Functions and Anatomy of Muscle Tissue

Comparison of Muscle Types

Muscle tissue is classified into three distinct types based on its location, appearance, and how it is controlled:

Skeletal Muscle Anatomy

To understand how a muscle functions, we must look at its hierarchical organization, from the whole organ down to the microscopic filaments.

Connective Tissue Layers

A whole skeletal muscle is organized by layers of connective tissue that provide structure and eventually fuse to form tendons, which attach the muscle to bone:

• Epimysium: The outermost sheath surrounding the entire muscle.
• Perimysium: A layer that bundles muscle fibers into groups called fascicles.
• Endomysium: A delicate layer that surrounds each individual muscle fiber.

The Muscle Fiber (Cell)

At the cellular level, the muscle fiber consists of the sarcoplasm (the cytoplasm) and the sarcolemma (the cell membrane). To facilitate rapid contraction, the cell contains specialized structures:

• T tubules (Transverse tubules): Inward extensions of the sarcolemma that carry electrical signals deep into the cell.
• Sarcoplasmic Reticulum: A specialized smooth endoplasmic reticulum that stores high concentrations of calcium ions.
• Terminal Cisternae: Enlarged sacs of the sarcoplasmic reticulum that sit adjacent to T tubules.
• Triad: A functional unit consisting of one T tubule and two adjacent terminal cisternae.

The Sarcomere and Myofilaments

The functional unit of contraction within a muscle fiber is the sarcomere, which is the region located between two Z disks. Within the sarcomere, two types of myofilaments interact to produce movement:

1. Actin (Thin Filaments): These filaments contain two regulatory proteins: tropomyosin, which covers the active binding sites when the muscle is at rest, and troponin, which binds to calcium ions to move the tropomyosin.
2. Myosin (Thick Filaments): These are shaped like golf clubs with protruding myosin heads. These heads form cross-bridges with actin to pull the filaments past one another.

Sarcomere Microstructure

The appearance of a sarcomere under a microscope is defined by specific bands and zones:

• I Band: A lighter region containing only actin filaments; it includes the Z disk.
• A Band: A darker region spanning the full length of the myosin filaments.
• H Zone: The center of the A band, which contains only myosin filaments.
• M Line: The very center of the H zone that holds the myosin filaments in place.

Study Tip: During contraction, the Z disks move closer together, the H zone disappears, and the I band shortens, but the length of the A band remains constant because the myosin filaments themselves do not change length.

Muscle Fiber Physiology: Electrical Signaling

Muscle contraction is initiated by electrical changes across the sarcolemma.

Resting Membrane Potential

At rest, a muscle cell maintains a resting membrane potential of approximately $-85\text{ mV}$. This negative charge inside the cell relative to the outside is maintained by the sodium-potassium pump, which actively moves $3\text{ Na}^+$ ions out of the cell for every $2\text{ K}^+$ ions moved in.

The Action Potential

When a stimulus occurs, an action potential is generated through three distinct phases:

1. Resting State: Voltage-gated sodium and potassium channels are closed.
2. Depolarization: Voltage-gated sodium channels open, allowing $\text{Na}^+$ to rush into the cell. This influx makes the internal membrane potential positive.
3. Repolarization: Sodium channels close and voltage-gated potassium channels open. $\text{K}^+$ diffuses out of the cell, restoring the negative resting potential.

This electrical signal travels down the T tubules, triggering the release of calcium from the sarcoplasmic reticulum into the sarcoplasm, which ultimately allows the myosin heads to bind to actin.