Chapter 1: Skeletal Muscle Structure

Every movement you make, from a blink to a sprint, ends with the same event: protein filaments inside a muscle cell sliding past one another. To understand how that happens (the subject of Chapter 2) and why contraction strength changes with stretch, recruitment and stimulus frequency (Chapter 3), you first need a clear picture of how skeletal muscle is built, from the whole organ down to single molecules.

This chapter follows the first half of the "Kontraksi Otot" teaching session: the definition and functions of muscle, the vocabulary (terminologi) of the muscle cell, and the three types of muscle tissue.

What muscle is and what it does

Skeletal (striated) muscle is tissue made of bundles of long, contractile cells (myofibers) that can shorten, move the body and hold body parts in position. Indonesian texts call it otot lurik ("striped muscle") after the bands visible under the microscope.

A skeletal muscle does far more than move bones:

Four properties make all of this possible:

• Excitability: the fiber responds to a stimulus (acetylcholine, or an electrical current in the practicum) with an action potential.
• Contractility: it can shorten and generate force.
• Extensibility: it can be stretched beyond its resting length.
• Elasticity: it recoils to its resting length when released.

Levels of organization

A muscle is built like a cable made of smaller cables. Each level is wrapped in its own connective tissue sheath, and all those sheaths merge at the ends of the muscle to form the tendon.

A useful way to remember the order is the "pyramid" shown in the session: muscle → muscle fibers (myofibers) → myofibrils → myofilaments (actin and myosin). The terms muscle cell, myocyte, myofiber and muscle fiber (Indonesian serabut otot) all mean the same thing, the single multinucleated cell.

Connective tissue wrappings

From outside to inside:

1. Deep fascia: a sheet of dense connective tissue that separates neighboring muscles and groups them into compartments.
2. Epimysium: surrounds the whole muscle.
3. Perimysium: surrounds each fascicle. Larger blood vessels and nerves run in it.
4. Endomysium: a delicate layer of reticular fibers around each individual fiber, carrying capillaries and nerve branches right up to the cell.
5. Sarcolemma: the cell's own plasma membrane, just inside the endomysium. It is not connective tissue, but the session lists it as the last "layer": epimisium → perimisium → endomisium → sarkolema.

The three connective tissue layers are continuous with each other and with the tendon, so the force made by every fiber is transmitted to bone. This is also why a torn fascicle does not simply "unravel" the whole muscle.

The muscle fiber

A skeletal muscle fiber is an unusual cell. It forms in the embryo by the fusion of many myoblasts, so it is multinucleated, with its nuclei pushed to the periphery just under the sarcolemma. It is long (millimeters to several centimeters, often the full length of the muscle) and 10–100 µm across. Most of its volume is packed with myofibrils; between them lie mitochondria, glycogen granules and a red oxygen-binding pigment, myoglobin.

Several structures have their own names because they are specialized versions of ordinary cell parts:

The T-tubules are the key to fast contraction. They carry the action potential from the surface into the center of a thick fiber within a millisecond or two, right next to the terminal cisternae where calcium is stored. In mammalian skeletal muscle there are two triads per sarcomere, at the junctions of the A and I bands.

Myofibrils and the sarcomere

Each fiber contains hundreds to thousands of myofibrils, cylindrical bundles of contractile proteins about 1–2 µm wide that run the length of the cell. A myofibril is a chain of repeating units called sarcomeres, joined end to end at the Z discs (Z lines).

The sarcomere is the smallest functional unit of striated muscle: the segment of a myofibril between two Z discs. When a muscle shortens, every sarcomere along every myofibril shortens a little, and those small changes add up.

The striped appearance comes from the way thick and thin filaments overlap inside each sarcomere:

A quick memory aid: "the A band stays constAnt; H and I get smaller." The A band equals the length of a thick filament, and filaments do not shorten. Only their overlap changes.

In three dimensions the filaments are arranged in a precise lattice. In cross-section through the overlap zone, each thick filament is surrounded by six thin filaments, and each thin filament sits between three thick filaments.

The myofilament proteins

The session groups the proteins of the sarcomere by job:

• Contractile proteins: actin and myosin, which produce force.
• Regulatory proteins: troponin and tropomyosin, which switch contraction on and off.
• Structural (accessory) proteins: titin, nebulin, myomesin and dystrophin, which hold the lattice together and give it elasticity.

