Chapter 9 A- Muscle Tissue: Structure, Function, and Contraction Mechanisms

Overview of Muscle Tissue

Muscle tissue is a fundamental component of the human body, responsible for movement, posture, and heat production. It transforms chemical energy (ATP) into mechanical energy, enabling force generation and motion.

• Nearly half of the body’s mass is muscle tissue.

• Muscle cells convert chemical potential energy into mechanical energy to generate force.

• ATP is essential for generating cycling cross bridges and tension in muscle fibers.

Types of Muscle Tissue

Muscle tissue is classified into three main types, each with distinct structure and function.

• Terminology: Prefixes such as myo-, mys-, and sarco- refer to muscle (e.g., sarcoplasm: muscle cell cytoplasm).

• Three types of muscle tissue:

  • Skeletal muscle

  • Cardiac muscle

  • Smooth muscle

• Only skeletal and smooth muscle cells are elongated and referred to as muscle fibers.

Skeletal Muscle

• Packaged into organs called skeletal muscles, attached to bones and skin.

• Fibers are the longest of all muscle types and have striations (stripes).

• Also called voluntary muscle (can be consciously controlled).

• Contracts rapidly; tires easily; powerful.

• Key words: skeletal, striated, voluntary.

Cardiac Muscle

• Found only in the heart; makes up the bulk of heart walls.

• Striated but involuntary (cannot be consciously controlled).

• Contracts at a steady rate due to the heart’s own pacemaker, but nervous system can increase rate.

Smooth Muscle

• Found in walls of hollow organs (e.g., stomach, urinary bladder, airways).

• Not striated; involuntary.

• Key words: visceral, nonstriated, involuntary.

Characteristics of Muscle Tissue

All muscle tissues share four main characteristics:

• Excitability (responsiveness): 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.

Muscle Functions

Muscles perform several essential functions in the body:

1. Produce movement (e.g., walking, digestion, pumping blood).

2. Maintain posture and body position.

3. Stabilize joints.

4. Generate heat as they contract.

Skeletal Muscle Anatomy

Skeletal muscle is an organ composed of various tissues, including nerve and blood supply, connective tissue sheaths, and attachments.

Nerve and Blood Supply

• Each muscle receives a nerve, artery, and veins.

• Consciously controlled skeletal muscle has nerves supplying every fiber to control activity.

• Contracting muscle fibers require large amounts of oxygen and nutrients and need waste products removed quickly.

Connective Tissue Sheaths

• Epimysium: Dense irregular connective tissue surrounding the entire muscle; may blend with fascia.

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

• Endomysium: Fine areolar connective tissue surrounding each muscle fiber.

Attachments

• Muscles span joints and attach to bones.

• Muscles attach to bone in at least two places:

  • Insertion: Attachment to movable bone.

  • Origin: Attachment to immovable or less movable bone.

• Attachments can be direct or indirect:

  • Direct (fleshy): Epimysium fused to periosteum of bone or perichondrium of cartilage.

  • Indirect: Connective tissue wrappings extend beyond muscle as rope-like tendon or sheet-like aponeurosis.

Muscle Fiber Microanatomy and Sliding Filament Model

Skeletal muscle fibers are long, cylindrical cells with multiple nuclei and specialized organelles for contraction.

• Sarcolemma: Muscle fiber plasma membrane.

• Sarcoplasm: Muscle fiber cytoplasm, containing glycosomes (for glycogen storage) and myoglobin (for O2 storage).

• Modified organelles:

  • Myofibrils

  • Sarcoplasmic reticulum

  • T tubules

Myofibrils

• Densely packed, rodlike elements; a single muscle fiber can contain thousands.

• Molecular composition of myofilaments:

  • Striations: Alternating dark (A bands) and light (I bands) regions.

    • A bands: Dark regions; include H zone (lighter region in middle), M line (protein myomesin bisecting H zone).

    • I bands: Light regions; Z disc (coin-shaped sheet of proteins on midline of I band).

  • Sarcomere: Smallest contractile unit; area between two Z discs.

  • Myofilaments: Arrangement of actin (thin) and myosin (thick) filaments within sarcomere.

    • Actin myofilaments: Thin filaments, anchored to Z discs.

    • Myosin myofilaments: Thick filaments, anchored at M line.

Molecular Composition of Myofilaments

• Thick filaments: Composed of myosin protein; myosin heads form cross bridges during contraction.

• Thin filaments: Composed of actin protein; actin subunits have binding sites for myosin head attachment.

