Skeletal muscles, also known as voluntary muscles or striated muscles, are a type of muscle tissue that is attached to bones by tendons and is under conscious control. These muscles are responsible for various voluntary movements of the body, including walking, running, grasping objects, and facial expressions.
1. Muscle Tissue Types:
Skeletal Muscle Tissue:
Skeletal muscles comprise skeletal muscle tissue, a type of striated muscle tissue under voluntary control.
The striations result from the organization of myofibrils within muscle fibers.
2. Muscle Fiber Structure
Muscle fiber (cell):
A muscle fiber is the term for a single skeletal muscle cell.
Each muscle fiber is multinucleated, containing multiple nuclei per cell.
The sarcolemma is the plasma membrane of the muscle fiber.
Sarcoplasm:
The sarcoplasm is the cytoplasm of the muscle fiber.
It contains the usual cellular components and is rich in glycogen, providing a rapid energy source for muscle contraction.
3. Myofibrils and Sarcomeres
Myofibrils:
The Myofibrils are long, cylindrical structures within the muscle fiber.
They are composed of repeating units called sarcomeres.
Sarcomeres:
Sarcomeres are the functional units of skeletal muscle.
Zlines mark the boundaries of sarcomeres.
Sarcomeres organize thin filaments (actin) and thick filaments (myosin).
4. Actin and Myosin Filaments:
Actin Filaments:
The protein actin primarily composes thin filaments known as actin filaments.
Actin filaments extend from the Zline towards the center of the sarcomere.
Myosin Filaments:
The Myosin filaments are thick filaments primarily composed of the protein myosin.
Myosin filaments are located in the center of the sarcomere, overlapping with actin filaments.
5. Sliding Filament Theory:
Muscle Contraction:
The sliding filament theory explains muscle contraction.
During contraction, actin and myosin filaments slide past each other, causing sarcomeres to shorten and the muscle to contract.
6. Connective Tissues:
Endomysium:
Surrounds individual muscle fibers.
Composed of areolar connective tissue.
Perimysium:
Surrounds groups of muscle fibers, forming fascicles.
Composed of dense irregular connective tissue.
Epimysium:
Surrounds the entire muscle.
Composed of dense irregular connective tissue.
Blends with tendons, attaching muscles to bones.
7. Blood Supply and Nerve Innervation:
Skeletal muscles are wellvascularized, ensuring an adequate supply of nutrients and oxygen.
Nerve fibers (motor neurons) innervate muscle fibers, controlling voluntary muscle contractions.
8. Satellite Cells:
Satellite cells are undifferentiated cells located between the sarcolemma and basal lamina.
They play a role in muscle growth, repair, and regeneration.
The histology of skeletal muscles involves a complex organization of muscle fibers, myofibrils, sarcomeres, and connective tissues. The sliding filament theory explains the molecular basis of muscle contraction, and the presence of satellite cells contributes to muscle growth and repair. The vascularization and nerve innervation ensure the proper functioning of these voluntary muscles.