Muscles and Muscle Tissue: Structure, Function, and Physiology

Muscle Tissue Overview

Introduction to Muscle Tissue

Muscle tissue comprises nearly half of the body's mass and is essential for movement, posture, and heat generation. Muscles convert chemical energy (ATP) into mechanical energy, enabling forceful contractions.

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

• Three types of muscle tissue: Skeletal, Cardiac, Smooth.

• Muscle fibers: Elongated muscle cells found in skeletal and smooth muscle, but not cardiac muscle.

Types of Muscle Tissue

Characteristic	Skeletal	Cardiac	Smooth
Body Location	Attached to bones or skin	Walls of the heart	Walls of hollow organs (except heart)
Cell Shape & Appearance	Long, cylindrical, multinucleate, striated	Branching chains, uni- or binucleate, striated	Spindle-shaped, uninucleate, non-striated
Control	Voluntary	Involuntary	Involuntary

Characteristics of Muscle Tissue

• 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.

Functions of Muscle Tissue

• Produce movement: Locomotion and manipulation (e.g., walking, digestion, pumping blood).

• Maintain posture and body position.

• Stabilize joints.

• Generate heat: As muscles contract.

Skeletal Muscle Structure

Organization and Connective Tissue Sheaths

Skeletal muscle is an organ composed of muscle fibers, nerves, blood vessels, and connective tissue sheaths. These sheaths support and reinforce the muscle.

• Epimysium: Dense irregular connective tissue surrounding the entire muscle.

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

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

Muscle Attachments

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

• Indirect attachment: Connective tissue wrappings extend beyond muscle as a tendon or aponeurosis.

Structural Organization

Level	Description	Connective Tissue
Muscle (organ)	Hundreds to thousands of muscle cells, blood vessels, nerves	Epimysium
Fascicle	Bundle of muscle cells	Perimysium
Muscle fiber (cell)	Elongated, multinucleate, striated cell	Endomysium

Skeletal Muscle Fiber Microanatomy

Microscopic Anatomy

Skeletal muscle fibers are long, cylindrical cells with multiple nuclei. The sarcolemma is the plasma membrane, and the sarcoplasm is the cytoplasm, containing glycosomes (glycogen storage) and myoglobin (O2 storage).

• Myofibrils: Densely packed, rodlike elements responsible for muscle contraction and striations.

• Sarcoplasmic reticulum (SR): Smooth ER that stores and releases Ca2+.

• T tubules: Invaginations of the sarcolemma that conduct impulses deep into the muscle fiber.

Sarcomere and Myofilaments

The sarcomere is the smallest contractile unit of muscle, defined as the region between two Z discs. It contains thick (myosin) and thin (actin) filaments arranged in a precise pattern.

• A bands: Dark regions containing thick filaments.

• I bands: Light regions containing thin filaments.

• H zone: Lighter region in the middle of the A band.

• M line: Line of protein that bisects the H zone.

• Z disc: Sheet of proteins that anchors thin filaments.

Molecular Composition of Myofilaments

• Thick filaments: Composed of myosin molecules with heads that form cross bridges during contraction.

• Thin filaments: Composed of actin, tropomyosin, and troponin. Actin has binding sites for myosin heads.

• Elastic filaments: Composed of titin, which helps maintain sarcomere structure and elasticity.

• Dystrophin: Links thin filaments to the sarcolemma.

Muscle Contraction Physiology

Sliding Filament Model of Contraction

Muscle contraction occurs when thin filaments slide past thick filaments, increasing their overlap. This process is powered by cross bridge cycling between actin and myosin.

• During contraction: I bands shorten, Z discs move closer, H zones disappear, and A bands move closer together.

Neuromuscular Junction and Muscle Fiber Excitation

The neuromuscular junction (NMJ) is the site where a motor neuron stimulates a muscle fiber. The neurotransmitter acetylcholine (ACh) is released, triggering an action potential in the muscle fiber.

• Ion channels: Chemically gated (ACh receptors) and voltage-gated channels are essential for action potential propagation.

