Muscles & 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 various physiological functions. Muscles transform chemical energy (ATP) into directed mechanical energy, enabling force generation and movement.

• Types of Muscle Tissue: Skeletal, cardiac, and smooth muscle.

• Key Terminologies: Myo, mys, and sarco are prefixes for muscle-related terms (e.g., sarcoplasm is muscle cell cytoplasm).

• Muscle Functions: Movement, posture maintenance, joint stabilization, and heat generation.

Types of Muscle Tissue

Skeletal Muscle

Skeletal muscle tissue is attached to bones and skin, responsible for voluntary movements. It is striated and can contract rapidly but tires easily.

• Features: Long, cylindrical, multinucleated fibers; striations; voluntary control.

• Functions: Locomotion, posture, manipulation of the environment.

Cardiac Muscle

Cardiac muscle tissue is found only in the heart, forming the bulk of heart walls. It is striated but involuntary, contracting at a steady rate due to the heart's pacemaker.

• Features: Branched fibers, single nucleus, striations, involuntary control.

• Functions: Pumping blood throughout the body.

Smooth Muscle

Smooth muscle tissue is located in the walls of hollow organs (e.g., stomach, bladder, airways). It is non-striated and involuntary.

• Features: Spindle-shaped cells, single nucleus, no striations, involuntary control.

• Functions: Movement of substances through internal body channels (e.g., peristalsis).

Characteristics of Muscle Tissue

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 after stretching.

Organization of Skeletal Muscle

Connective Tissue Sheaths

Skeletal muscle is organized into bundles surrounded by connective tissue sheaths:

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

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

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

Attachments

• Origin: Attachment to immovable or less movable bone.

• Insertion: Attachment to movable bone.

• Direct Attachment: Epimysium fused to periosteum or perichondrium.

• Indirect Attachment: Connective tissue wrappings extend as tendon or aponeurosis.

Microscopic Anatomy of Skeletal Muscle

Muscle Fiber Structure

• Muscle Fiber: Also called myofiber; long, cylindrical cell with multiple nuclei.

• Sarcolemma: Muscle cell membrane.

• Sarcoplasm: Cytoplasm of muscle cell, contains glycosomes (glycogen storage) and myoglobin (oxygen storage).

Myofibrils and Sarcomeres

• Myofibrils: Densely packed, rodlike elements; account for ~80% of cell volume.

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

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

Myofilaments

• Thick Filaments: Composed of myosin protein.

• Thin Filaments: Composed of actin, tropomyosin, and troponin.

• Titin: Elastic protein that holds thick filaments in place and allows recoil.

• Dystrophin: Links thin filaments to sarcolemma; important for muscle integrity.

Neuromuscular Junction and Muscle Contraction

Neuromuscular Junction

• Motor Neuron: Nerve cell that stimulates muscle fiber.

• Axon Terminal: End of motor neuron; releases neurotransmitter acetylcholine (ACh).

• Synaptic Cleft: Space between axon terminal and muscle fiber.

• Motor End Plate: Region of sarcolemma with ACh receptors.

Events at the Neuromuscular Junction

1. Action potential arrives at axon terminal.

2. Voltage-gated calcium channels open; Ca2+ enters terminal.

3. ACh is released into synaptic cleft.

4. ACh binds to receptors on sarcolemma, opening Na+ channels.

5. Local depolarization (end plate potential) occurs.

6. ACh is broken down by acetylcholinesterase.

Action Potential Generation

• Depolarization: Na+ influx makes inside of cell less negative.

• Repolarization: K+ efflux restores resting membrane potential.

• Refractory Period: Time during which muscle fiber cannot be restimulated.

Excitation-Contraction Coupling

Mechanism

Excitation-contraction coupling links the action potential in the sarcolemma to the sliding of myofilaments, resulting in muscle contraction.

• Action potential travels along sarcolemma and down T tubules.

• Voltage-sensitive proteins trigger Ca2+ release from sarcoplasmic reticulum (SR).

• Ca2+ binds to troponin, moving tropomyosin and exposing actin binding sites.

• Myosin heads bind to actin, forming cross bridges and initiating contraction.

Cross Bridge Cycle

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

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

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

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

Whole Muscle Contraction

Motor Units

• Motor Unit: One motor neuron and all muscle fibers it innervates.

• Smaller motor units allow fine control; larger units generate more force.

• Motor units contract asynchronously to prevent fatigue.

Muscle Twitch

• Phases: Latent period (excitation-contraction coupling), contraction (cross bridge formation), relaxation (Ca2+ reentry into SR).

