Characteristics and Structure of Muscular Tissue

Skeletal muscle cells are exceptionally large and are often referred to as muscle fibers or myofibers. Because of their unique developmental history, they are multinucleated; they form from the fusion of many individual stem cells called myoblasts.

Connective Tissue Layers

A functional muscle is not just a collection of loose cells; it is organized by three distinct layers of connective tissue that contain collagen to provide strength and elasticity.

• Endomysium: A thin layer that surrounds each individual muscle fiber.
• Perimysium: A layer that bundles groups of muscle fibers together into units called fascicles.
• Epimysium: The outermost layer that surrounds the entire muscle.

Exam Tip: Remember the hierarchy of organization: Muscle $\rightarrow$ Fascicle $\rightarrow$ Muscle Fiber. The connective tissues follow this same order: Epimysium $\rightarrow$ Perimysium $\rightarrow$ Endomysium.

The Anatomy of a Muscle Fiber

The internal structure of a muscle fiber is highly specialized to facilitate rapid contraction and efficient energy use.

The Membrane and Cytoplasm

The cell is enclosed by a plasma membrane called the sarcolemma. Inside the cell lies the sarcoplasm (the cytoplasm), which is packed with essential components:

• Myofibrils: Long, protein-filled cords that occupy most of the sarcoplasm and are the actual contractile elements.
• Glycogen: Stored carbohydrates that provide a quick source of energy for exercise.
• Myoglobin: A red pigment that stores oxygen directly within the cell for muscle activity.
• Mitochondria: Numerous bean-shaped or tubular organelles that produce the ATP required for contraction.

The Excitation-Contraction Machinery

To ensure that an electrical signal can trigger a contraction throughout the entire thickness of the large muscle fiber, the cell utilizes a specialized system of tubules and reservoirs:

• Transverse (T) tubules: These are inward foldings of the sarcolemma that penetrate deep into the cell, allowing electrical signals to reach the interior.
• Sarcoplasmic Reticulum (SR): A specialized form of smooth endoplasmic reticulum that forms a network around each myofibril. It acts as a calcium reservoir, releasing calcium to activate contraction.
• Terminal Cisterns: These are dilated end-sacs of the SR that cross the muscle fiber. They store calcium bound to a protein called calsequestrin.

Cellular Repair and Regeneration

Because skeletal muscle fibers are so large and undergo significant mechanical stress, they require a mechanism for repair. While the multinucleated nature of the fiber helps in protein synthesis and repair, the primary agents of regeneration are satellite cells. These are unspecialized myoblasts located between the sarcolemma and the endomysium. When muscle tissue is damaged, these cells can activate to assist in the regeneration of the muscle fiber.