Thick filaments: myosin

A thick filament is a bundle of about 250–300 myosin II molecules. Each molecule has two intertwined heavy chains, forming a long tail, and two globular heads, attached to the tail by a flexible hinge (neck) region. The heads carry the two sites that matter for contraction:

• an actin-binding site, and
• an ATP-binding site with ATPase activity, which splits ATP to power movement.

The myosin molecules are arranged tail-to-center, with the heads pointing toward the Z discs on each side. This leaves a short bare zone in the middle (at the M line) with no heads. Because the two halves face opposite directions, both halves pull their thin filaments toward the center. The heads that project from the filament and bind actin are the crossbridges.

Thin filaments: actin, tropomyosin and troponin

• Actin. Globular G-actin subunits polymerize into long strands of F-actin; two strands twist together like two strings of pearls. Every G-actin carries a binding site for a myosin head.
• Tropomyosin. A long, thread-like protein lying in the groove of the actin helix. Each tropomyosin spans about seven actin subunits and, in resting muscle, covers their myosin-binding sites.
• Troponin. A complex of three subunits attached to tropomyosin at regular intervals:
  • troponin C (TnC) binds calcium;
  • troponin I (TnI) inhibits the actin–myosin interaction;
  • troponin T (TnT) binds the complex to tropomyosin.

Together, troponin and tropomyosin are a calcium-operated switch. At rest they hide the binding sites; when Ca²⁺ binds TnC, the complex changes shape and pulls tropomyosin aside (Chapter 2).

Structural proteins

Clinical link: dystrophin. Loss of dystrophin makes the sarcolemma fragile, so fibers are damaged with each contraction and are slowly replaced by fat and fibrous tissue. This is Duchenne muscular dystrophy, an X-linked disorder of boys that presents with difficulty rising from the floor (Gowers' sign) and calf pseudohypertrophy. A milder partial deficiency causes Becker muscular dystrophy.

Three types of muscle

The body has three muscle tissues. Skeletal and cardiac muscle are striated because their filaments are organized into sarcomeres; smooth muscle is not.

Three rows in this table are favorite exam questions:

• Only skeletal muscle needs a nerve to contract. Cut the motor nerve and the muscle is paralyzed, then wastes (denervation atrophy). Cardiac and smooth muscle keep working because they have their own pacemakers or respond to local signals.
• Only smooth muscle lacks troponin. Its calcium sensor is calmodulin, which activates myosin light-chain kinase.
• Skeletal fibers are not electrically coupled. Each fiber contracts only when its own motor neuron fires. This lets the nervous system grade whole-muscle force by choosing how many fibers to activate, which is the principle of recruitment in Chapter 3.

Putting it together

The summary map from the session is worth being able to draw from memory:

• A skeletal muscle is composed of connective tissue, blood vessels, nerves and fascicles.
• Fascicles are made of individual muscle fibers (cells).
• Each fiber has a sarcolemma (continuous with the T-tubules), multiple nuclei and sarcoplasm.
• The sarcoplasm holds the sarcoplasmic reticulum (functionally linked to the T-tubules), mitochondria, glycogen granules and the myofibrils.
• Myofibrils are composed of troponin, actin and tropomyosin (the thin filaments), myosin (the thick filaments), and titin and nebulin, all organized into sarcomeres.

Key points

• Skeletal muscle is organized as muscle → fascicle → fiber (cell) → myofibril → sarcomere → myofilament, with epimysium, perimysium and endomysium wrapping the first three levels.
• The fiber is a multinucleated cell with a sarcolemma, sarcoplasm and sarcoplasmic reticulum; T-tubules bring the action potential inside, and a triad is one T-tubule plus two terminal cisternae.
• The sarcomere (Z disc to Z disc) is the smallest contractile unit. During contraction the A band stays the same while the I band and H zone shorten.
• Contractile proteins are actin and myosin; regulatory proteins are troponin and tropomyosin; structural proteins include titin, nebulin, myomesin and dystrophin.
• Skeletal muscle is striated, multinucleated, voluntary, uses troponin, and its fibers are not electrically coupled. Smooth muscle has no sarcomeres and uses calmodulin; cardiac muscle is striated, autorhythmic and coupled by gap junctions.