• Tropomyosin and troponin: Regulatory proteins bound to actin.

• Other proteins: Titin (elastic filament), dystrophin (links thin filaments to sarcolemma), nebulin, myomesin, C proteins (bind filaments or sarcomeres together).

Sarcoplasmic Reticulum and T Tubules

The sarcoplasmic reticulum (SR) and T tubules play crucial roles in muscle contraction by regulating calcium ion storage and release.

• Sarcoplasmic reticulum: Network of smooth endoplasmic reticulum surrounding each myofibril; stores and releases Ca2+.

• T tubules: Tube-like invaginations of the sarcolemma that penetrate the cell’s interior at each A-I band junction.

• Triad relationships: Each T tubule is flanked by two terminal cisterns of the SR, forming a triad.

Sliding Filament Model of Contraction

The sliding filament model explains how muscle fibers contract by the sliding of actin and myosin filaments past each other.

• Contraction is initiated when cross bridges form between actin and myosin, generating force.

• Shortening occurs when tension generated by cross bridges exceeds opposing forces.

• During contraction, thin filaments slide past thick filaments, causing actin and myosin to overlap more.

• When nervous stimulation ceases, Ca2+ is pumped back into the SR, and contraction ends.

Steps of the Cross Bridge Cycle

1. Cross bridge formation: Myosin head attaches to actin filament.

2. Power (working) stroke: Myosin head pivots, pulling actin filament toward M line.

3. Cross bridge detachment: ATP attaches to myosin head, causing detachment from actin.

4. Cocking of myosin head: ATP hydrolysis "cocks" the myosin head for the next cycle.

Muscle Fiber Contraction: Excitation-Contraction Coupling

Excitation-contraction (E-C) coupling is the process by which an action potential in the sarcolemma leads to contraction of the muscle fiber.

• Action potential (AP) is propagated along the sarcolemma and down T tubules.

• Voltage-sensitive proteins in T tubules cause SR to release Ca2+ into the cytosol.

• Ca2+ binds to troponin, causing tropomyosin to move away from myosin-binding sites on actin.

• Myosin heads bind to actin, initiating contraction.

• When stimulation ends, Ca2+ is pumped back into the SR, and contraction ceases.

Role of Calcium in Muscle Contraction

• Calcium ions are essential for the interaction between actin and myosin.

• Increased Ca2+ leads to more muscle response; too much or too little can impair contraction.

Neuromuscular Junction and Action Potential

The neuromuscular junction (NMJ) is the site where a motor neuron communicates with a skeletal muscle fiber to initiate contraction.

• Each axon divides into branches, forming a NMJ or motor end plate with each muscle fiber.

• The axon terminal contains synaptic vesicles filled with the neurotransmitter acetylcholine (ACh).

• ACh binds to receptors on the muscle fiber, opening ion channels and generating an end plate potential.

• If threshold is reached, an action potential is generated and propagated along the sarcolemma.

Events at the Neuromuscular Junction

1. AP arrives at axon terminal.

2. Voltage-gated Ca2+ channels open, Ca2+ enters motor neuron.

3. Ca2+ entry causes release of ACh into synaptic cleft.

4. ACh binds to receptors, opening Na+ channels and generating end plate potential.

5. ACh is degraded by acetylcholinesterase, terminating the signal.

Generation of an Action Potential Across the Sarcolemma

• Resting sarcolemma is polarized (negative inside relative to outside).

• Depolarization: Na+ channels open, Na+ enters cell, making inside less negative.

• Repolarization: K+ channels open, K+ exits cell, restoring negative membrane potential.

• Refractory period: Muscle fiber cannot be stimulated until repolarization is complete.

Clinical Application: Rigor Mortis

• Rigor mortis occurs 3–4 hours after death as muscles begin to stiffen.

• Peak rigidity occurs about 12 hours postmortem.

• ATP is no longer synthesized, so Ca2+ cannot be pumped back into SR, resulting in sustained contraction.

• ATP is also needed for cross bridge detachment; without it, myosin heads remain bound to actin, causing stiffness until muscle proteins break down.

Summary Table: Types of Muscle Tissue

Type	Location	Striations	Control	Key Features
Skeletal	Attached to bones/skin	Yes	Voluntary	Longest fibers, rapid contraction, tires easily
Cardiac	Heart	Yes	Involuntary	Steady rate, pacemaker, nervous system modulation
Smooth	Walls of hollow organs	No	Involuntary	Visceral, nonstriated, slow sustained contractions