• Different muscles have varying twitch durations based on metabolic properties.

Graded Muscle Responses

• Temporal Summation: Increased frequency of stimulation leads to greater force.

• Recruitment: Increased stimulus strength recruits more motor units.

• Tetanus: Sustained contraction due to high-frequency stimulation.

Types of Muscle Contractions

Isotonic vs. Isometric Contractions

• Isotonic Contraction: Muscle changes length and moves a load.

• Concentric: Muscle shortens (e.g., lifting a weight).

• Eccentric: Muscle lengthens while generating force (e.g., lowering a weight).

• Isometric Contraction: Muscle tension increases but length does not change (e.g., holding a weight steady).

Energy for Muscle Contraction

ATP Sources

• Direct Phosphorylation: Creatine phosphate (CP) donates phosphate to ADP to form ATP.

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

• Aerobic Respiration: Glucose and fatty acids are broken down in mitochondria to produce ATP (requires oxygen).

Equation for Direct Phosphorylation:

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

Muscle Fatigue and Recovery

Muscle Fatigue

• Physiological inability to contract despite stimulation.

• Caused by ionic imbalances, accumulation of inorganic phosphate, decreased ATP, and other factors.

Recovery

• Excess post-exercise oxygen consumption (EPOC) restores muscle to pre-exercise state.

• Replenishes oxygen reserves, converts lactic acid to pyruvate, replaces glycogen stores, and resynthesizes ATP and CP.

Factors Affecting Muscle Contraction

• Frequency of stimulation

• Number of muscle fibers recruited

• Relative size of muscle fibers (hypertrophy)

• Degree of muscle stretch

Types of Skeletal Muscle Fibers

Classification

• Slow Oxidative (Type I): Slow contraction, high endurance, rich in mitochondria and myoglobin.

• Fast Oxidative (Type IIa): Fast contraction, moderate endurance, aerobic metabolism.

• Fast Glycolytic (Type IIb): Fast contraction, low endurance, anaerobic metabolism, large glycogen stores.

Functional Implications

• Posture muscles: Slow oxidative fibers.

• Leg muscles: Fast oxidative fibers.

• Eye muscles: Fast glycolytic fibers.

Adaptation to Exercise

Endurance (Aerobic) Exercise

• Increases capillaries, mitochondria, and myoglobin synthesis.

• Improves endurance, strength, and resistance to fatigue.

Resistance Exercise

• Leads to hypertrophy (increase in fiber size), more mitochondria, myofilaments, glycogen stores, and connective tissue.

• Increases muscle strength and size.

Muscle Atrophy and Aging

• Muscles must be active to remain healthy; immobilization or loss of neural stimulation leads to atrophy.

• Muscle strength can decline rapidly; severe atrophy may be irreversible.

• With aging, connective tissue increases and muscle fibers decrease, leading to sarcopenia.

Cardiac and Smooth Muscle

Cardiac Muscle

• Found only in the heart; striated, involuntary, branched fibers, single nucleus.

• Contracts at a steady rate; regulated by pacemaker and nervous system.

Smooth Muscle

• Found in walls of hollow organs; non-striated, involuntary, spindle-shaped cells, single nucleus.

• Arranged in longitudinal and circular layers; contraction causes peristalsis.

• Connected by gap junctions; contracts in response to neural, hormonal, or chemical stimuli.

• Contains calmodulin (not troponin) for Ca2+ binding.

Comparison of Skeletal, Cardiac, and Smooth Muscle

Structural and Functional Differences

Feature	Skeletal Muscle	Cardiac Muscle	Smooth Muscle
Location	Attached to bones	Heart walls	Walls of hollow organs
Striations	Present	Present	Absent
Control	Voluntary	Involuntary	Involuntary
Cell Shape	Long, cylindrical	Branched	Spindle-shaped
Nuclei	Multiple	Single	Single
Special Features	T-tubules, sarcomeres	Intercalated discs, pacemaker cells	Gap junctions, calmodulin

Developmental Aspects of Muscle

• Muscle tissues develop from embryonic myoblasts.

• Skeletal muscle cells form by fusion of myoblasts (multinucleated).

• Cardiac and smooth muscle myoblasts do not fuse; develop gap junctions.

• Muscle mass differs between sexes due to testosterone; strength per unit mass is similar.

• With age, muscle fibers decrease and connective tissue increases (sarcopenia).

*Additional info: Some explanations and definitions have been expanded for clarity and completeness. Table entries and some details inferred from standard textbook knowledge where original content was fragmented or unclear.*