Chapter 12: Muscles – Structure, Function, and Physiology

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Muscle Overview

Types of Muscle Tissue

Muscle tissue is specialized for contraction and is essential for movement, posture, and various physiological functions. There are three main types of muscle tissue in the human body:

Skeletal Muscle: Voluntary, striated muscle attached to bones for movement and posture.
Cardiac Muscle: Involuntary, striated muscle found only in the heart, responsible for pumping blood.
Smooth Muscle: Involuntary, non-striated muscle found in walls of internal organs, responsible for movements such as peristalsis and vasoconstriction.

Microscopic comparison of skeletal, cardiac, and smooth muscle

Organization of Skeletal Muscle

Hierarchical Structure

Skeletal muscle is organized into a hierarchy of structures, from the whole muscle down to the molecular level:

Muscle: Surrounded by connective tissue, containing bundles of fascicles.
Fascicle: Bundle of muscle fibers (cells).
Muscle Fiber: Multinucleated cell containing myofibrils.
Myofibril: Composed of repeating units called sarcomeres, the contractile units of muscle.
Sarcomere: Contains thick (myosin) and thin (actin) filaments, along with regulatory and structural proteins.

Skeletal muscle structure from gross to microscopic Flowchart of skeletal muscle organization Muscle cross-section to myofibril structure

Muscle Fiber Structure

Sarcolemma: Plasma membrane of the muscle fiber.
T-tubules: Invaginations of the sarcolemma that transmit action potentials into the fiber.
Sarcoplasmic Reticulum (SR): Specialized endoplasmic reticulum that stores and releases Ca2+ for contraction.
Mitochondria: Provide ATP for contraction.

Ultrastructure of muscle fiber T-tubule and sarcoplasmic reticulum

Sarcomere Structure and Proteins

Sarcomere Organization

The sarcomere is the functional unit of muscle contraction, defined by the region between two Z-disks. It contains:

Z disk: Anchors thin filaments and defines sarcomere boundaries.
I band: Contains only thin filaments (actin).
A band: Contains the entire length of thick filaments (myosin), including regions of overlap with thin filaments.
H zone: Central region of A band with only thick filaments.
M line: Center of the sarcomere, where thick filaments are linked.

Sarcomere structure and protein arrangement Sarcomere bands and lines Detailed sarcomere structure Titin and nebulin in sarcomere

Key Sarcomeric Proteins

Actin: Forms thin filaments; provides binding sites for myosin.
Myosin: Motor protein forming thick filaments; interacts with actin for contraction.
Tropomyosin: Blocks myosin-binding sites on actin in resting muscle.
Troponin: Binds Ca2+ and moves tropomyosin to expose binding sites.
Titin: Provides elasticity and stabilizes myosin.
Nebulin: Helps align actin filaments.

Titin and nebulin alignment Sarcomere protein arrangement

Sliding Filament Theory of Muscle Contraction

Mechanism of Contraction

Muscle contraction occurs through the sliding of thin filaments past thick filaments, shortening the sarcomere without changing the length of the filaments themselves. This process is powered by ATP hydrolysis and regulated by Ca2+ binding to troponin.

ATP binds to myosin, causing it to release actin.
Myosin hydrolyzes ATP, cocks its head, and binds to a new position on actin.
Ca2+ binds to troponin, shifting tropomyosin and exposing myosin-binding sites on actin.
Myosin executes a power stroke, pulling actin toward the M line.
ADP and Pi are released; myosin remains bound until another ATP binds.

Sliding filament theory steps Molecular basis of contraction Power stroke and rigor state Sliding filament cycle

Excitation-Contraction Coupling

Sequence of Events

Excitation-contraction coupling links the action potential in the muscle fiber to the initiation of contraction:

Action potential arrives at the neuromuscular junction, releasing acetylcholine (ACh).
ACh binds to nicotinic receptors, causing Na+ influx and depolarization of the sarcolemma.
Action potential travels down T-tubules, activating DHP receptors.
DHP receptors mechanically open RyR channels on the SR, releasing Ca2+ into the sarcoplasm.
Ca2+ binds to troponin, initiating contraction.
Relaxation occurs when Ca2+ is pumped back into the SR by Ca2+-ATPase.

Excitation-contraction coupling Relaxation phase

Muscle Metabolism and Fatigue

Sources of ATP

Aerobic Respiration: Uses oxygen, glucose, and fatty acids to produce 30–32 ATP per glucose.
Anaerobic Metabolism: Fast, produces 2 ATP per glucose (lactic acid fermentation).
Phosphocreatine: Provides a rapid but short-term buffer for ATP (2–7 seconds).

Phosphocreatine structure

Muscle Fatigue

Central Fatigue: Originates in the CNS; associated with the perception of tiredness.
Peripheral Fatigue: Due to changes at the neuromuscular junction or within the muscle fiber (e.g., glycogen depletion, high Pi, K+ imbalance, ACh depletion).

Muscle Fiber Types

Classification by Contraction Speed and Metabolism

Fast-Twitch Glycolytic Fibers: Rapid contraction, fatigue quickly, rely on anaerobic metabolism.
Fast-Twitch Oxidative Fibers: Intermediate speed, use both aerobic and anaerobic metabolism.
Slow-Twitch Oxidative Fibers: Slow contraction, resistant to fatigue, high in mitochondria and myoglobin, suited for endurance.

Fast and slow twitch muscle fibers

Control of Muscle Force

Motor Units and Recruitment

Motor Unit: A single motor neuron and all the muscle fibers it innervates; all fibers contract together when the neuron fires.
Recruitment: Increasing the number of active motor units increases muscle force; slow-twitch fibers are recruited first, followed by fast-twitch as needed.

Motor unit recruitment

Summation and Tetanus

Summation: Increased frequency of stimulation leads to greater force as contractions build on each other.
Tetanus: Sustained maximal contraction when stimuli are so frequent that relaxation does not occur.

Summation and tetanus

Muscle Mechanics

Isotonic vs. Isometric Contractions

Isotonic Contraction: Muscle changes length (shortens or lengthens) while tension remains constant; produces movement.
Isometric Contraction: Muscle develops tension without changing length; stabilizes joints and maintains posture.

Isotonic and isometric contractions

Levers and Body Movement

Bones act as levers and joints as fulcrums to amplify force or speed of movement.
Muscle origin is proximal, insertion is distal; arrangement affects mechanical advantage.

Levers and fulcrum in the arm

Smooth and Cardiac Muscle

Smooth Muscle

Found in walls of hollow organs; contracts slowly and can sustain contractions with little fatigue.
Cells are spindle-shaped, have a single nucleus, and lack striations.
Contraction can be initiated by electrical, chemical, or mechanical signals; Ca2+ binds to calmodulin (not troponin).

Smooth muscle structure

Cardiac Muscle

Found only in the heart; striated like skeletal muscle but involuntary.
Cells are branched, have a single nucleus, and are connected by intercalated discs (containing gap junctions and desmosomes).
Can generate action potentials spontaneously (pacemaker cells); regulated by autonomic nervous system and hormones.

Cardiac muscle structure

Summary Table: Comparison of Muscle Types

Feature	Skeletal Muscle	Cardiac Muscle	Smooth Muscle
Striations	Yes	Yes	No
Control	Voluntary	Involuntary	Involuntary
Cell Shape	Long, cylindrical, multinucleated	Branched, single nucleus	Spindle-shaped, single nucleus
Location	Attached to bones	Heart	Walls of hollow organs
Special Features	Motor units, rapid contraction	Intercalated discs, pacemaker cells	Gap junctions, slow sustained contraction