Skeletal Muscle Histology

Unit 7: Muscle Tissue and Muscular System

The muscular system is responsible for movement, posture, and heat production. This unit explores the different types of muscle tissue, their functions, properties, and the detailed histology and physiology of skeletal muscle contraction.

7.1. Types of Muscular Tissue

Muscle tissue is specialized for contraction and is responsible for virtually all body movements. There are three primary types of muscle tissue, each with distinct anatomical, histological, and physiological characteristics.

• Skeletal Muscle:

  • Location: Primarily attached to bones, but also to skin (e.g., facial muscles).

  • Appearance: Striated (due to alternating light and dark bands) and fibers are variable in length , typically long and cylindrical.

  • Nuclei: Multinucleated (many nuclei per cell), located peripherally.

  • Control: Voluntary, meaning contractions are consciously controlled.

  • Function: Responsible for locomotion, maintaining posture, generating heat, and protecting organs.

• Cardiac Muscle:

  • Location: Found exclusively in the wall of the heart.

  • Appearance: Striated and branched fibers.

  • Nuclei: Usually one or two centrally located nuclei per cell.

  • Unique features: Contains intercalated discs, which are specialized junctions (gap junctions and desmosomes) that connect individual cardiac muscle cells, allowing for rapid communication and coordinated contraction.

  • Control: Involuntary and autorhythmic (generates its own beats), not consciously controlled.

  • Function: Pumps blood throughout the body.

• Smooth Muscle:

  • Location: Found in the walls of hollow internal organs (e.g., blood vessels, airways, gastrointestinal tract, uterus, bladder, iris of the eye) and attached to hair follicles in the skin.

  • Appearance: Non-striated (lacks visible bands). Fibers are spindle-shaped.

  • Nuclei: Single, centrally located nucleus per cell.

  • Unique features: Cells are bound together by gap junctions, allowing coordinated contractions.

  • Control: Involuntary, not consciously controlled.

  • Function: Moves substances through internal body passages (e.g., food through the digestive tract), regulates blood vessel diameter, and controls pupil size.

7.2. Functions of Muscular Tissue

Muscular tissue performs several vital functions for the body:

• Producing Body Movements: Skeletal muscles contract to move bones, allowing for walking, running, and all other physical activities.

• Stabilizing Body Positions: Sustained contractions of skeletal muscles maintain posture and stabilize joints.

• Storing and Moving Substances within the Body:

  • Cardiac muscle pumps blood.

  • Smooth muscle moves food through the digestive tract, propels urine, and controls blood flow.

  • Skeletal muscles facilitate the flow of lymph and aid in venous blood return.

• Generating Heat: Muscle contraction generates heat as a by product, which is crucial for maintaining normal body temperature (e.g., shivering).

7.3. Properties ofMuscular Tissue

All muscle tissues share several key properties that enable them to perform their functions:

• Excitability: The ability to respond to electrical stimuli (e.g., neurotransmitters, hormones, or autorhythmic activity) by generating an electrical impulse called a muscle action potential.

• Conductivity: The ability to propagate electrical signals (action potentials) rapidly along the muscle cell membrane.

• Contractility: The ability to shorten and thicken (contract forcefully) when stimulated by an action potential. This ability allows muscle tissue to generate tension and perform work. It transforms chemical/electrical energy into mechanical energy.

• Extensibility: The ability to be stretched or extended without being damaged. This physical property allows muscles to stretch when opposing muscles contract.

• Elasticity: The ability to return to its original length and shape after contraction or extension. This physical property aids muscles in resuming their resting state.

7.4. General Structure and Classification of Striated Muscle

The scientific study of muscles is known as myology. Motion results from the alternating contraction and relaxation of muscles.

Skeletal (striated) muscles are the muscles we can voluntarily move. Each skeletal muscle is considered a separate organ, comprising not only muscle cells (muscle fibers) but also connective tissue, blood vessels, and nerves.

Tissular Components of Skeletal Muscle

Skeletal muscles have an organized structure involving several layers of connective tissue:

• Connective Tissue in Muscle:

  • Superficial Fascia:

    • Also known as the hypodermis.

    • Composed of areolar connective tissue and adipose tissue.

    • Located between the skin and the muscle.

    • Function: Provides a pathway for nerves, blood vessels, and lymphatic vessels to enter and exit muscles, and acts as insulation and mechanical protection.

  • Deep Fascia:

    • A dense sheet of irregular connective tissue.

    • Functions:

      • Surrounds and holds muscle fibers, fascicles, and entire muscles together.

      • Allows free movement of muscles.

      • Provides a pathway for nerves, blood, and lymphatic vessels inside the muscle.

      • Fills spaces between muscles.

• Layers of Connective Tissue within the Muscle: Three layers of connective tissue, composed of collagen and elastic fibers, extend from the deep fascia to protect and strengthen the skeletal muscle.

