Muscles and Muscle Tissue: Part A

Introduction to Muscle Tissue

Muscle tissue is specialized for contraction, enabling movement and force generation in the body. It transforms chemical energy (ATP) into directed mechanical energy, which exerts force. There are three main types of muscle tissue, each with distinct structure and function.

• Skeletal muscle

• Cardiac muscle

• Smooth muscle

Muscle terminology often uses prefixes such as myo-, mys-, and sacro-.

Types of Muscle Tissue

Skeletal Muscle

Skeletal muscles are attached to bones and skin, responsible for voluntary movements and posture.

• Elongated cells called muscle fibers

• Striated (striped appearance under microscope)

• Voluntary (conscious control)

• Contract rapidly; tire easily; powerful

• Require nervous system stimulation

Cardiac Muscle

Cardiac muscle is found only in the heart, forming the bulk of the heart walls.

• Striated

• Involuntary (cannot be consciously controlled)

• Can contract without nervous system stimulation

More details are covered in cardiovascular system chapters.

Smooth Muscle

Smooth muscle is found in the walls of hollow organs such as the stomach, urinary bladder, and airways.

• Not striated

• Involuntary

• Can contract without nervous system stimulation

Special Characteristics of Muscle Tissue

• Excitability (responsiveness or irritability): ability to receive and respond to stimuli

• Contractility: ability to shorten forcibly when stimulated

• Extensibility: ability to be stretched

• Elasticity: ability to recoil to resting length

Functions of Muscle Tissue

• Movement of bones or fluids (e.g., blood)

• Maintaining posture and body position

• Stabilizing joints

• Heat generation (especially skeletal muscle)

• Additional functions: protects organs, forms valves, controls pupil size, causes "goosebumps"

Skeletal Muscle: Structure and Organization

Gross Anatomy

• Each muscle served by one artery, one nerve, and one or more veins

• Connective tissue sheaths support cells and reinforce the whole muscle

• Connective tissue layers:

  • Epimysium: dense irregular connective tissue surrounding entire muscle

  • Perimysium: fibrous connective tissue surrounding fascicles (groups of muscle fibers)

  • Endomysium: fine areolar connective tissue surrounding each muscle fiber

Muscle Attachments

• Muscles attach to bones at at least two places:

  • Insertion: movable bone

  • Origin: immovable (or less movable) bone

• Attachments can be:

  • Direct: epimysium fused to periosteum of bone or perichondrium of cartilage

  • Indirect: connective tissue wrappings extend beyond muscle as tendon or aponeurosis

Microscopic Anatomy of Skeletal Muscle Fiber

• Long, cylindrical cell (10–100 μm in diameter; up to 30 cm long)

• Multiple peripheral nuclei

• Sarcolemma: plasma membrane

• Sarcoplasm: cytoplasm

• Contains glycosomes (for glycogen storage) and myoglobin (for O2 storage)

• Modified structures: myofibrils, sarcoplasmic reticulum, and T tubules

Myofibrils

• Densely packed, rodlike elements (~80% of cell volume)

• Contain sarcomeres: contractile units

• Exhibit striations: perfectly aligned repeating series of dark A bands and light I bands

Sarcomere

• Smallest contractile unit (functional unit) of muscle fiber

• Align along myofibril like boxcars on a train

• Composed of thick and thin myofilaments made of contractile proteins

• Contains an A band with half an I band at each end

Striations and Bands

• H zone: lighter region in midsection of dark A band where filaments do not overlap

• M line: line of protein myomesin bisects H zone

• Z disc: coin-shaped sheet of proteins on midline of I band that anchors thin filaments and connects myofibrils to one another

• Thick filaments: run entire length of A band

• Thin filaments: run length of I band and partway into A band

• Sarcomere: region between two successive Z discs

Myofilament Arrangement

• Thick filaments: composed of protein myosin

• Thin filaments: composed of protein actin

• Elastic filaments: composed of titin; help maintain organization and elasticity

Ultrastructure of Filaments

• Thick filament (myosin):

  • Each myosin molecule has two heavy and four light polypeptide chains

  • Heads act as cross bridges during contraction

  • Binding sites for actin, ATP, and ATPase enzymes

• Thin filament (actin):

  • Twisted double strand of fibrous protein F actin

  • F actin consists of G (globular) actin subunits

  • G actin bears active sites for myosin head attachment

  • Regulatory proteins: tropomyosin and troponin

Sarcoplasmic Reticulum (SR) and T Tubules

• Sarcoplasmic reticulum (SR): network of smooth endoplasmic reticulum surrounding each myofibril; regulates intracellular Ca2+ levels

• T tubules: continuations of sarcolemma; penetrate cell's interior at each A band–I band junction

• Triad: T tubule and two terminal cisterns of SR; important for excitation-contraction coupling

Sliding Filament Model of Contraction

Muscle contraction occurs when myosin heads bind to actin and pull thin filaments toward the center of the sarcomere, shortening the muscle fiber.

• Generation of force does not necessarily cause shortening of the fiber

• Shortening occurs when tension generated by cross bridges on thin filaments exceeds forces opposing shortening

• In relaxed state, thick and thin filaments overlap only at ends of A band

• During contraction, thin filaments slide past thick filaments so that actin and myosin overlap more

Key equation (muscle force):

Summary Table: Types of Muscle Tissue

Additional info:

• Some details about muscle diseases, repair, and clinical applications may be covered in later chapters.