An elastic protein that connects the Z disc and M line, crucial for maintaining structure and elasticity during muscle contraction and relaxation.

• Nebulin: A large protein that stabilizes thin filaments throughout their length, contributing to the structural integrity of the sacromere.

• Obscurin: Links proteins in the M line to the sarcoplasmic reticulum, playing a role in organizing myofibrils.

Muscle Contraction Mechanism

• Sliding Filament Theory: The mechanism of muscle contraction whereby thick myofilaments (myosin) slide past thin myofilaments (actin), resulting in shortening of the muscle fiber.

Cross-Bridge Cycle:

1. Myosin binds to actin, powered by ATP, moving the actin filament approximately 10 nm towards the sarcomere center.

2. Myosin heads undergo a power stroke when ADP and inorganic phosphate are released, pulling actin along.

3. Calcium ions play a pivotal role in activating contraction by binding to troponin to expose binding sites on actin.

Excitation-Contraction Coupling

Mechanism:

• Action potential arrives at the neuromuscular junction, releasing acetylcholine (ACh).
• ACh binds to receptors on the sarcolemma, depolarizing the muscle fiber and generating an action potential.
• Action potential travels quickly along T-tubules, triggering the sarcoplasmic reticulum to release Ca2+ into the cytoplasm.
• Calcium binds to troponin, thus moving tropomyosin and exposing binding sites on actin.
• Myosin heads attach to actin, initiating muscle contraction.

Muscle Fiber Types and Functionality

Three Main Muscle Types:

• Skeletal Muscle: Voluntary muscle responsible for movement at joints, with fibers arranged in a striated pattern.

• Cardiac Muscle: Involuntary muscle found exclusively in the heart, featuring striated fibers and rhythmic contractions that are crucial for pumping blood.

• Smooth Muscle: Involuntary muscle located in hollow organs (e.g., intestines, blood vessels) that facilitates peristalsis and regulates blood flow.

ATP Regeneration for Muscle Activity

Metabolic Pathways:

1. Phosphagen System: Provides rapid ATP generation for short bursts of activity lasting only seconds, utilizing creatine phosphate.

2. Anaerobic Glycolysis: Translates glucose into ATP rapidly without the need for oxygen, resulting in the production of lactic acid as a byproduct.

3. Aerobic Respiration: The main pathway for long-term ATP production, requiring oxygen and involving the breakdown of carbohydrates, fats, and proteins.

Muscle activity and performance depend on fiber type, training, and metabolic capacity.

Tension and Muscle Action

• Twitch vs. Tetanus:

  • Single Twitch: A quick contraction and relaxation cycle triggered by a single action potential.
  • Tetanus: A sustained muscular contraction resulting from rapid stimulation, which can be complete (no relaxation) or incomplete (some relaxation).

• Tension is modulated by the frequency of action potentials, affecting force generation in skeletal muscle.

Smooth Muscle Characteristics

Differences from Skeletal Muscle:

• Lack of striations; muscle contractions are controlled by the autonomic nervous system.

• Contracts slower, but can sustain contractions for longer periods without fatigue.

• Regulated by the phosphorylation of myosin via calmodulin in response to calcium ions, allowing for different contraction mechanisms.

Cardiac Muscle Characteristics

Unique Features:

• Branched cells interconnected by intercalated discs, which facilitate synchronized contraction essential for effective heart function.

• Striated appearance; cardiac muscle fibers support rhythmic contractions and are less prone to fatigue.

• Contains pacemaker cells that initiate contractions autonomously, maintaining heart rhythm even in the absence of neural input.

Summary of Muscle Types Characteristics Table

Contraction Regulation

• Calcium ions regulate contractions differently in smooth muscle compared to skeletal muscle; in smooth muscle, calcium binding causes phosphorylation of myosin, allowing cross-bridge interactions without the involvement of troponin, which is pivotal in skeletal muscle contraction.

Conclusion

The study of muscular physiology is essential to understand how muscles contract, regenerate energy, and function cooperatively to produce movements critical for life, physical activity, and interaction with the environment.

Excitation-Contraction Coupling is the physiological process that links the electrical signal from the nervous system to the mechanical action of muscle fibers. The mechanism involves the following steps:

• An action potential arrives at the neuromuscular junction, releasing acetylcholine (ACh).
• ACh binds to receptors on the sarcolemma, depolarizing the muscle fiber and generating a new action potential.
• The action potential travels quickly along T-tubules, triggering the sarcoplasmic reticulum to release calcium ions (Ca2+) into the cytoplasm.
• Calcium ions bind to troponin, causing a conformational change that moves tropomyosin, exposing binding sites on actin filaments.
• Myosin heads then attach to these exposed binding sites on actin, leading to muscle contraction.

This coupling is essential for the proper functioning of muscle tissue, allowing for coordinated contractions necessary for movement and physical activity.