  • Epimysium: The outermost layer of dense irregular connective tissue that surrounds the entire muscle.

  • Perimysium: A layer of dense irregular connective tissue that surrounds groups of 10-100 muscle cells, forming bundles called fascicles. These fascicles contain extensive blood vessels and nerves (neurovascular bundles).

  • Endomysium: A delicate layer of loose connective tissue with reticular fibers that surrounds and separates individual muscle cells (fibers) within a fascicle.

• Tendons and Aponeuroses: These are extensions of the three connective tissue layers (epimysium, perimysium, and endomysium) beyond the muscle belly.

  • Tendon: A cord of dense regular connective tissue that attaches a muscle to the periosteum of a bone. It transfers the force of muscle contraction to the bone, causing movement. (Force - muscle contraction-bone = mvt)

  • Aponeurosis: Similar to a tendon but formed as a broad, flat layer, providing a wider area of attachment to bone or other muscles.

Blood Supply and Innervation of Skeletal Muscle

• Skeletal muscles are highly vascularized and innervated, reflecting their high metabolic demand and precise control.

• Nerves:

  • Each skeletal muscle receives nerves from somatic motor neurons.

  • The axons of these motor neurons branch extensively within the muscle to reach multiple muscle cells, forming a motor unit.

  • Thepoint of contact between an axon terminal and a muscle fiber is called the neuromuscular junction (NMJ).

  • Nerve terminals are found in the endomysium.

• Blood Vessels:

  • Every nerve bundleentering a muscle is accompanied by one artery and one or two veins.

  • Each muscle fiber is in close contact with one or more capillaries.

  • This extensive blood supply is necessary to deliver abundant nutrients (e.g., oxygen, glucose, fatty acids) and removewaste products, crucial for the energy-intensive process of muscle contraction.

  • Capillaries are also located within the endomysium.

7.5. Histology of Skeletal Muscle

The detailed cellular structure of skeletal muscle fibershighlights the specialized components enabling contraction.

Muscle Cells = Muscle Fibers

• Development: Muscle fibers arise from the fusion of many (hundreds or more) mesodermal cells called myoblasts during embryonic development.

• Multinucleated: Due to the fusion of myoblasts, mature skeletal muscle fibers are multinucleated.

• Satellite Cells: Some embryonic myoblasts do not fuse during development and remain as immature cells called satellite cells. These cells are crucial for muscle regeneration and repair after injury, although mature muscle cells themselves do not typically divide.

• Growth: Muscle growth primarily occurs through hypertrophy (enlargement of existing muscle fibers) rather than hyperplasia (increase in the number of muscle fibers).

Key Components of a Muscle Fiber

• Sarcolemma: The plasma membrane of a muscle cell. It contains ion channels vital for propagating muscle action potentials.

• Transverse Tubules (T-tubules): Tiny invaginations of the sarcolemma that extend deep into the muscle fiber. They ensure rapid and synchronous spreading of the muscle action potential throughout the entire fiber. T-tubules form triads with two terminal cisternae of the sarcoplasmic reticulum.

• Sarcoplasm: The cytoplasm of the muscle fiber. It contains:

  • A large amount of glycogen, an energy storage molecule (for glucose).

  • Myoglobin, an oxygen-binding protein similar to hemoglobin, which stores oxygen within the muscle fiber for ATP production.

  • Numerous mitochondria, reflecting the high ATPdemand for muscle contraction.

  • Tiny threads called myofibrils, the primary contractile organelles that give skeletal muscle its striated appearance.

• Sarcoplasmic Reticulum (SR): A specialized endoplasmic reticulum that encircles each myofibril. Its primary function is to store andrelease calcium ions (), which are essential for initiating muscle contraction.

  • Terminal Cisternae: Enlarged end sacs of the SR that abut the T-tubules, forming the triad.

Myofibrils and Sarcomeres

Myofibrils are the contractile elements of skeletal muscle fibers. They are composed of repeating units called sarcomeres.

• Sarcomere: The basic functional and contractile unit of a myofibril. It extends from one Zdisc to the next Z disc. The arrangement of thick and thin filaments within the sarcomere creates the characteristic striations.

Protein Components of Myofibrils

Myofibrils are made up of three types of proteins: contractile, regulatory, and structural.

1. Contractile Proteins: Generate force during contraction.

• Actin (in thin filaments):

  • Consists of two interlocked helical strands of fibrous (F) actin.

  • Each F-actin strand is composed of many globular (G) actin monomers (300 or more units).

  • Each G-actin monomer has a myosin-binding site where the myosin head attaches.

• Myosin (in thick filaments):

  • Functions as a motor protein in all three types of muscle tissue.

  • Each myosin molecule resembles two golf clubs twisted together, consisting of a tail and two globular heads.

