Muscular System UPDATED

Functions of the Muscular System

• Primary Functions:

  • Movement: Enables voluntary and involuntary movements of the body.

  • Posture: Maintains body posture and stability.

  • Heat Production: Generates heat through muscle contractions to help maintain body temperature.

• Secondary Functions:

  • Joint Stability: Provides support and stability to joints during movement.

  • Circulation: Assists in the circulation of blood through the contraction of cardiac and smooth muscles.

  • Respiration: Facilitates breathing by enabling the contraction of respiratory muscles.

The Structure of Skeletal Muscle

• All skeletal muscles are surrounded and penetrated by three main connective tissue layers:

  • Epimysium: The outermost layer, encasing the entire muscle and continuous with a tendon.

  • Perimysium: The intermediate layer that surrounds a fascicle, which is a bundle of muscle cells.

  • Endomysium: The innermost layer that surrounds the plasma membrane of a muscle cell.

• Muscle cell and muscle fiber are interchangeable terms.

Muscle Fiber Anatomy

• Myoblasts fuse to create long, cylindrical, multinucleate muscle fibers.

• Some muscle fibers can reach lengths of 30 cm and diameters of 100 micrometers.

• Muscle fibers are organized into bundles called fascicles, which are encased in the perimysium.

• The plasma membrane of muscle cells is referred to as the sarcolemma.

• The sarcolemma connects with the endomysium through cellular adhesion proteins.

• The inner contents of muscle cells are known as sarcoplasm instead of cytoplasm.

• The structure of skeletal muscle facilitates interaction between the sarcolemma and sarcoplasmic reticulum.

• Transverse tubules, which are extensions of the sarcolemma, interface with the sarcoplasmic reticulum at transverse-cisternae.

• These tubules enable signals from the nervous system to enter the sarcoplasm, initiating muscle contraction.

  The Functional Unit of Muscle

• Each muscle fibers contain myofibrils or fibrils, where repeating units of sarcomeres are found.

• The sarcomere is the functional, contractile unit of the muscle fiber and consists of myofilaments: actin and myosin.

Histology: Sarcomere

• The sarcomere contributes to the banded appearance of skeletal and cardiac muscle.

• The anatomy of the sarcomere includes:

  • Z disc: Serves as an anchoring protein for thin filaments; delineates the boundaries of a sarcomere.

  • A Bands: Dark bands containing thick myosin filaments that overlap with thin actin filaments.

  • I bands: Light bands composed solely of thin actin filaments.

Sarcomere

• H zone: A light band located at the center of an A band, lacking thin filaments.

• M line: A dark band at the center of the H zone, composed of anchoring proteins; myosin projects towards the Z-line.

• Titin: An elastic protein that connects the thick filaments of neighboring sarcomeres.