Skeletal muscles are a type of striated muscle tissue that is under voluntary control. They are responsible for body movements and posture and are connected to bones via tendons. The histological structure of skeletal muscles is highly specialized to enable contraction and movement.

1. Structure of Skeletal Muscle

Skeletal muscles are made up of bundles of muscle fibers (muscle cells), and each muscle fiber has a highly organized internal structure that is critical to its function. Here is a breakdown of the skeletal muscle structure at different levels:

1. Muscle (Organ Level)

• Epimysium: The outermost connective tissue layer that surrounds the entire muscle. It is composed of dense connective tissue and helps to protect and support the muscle.

1. Fascicle (Bundle of Muscle Fibers)

• Perimysium: Connective tissue that surrounds each fascicle (group of muscle fibers). It contains blood vessels and nerves that supply the muscle fibers.

1. Muscle Fiber (Cell Level)

• Endomysium: The delicate connective tissue surrounding each individual muscle fiber (cell), providing structural support and containing capillaries that nourish the fibers.

2. Muscle Fiber (Muscle Cell) Structure

Each skeletal muscle fiber is a long, cylindrical, multinucleated cell that extends the length of the muscle. The key components of a muscle fiber include:

1. Sarcolemma

• The sarcolemma is the plasma membrane of the muscle cell. It surrounds the entire muscle fiber and contains t-tubules (transverse tubules), which are invaginations of the membrane that help conduct the action potential deep into the muscle fiber.

1. Sarcoplasm

• The sarcoplasm is the cytoplasm of the muscle cell and contains the organelles and cytosolic structures, including the nuclei and mitochondria.

1. Nuclei

• Skeletal muscle fibers are multinucleated, meaning they contain many nuclei located at the periphery of the fiber, beneath the sarcolemma. This characteristic is due to the fusion of myoblasts during muscle development.

1. Myofibrils

• The myofibrils are long, thread-like structures that run the length of the muscle fiber and are responsible for muscle contraction. They are made up of repeating contractile units called sarcomeres.

3. Sarcomere: The Functional Unit of Muscle Contraction

The sarcomere is the smallest functional unit of a muscle, and it is responsible for muscle contraction. It is a highly organized structure composed of actin and myosin filaments, and it is the basic unit of the myofibrils.

Key Components of the Sarcomere:

1. Z-lines (Z-discs): The boundaries of the sarcomere, where the thin actin filaments are anchored. The sarcomere stretches from one Z-line to the next.
2. A-band (Anisotropic band): The darker region of the sarcomere, which contains the entire length of the thick myosin filaments, as well as the overlapping thin actin filaments. It appears dark because the thick and thin filaments overlap.
3. I-band (Isotropic band): The lighter region of the sarcomere that contains only the thin actin filaments. The I-band gets smaller during muscle contraction as the actin filaments slide past the myosin filaments.
4. H-zone: The central part of the A-band where there is no overlap between actin and myosin filaments. During contraction, the H-zone shrinks as the actin filaments slide over the myosin filaments.
5. M-line: A thin line in the center of the sarcomere that holds the thick myosin filaments in place.

4. Myofilaments

The contractile proteins in the sarcomere are organized into two main types of filaments:

1. Thick Filaments (Myosin)

• Myosin is a protein that forms thick filaments. It has a tail region and two globular heads. The heads of the myosin filaments bind to actin during muscle contraction and “walk” along the actin filaments, resulting in the sliding filament mechanism of contraction.

1. Thin Filaments (Actin)

• Actin is a protein that forms thin filaments. Each actin filament consists of a helical arrangement of actin subunits. The thin filaments also contain two regulatory proteins, tropomyosin and troponin, which control the binding of myosin to actin during muscle contraction.
  • Tropomyosin: A protein that blocks the binding sites on actin, preventing interaction with myosin in a relaxed muscle.
  • Troponin: A protein complex that binds to calcium ions and induces a conformational change in tropomyosin, exposing the myosin-binding sites on actin during contraction.

5. Muscle Contraction Mechanism

The contraction of skeletal muscles follows the sliding filament theory, where the thick and thin filaments slide past each other to shorten the sarcomere, leading to muscle contraction.

Steps of Muscle Contraction:

1. Action Potential: An electrical impulse (action potential) travels along the motor neuron to the neuromuscular junction, where it stimulates the release of acetylcholine (ACh).
2. Depolarization of Sarcolemma: ACh binds to receptors on the sarcolemma, generating an action potential that travels along the T-tubules to the interior of the muscle fiber.
3. Calcium Release: The action potential triggers the release of calcium ions (Ca²⁺) from the sarcoplasmic reticulum (SR) into the sarcoplasm.
4. Troponin-Ca²⁺ Interaction: Calcium binds to troponin, causing a conformational change that moves tropomyosin and exposes the myosin-binding sites on actin.
5. Cross-Bridge Formation: Myosin heads bind to the exposed actin sites, forming cross-bridges.
6. Power Stroke: Myosin heads pivot, pulling the actin filaments toward the center of the sarcomere, resulting in contraction (this is called the power stroke).
7. ATP Binding: ATP binds to the myosin head, causing it to release from the actin filament and reset for another cycle of contraction.
8. Relaxation: When the action potential stops, calcium is pumped back into the sarcoplasmic reticulum, tropomyosin covers the binding sites on actin, and the muscle relaxes.

6. Other Important Features of Skeletal Muscle Histology
7. Mitochondria

• Skeletal muscle fibers have a large number of mitochondria, providing the energy (ATP) needed for continuous muscle contractions.

1. Satellite Cells

• Satellite cells are undifferentiated cells located near muscle fibers that can differentiate into muscle cells for repair and regeneration after injury.

1. Blood Supply

• Skeletal muscles are richly vascularized to supply oxygen and nutrients to muscle fibers and remove waste products from metabolism.

7. Types of Skeletal Muscle Fibers

Skeletal muscle fibers can be classified into different types based on their contractile and metabolic properties:

1. Type I Fibers (Slow-Twitch)

• Red fibers: High in mitochondria and myoglobin.
• Fatigue-resistant: These fibers are adapted for long-term, endurance activities (e.g., posture maintenance, long-distance running).

1. Type IIa Fibers (Fast-Twitch, Oxidative)

• Intermediate fibers that have both endurance and strength, combining some characteristics of Type I and Type IIb fibers.

1. Type IIb Fibers (Fast-Twitch, Glycolytic)

• White fibers: Low in mitochondria and myoglobin.
• Powerful but fatigable: These fibers are used for rapid, powerful movements (e.g., sprinting, weightlifting).

Conclusion

Skeletal muscles are composed of muscle fibers that are highly specialized for contraction. The histology of skeletal muscle reveals its complex organization, from the macro-level muscle structure down to the microscopic sarcomere, which is the functional unit of contraction. The sliding filament mechanism, regulated by calcium ions and proteins like actin, myosin, tropomyosin, and troponin, facilitates muscle contraction. Additionally, skeletal muscles are classified into different fiber types based on their function and metabolic characteristics, allowing for a wide range of movements and activities.