  • The myosin heads (also called cross-bridges) extend toward the thin filaments.

  • Myosin heads contain:

    • A binding site for actin.

    • A binding site for ATP.

    • ATPase activity, an enzyme that hydrolyzes ATP into ADP and to release energy.

  • The backbone of the thick filament is formed bythe myosin tails, while the heads project outward.

  • Thick filaments are held in place by the M line proteins.

2. Regulatory Proteins: Help switch the contraction process on and off.

• Tropomyosin (in thin filaments):

  • A long, filamentous protein that forms two alpha-helical chains spiraling around the F-actin strands.

  • In a relaxed muscle, tropomyosin covers the myosin-binding sites on the actin molecules, preventing myosin from binding.

• Troponin (in thin filaments):

  • A complex of three globular proteins attached to both actin and tropomyosin.

  • It has three subunits:

    • Troponin C (TnC): Binds to ions.

    • Troponin T (TnT): Binds to tropomyosin, providing stability to the complex.

    • Troponin I (TnI): The inhibitory domain that binds to actin and inhibits the interaction between actin and myosin.

  • When binds to Troponin C, it causes a conformational change in the troponin-tropomyosin complex, moving tropomyosin away from the myosin-binding sites on actin, thereby initiating contraction.

3. Structural Proteins: Contribute to the alignment, stability, elasticity, and extensibility of myofibrils.

• Titin:

  • An enormous protein, one of the largest known.

  • Connects a Z disc to an M line within the sarcomere.

  • Anchors the thick filaments to both the Z disc and the M line.

  • Possesses elastic properties, contributing significantly to muscle elasticity and extensibility. It helps the sarcomere return to its resting length after contraction or stretching. The portion between the Z disk and the end of the thick filament can stretch up to fourtimes its resting length without harm.

• Nebulin:

  • A long, inelastic protein that wraps around the entire length of each thin filament.

  • Helps to maintain the precise alignment of the thin filaments within the sarcomere and anchors them to the Z discs.

Physiology of Muscular Contraction: Skeletal Muscle (Sliding Filament Theory)

Muscle contraction occurs through the sliding of thin filaments past thick filaments, shortening the sarcomere.

Contraction Cycle Steps (Cross-bridgeCycle):

1. ATP Hydrolysis: Before contraction, the myosin head contains an ATP molecule. An ATPase on the myosin head hydrolyzes ATP into ADP and inorganic phosphate (). This reaction reorients and energizes the myosin head, cocking it into itshigh-energy position. ADP and remain attached to the myosin head.

2. Myosin Head Binds to Actin (Cross-bridge Formation): With the myosin head energized and the myosin-binding sites on actin exposed (due to binding to troponinand moving tropomyosin), the myosin head attaches to actin, forming a cross-bridge.

3. Power Stroke: The release of the from the myosin head triggers the power stroke. The myosin head pivots and pulls the thin filament towards the M line (center of the sarcomere).During this movement, the ADP is also released from the myosin head.

4. Myosin Head Binds Fresh ATP and Detaches from Actin: At the end of the power stroke, the myosin head remains firmly attached to actin. A new molecule of ATP binds to the myosin head, causing it to detachfrom the actin filament. This detachment is crucial for allowing the cycle to repeat. Without ATP (e.g., in rigor mortis), the cross-bridges cannot detach.

The contraction cycle continues as long as ATP is available and the level in the sarcoplasm is high enoughto keep the myosin-binding sites on actin exposed. Each myosin head acts like a "paddle," repeatedly binding, advancing, and detaching, causing the thin filaments to glide over the thick filaments towards the M line, thus shortening the sarcomere.

Key Takeaways

• There are three types of muscle tissue: skeletal (voluntary, striated), cardiac (involuntary, striated, branched, intercalated discs), and smooth (involuntary, non-striated, single nucleus).

• Muscle functions include movement, posture, internal substance movement, and heat generation.

• Key muscle properties are excitability, conductivity, contractility, extensibility, and elasticity.

• Skeletal muscle is a complex organ with specialized connective tissue layers: epimysium (surrounds entire muscle), perimysium (surrounds fascicles), and endomysium (surrounds individual fibers). These layers extend to form tendons and aponeuroses.

• Skeletal muscle fibers are multinucleated and derive from fused myoblasts; satellite cells provide regeneration capacity.

• Within skeletal muscle fibers, myofibrils are composed of repeating contractile units called sarcomeres.

• The sarcomere's banding pattern (A bands, I bands, H zone) is due to the organized arrangement of thick filaments (myosin) and thin filaments (actin).

• Muscle contraction occurs via the sliding filament theory, driven by the interaction between myosin heads and actin, regulated by troponin and tropomyosin, and powered by ATP hydrolysis and calcium ions ().

• Titin and nebulin are critical structural proteins providing elasticity and alignment to the myofibrils.