Skeletal Muscle Tissue

Introduction to Muscle Tissue

Muscle tissue is a fundamental component of the human body, responsible for movement, posture, and heat generation. There are three main types of muscle tissue: skeletal, cardiac, and smooth muscle. Skeletal muscle is the most abundant, making up nearly half of the body's mass, and is under voluntary control.

Properties of Muscle Tissue

• Contractility: Muscle cells shorten due to the action of myofilaments (actin and myosin).

• Excitability: Muscle cells respond to nerve signals, generating electrical impulses along their plasma membrane.

• Extensibility: Muscles can be stretched by opposing muscle contractions or by substances within hollow organs.

• Elasticity: Muscles recoil to their resting length after being stretched.

Terminology Specific to Muscle Tissue

• Myo and mys: Prefixes meaning "muscle"

• Sarco: Prefix meaning "flesh"

• Sarcolemma: Plasma membrane of muscle cells

• Sarcoplasm: Cytoplasm of muscle cells

Functions of Muscle Tissue

• Produce Movement: Skeletal muscle moves bones; smooth muscle moves substances through organs.

• Open and Close Body Passageways: Sphincter muscles act as valves.

• Maintain Posture and Stabilize Joints: Muscle tone stabilizes synovial joints.

• Heat Generation: Muscle contractions produce heat, maintaining body temperature.

Types of Muscle Tissue

Skeletal Muscle Tissue

Skeletal muscle tissue is found in organs that attach to and move the skeleton. It is striated and under voluntary control.

Cardiac and Smooth Muscle Tissue

• Cardiac Muscle: Found only in the heart; striated; involuntary.

• Smooth Muscle: Found in walls of hollow organs; non-striated; involuntary.

• Both are called visceral muscle due to their location and involuntary control.

Gross Anatomy of Skeletal Muscle

Connective Tissue Sheaths and Fascicles

Skeletal muscle is organized into layers of connective tissue:

• Epimysium: Surrounds the entire muscle (dense irregular connective tissue).

• Perimysium: Surrounds each fascicle (group of muscle fibers).

• Endomysium: Wraps each individual muscle cell.

Muscle Attachments

• Origin: Less movable attachment point.

• Insertion: More movable attachment point.

• Attachments can be direct (short connective tissue fibers) or indirect (tendons or aponeuroses).

Microscopic and Functional Anatomy of Skeletal Muscle Tissue

Skeletal Muscle Fiber

Muscle fibers are long, cylindrical, multinucleate cells formed by fusion of embryonic cells. Nuclei are located peripherally.

Myofibrils and Sarcomeres

Myofibrils are specialized contractile organelles composed of repeating units called sarcomeres, which are the functional units of muscle contraction.

• Z disc: Boundary of each sarcomere.

• Thin (actin) filaments: Extend from Z disc toward center; contain troponin and tropomyosin.

• Thick (myosin) filaments: Located in center; contain ATPase enzymes.

• A band: Full length of thick filament.

• I band: Region with only thin filaments.

• H zone: Center of A band with no thin filaments.

• M line: Center of H zone; holds thick filaments together.

Titin and Other Myofibrils

Titin is a springlike molecule that resists overstretching, holds thick filaments in place, and unfolds when muscle is stretched.

Sarcoplasmic Reticulum and T Tubules

The sarcoplasmic reticulum (SR) is a specialized smooth endoplasmic reticulum that stores calcium ions, which are released to trigger muscle contraction. T tubules are deep invaginations of the sarcolemma, forming a triad with two terminal cisterns of the SR.

Mechanism of Muscle Contraction

Types of Contraction

• Concentric contraction: Muscle shortens to do work.

• Eccentric contraction: Muscle generates force as it lengthens, acting as a brake against gravity.

Sliding Filament Mechanism

During contraction, myosin heads attach to actin filaments and pull them toward the center of the sarcomere. This process is powered by ATP and initiated by calcium ions binding to troponin.

Changes in Striation Pattern

• Fully relaxed: Thin filaments partially overlap thick filaments.

• Contracted: Z discs move closer, sarcomere shortens, I bands shorten, H zone disappears, A band remains unchanged.

Structure and Organizational Levels of Skeletal Muscle

Hierarchical Organization

Skeletal muscle is organized from the whole muscle down to the molecular level:

Innervation of Skeletal Muscle

Neuromuscular Junction

Motor neurons innervate skeletal muscle tissue at the neuromuscular junction, where nerve endings meet muscle fibers. Terminal boutons store neurotransmitters, and the synaptic cleft is the space between the axon terminal and sarcolemma.

Motor Units

A motor unit consists of a motor neuron and all the muscle fibers it innervates. Each axon branches to form neuromuscular junctions with muscle fibers scattered throughout the muscle.

Types of Skeletal Muscle Fibers

Classification by Energy Production and Contraction Speed

• Slow oxidative fibers: Red, abundant myoglobin, aerobic metabolism, many mitochondria, contract slowly, resistant to fatigue, small diameter.

• Fast glycolytic fibers: White, little myoglobin, anaerobic metabolism, few mitochondria, large diameter, contract rapidly, tire quickly.

• Fast oxidative fibers: Intermediate diameter, contract quickly, oxygen dependent, high myoglobin, rich capillary supply, somewhat fatigue resistant.

Comparison of Skeletal, Cardiac, and Smooth Muscle

Body Location

Cell Shape and Appearance

Connective Tissue Components

Presence of T Tubules and Site of Invagination

Sarcoplasmic Reticulum, Calcium Source, and Cell Junctions

Regulation of Contraction and Energetics

Additional info: The notes above expand on brief points from the original material, providing definitions, examples, and comparisons to ensure completeness and academic quality for exam preparation.