Overview
This lecture explains the detailed anatomy of a skeletal muscle cell, tracing the structure from whole muscle down to the microscopic sarcomere, and describes how muscle contraction happens at the molecular level.
Gross Anatomy of Skeletal Muscle
- Skeletal muscles attach to bones via tendons, which are types of connective tissue.
- The outermost layer covering muscle is called the epimysium, which is continuous with the tendon.
- Beneath the epimysium is the perimysium, which surrounds bundles of muscle fibers called fascicles (or fasciculus).
- Each individual muscle cell (muscle fiber or myofiber) is surrounded by the endomysium.
Structure of a Muscle Cell (Myofiber)
- Muscle fibers are long, cylindrical cells with nuclei located at their periphery.
- The plasma membrane of a muscle cell is called the sarcolemma.
- The cytoplasm inside a muscle cell is known as the sarcoplasm.
- Within each muscle fiber are myofibrils, which contain the contractile machinery.
Myofibrils and the Sarcomere
- Myofibrils show striations (bands) visible under a microscope.
- The basic unit of contraction is the sarcomere, defined as the segment between two Z-lines.
- Sarcomeres contain actin (thin) filaments anchored to Z-lines and myosin (thick) filaments in the center.
- Titin proteins hold the myosin in place within the sarcomere.
Sarcomere Bands and Contraction
- The A-band includes areas where actin and myosin overlap (length of myosin).
- The I-band contains only actin filaments and spans two adjacent sarcomeres.
- During contraction, myosin heads use ATP to crawl along actin, pulling actin filaments toward the center.
- The I-band shortens during contraction, but the A-band stays the same length.
- The result is the shortening of the sarcomere and contraction of the muscle.
Initiation of Contraction
- An action potential travels down a motor neuron axon to the muscle cell, depolarizing the sarcolemma.
- T-tubules in the sarcolemma transmit the action potential deep into the cell, triggering calcium release.
- Calcium binds to troponin on actin, causing tropomyosin to move and exposing binding sites for myosin.
- Myosin can then bind actin, causing muscle contraction.
Key Terms & Definitions
- Epimysium — connective tissue layer surrounding entire muscle.
- Perimysium — connective tissue surrounding muscle fascicles.
- Endomysium — connective tissue around individual muscle fibers.
- Myofiber (Muscle fiber) — single muscle cell.
- Sarcolemma — plasma membrane of a muscle cell.
- Sarcoplasm — cytoplasm of a muscle cell.
- Myofibril — contractile thread within muscle fibers.
- Sarcomere — basic contractile unit between two Z-lines.
- Actin — thin filament, anchored to Z-line.
- Myosin — thick filament, interacts with actin for contraction.
- A-band — area with overlapping actin and myosin filaments.
- I-band — area with only actin filaments.
- T-tubule — invagination of sarcolemma bringing action potentials into the cell.
Action Items / Next Steps
- Review the roles and locations of connective tissue layers: epimysium, perimysium, and endomysium.
- Study the structure and function of the sarcomere, including A-band, I-band, and Z-line.
- Understand the molecular steps of skeletal muscle contraction and the roles of calcium, troponin, and tropomyosin.