Generation of an Action Potential

1. End plate potential: ACh binding opens ion channels, causing Na+ influx and local depolarization.

2. Depolarization: If threshold is reached, voltage-gated Na+ channels open, generating an action potential.

3. Repolarization: Na+ channels close, K+ channels open, restoring resting membrane potential.

Excitation-Contraction (E-C) Coupling

E-C coupling links the action potential to muscle contraction. The AP travels along the sarcolemma and T tubules, triggering Ca2+ release from the SR, which initiates contraction.

Cross Bridge Cycle

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

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

3. Cross bridge detachment: ATP binds to myosin, causing detachment.

4. Cocking of myosin head: ATP hydrolysis re-energizes the myosin head.

Whole Muscle Contraction

Motor Units and Muscle Twitch

• Motor unit: A motor neuron and all the muscle fibers it innervates.

• Muscle twitch: Response of a muscle to a single stimulus, consisting of latent, contraction, and relaxation phases.

Graded Muscle Contractions

• Temporal (wave) summation: Increased frequency of stimulation increases contraction strength.

• Recruitment: Increasing stimulus strength recruits more motor units for stronger contractions.

Isotonic and Isometric Contractions

• Isotonic contraction: Muscle changes length and moves a load (concentric: shortens; eccentric: lengthens).

• Isometric contraction: Muscle tension increases but does not exceed load; muscle does not change length.

Energy for Muscle Contraction

ATP Regeneration Pathways

• Direct phosphorylation: Creatine phosphate donates phosphate to ADP to form ATP.

• Anaerobic pathway: Glycolysis and lactic acid formation (no oxygen required).

• Aerobic pathway: Glycolysis followed by aerobic respiration in mitochondria (requires oxygen).

Equation for direct phosphorylation:

$\text{Creatine~phosphate} + \text{ADP} \rightarrow \text{Creatine} + \text{ATP}$

Muscle Fatigue and Recovery

• Muscle fatigue: Inability to contract despite stimulation, often due to ionic imbalances, increased inorganic phosphate, or decreased ATP.

• Excess postexercise oxygen consumption (EPOC): Oxygen required to restore muscle to resting state.

Factors Affecting Muscle Contraction

Force, Velocity, and Duration

• Force: Depends on frequency of stimulation, number of fibers recruited, size of fibers, and degree of stretch.

• Velocity and duration: Influenced by muscle fiber type, load, and recruitment.

Fiber Type	Speed	ATP Pathway	Fatigue Resistance	Best Suited For
Slow Oxidative	Slow	Aerobic	High	Endurance (posture, marathon)
Fast Oxidative	Fast	Aerobic (some anaerobic)	Intermediate	Sprinting, walking
Fast Glycolytic	Fast	Anaerobic glycolysis	Low	Short-term intense movements

Smooth Muscle

Structure and Function

• Found in walls of hollow organs (except heart).

• Spindle-shaped, uninucleate, non-striated cells.

• Organized in longitudinal and circular layers for peristalsis.

• Innervated by autonomic nervous system; uses varicosities instead of neuromuscular junctions.

• Contraction is slow, sustained, and energy-efficient.

Comparison Table: Skeletal, Cardiac, and Smooth Muscle

Characteristic	Skeletal	Cardiac	Smooth
Control	Voluntary	Involuntary	Involuntary
Striations	Yes	Yes	No
Cell Shape	Long, cylindrical	Branched	Spindle-shaped
Connective Tissue	Epimysium, perimysium, endomysium	Endomysium	Endomysium
Calcium Regulation	Troponin	Troponin	Calmodulin
Contraction Speed	Slow to fast	Slow	Very slow

Development and Clinical Aspects

Muscle Development and Regeneration

• All muscle tissues develop from embryonic mesoderm cells called myoblasts.

• Skeletal muscle fibers form by fusion of myoblasts; cardiac and smooth muscle do not fuse but develop gap junctions.

• Regeneration: Skeletal muscle has limited regenerative ability; smooth muscle regenerates throughout life.

Clinical Conditions

• Duchenne muscular dystrophy (DMD): X-linked disorder causing muscle degeneration due to defective dystrophin.

• Myasthenia gravis: Autoimmune disease where ACh receptors are destroyed, leading to muscle weakness.

• Rigor mortis: Postmortem muscle stiffening due to lack of ATP for cross bridge detachment.

• Disuse atrophy: Muscle wasting due to inactivity or loss of neural stimulation.