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Muscle

Primary tissue of the heart, hollow organs, and skeletal system; name derives from Latin for “little mouse.”

Myo / Mys

Prefixes meaning “muscle,” as in myofilament or myofibril.

Sarco

Prefix meaning “flesh”; appears in terms related to muscle cytology (e.g., sarcolemma, sarcoplasm).

Sarcolemma

The plasma membrane of a muscle cell.

Sarcoplasm

The cytoplasm of a muscle cell.

Contractility

Property of muscle that allows it to shorten when myofilaments (actin, myosin) slide past each other.

Excitability

Ability of muscle to respond to nerve signals by producing electrical impulses along the sarcolemma.

Extensibility

Ability of muscle to be stretched by opposing muscles or substances within an organ.

Elasticity

Ability of muscle to recoil to its resting length after being stretched.

Sphincter

Ring-like muscle that opens to allow passage of a substance and contracts to close a passageway.

Muscle Tone

Continuous, low-level contraction that stabilizes posture and synovial joints.

Skeletal Muscle Tissue

Striated, multinucleate fibers under voluntary control; makes up ~40 % of body weight.

Cardiac Muscle Tissue

Striated, uni- or bi-nucleate fibers in heart walls; involuntary control.

Smooth Muscle Tissue

Non-striated, uninucleate fibers in walls of hollow organs; involuntary control.

Visceral Muscle

Collective term for cardiac and smooth muscle found in visceral organs and innervated involuntarily.

Epimysium

Dense regular connective tissue sheath surrounding an entire muscle.

Perimysium

Fibrous connective tissue sheath surrounding each fascicle.

Endomysium

Fine connective tissue sheath enclosing each individual muscle fiber.

Fascicle

Bundle of muscle fibers within a skeletal muscle.

Muscle Fiber

Elongated, multinucleate skeletal muscle cell; contains myofibrils and is striated.

Myofibril

Rod-like contractile organelle composed of repeating sarcomeres; fills ~80 % of a muscle fiber.

Sarcomere

Smallest contractile unit of a muscle; segment between two Z discs.

Z Disc

Boundary line of a sarcomere where thin filaments anchor.

Thin Filament (Actin)

Filament extending from Z disc toward sarcomere center; composed mainly of actin.

Thick Filament (Myosin)

Filament in sarcomere center; contains myosin molecules with ATPase heads.

A Band

Dark band corresponding to full length of thick filaments, including overlapping thin filaments.

I Band

Light band containing only thin filaments; spans two adjacent sarcomeres.

H Zone

Central region of the A band where thick filaments have no overlapping thin filaments.

M Line

Line in center of H zone that holds adjacent thick filaments together.

Titin

Elastic protein that anchors thick filaments to Z disc and provides recoil.

Sarcoplasmic Reticulum (SR)

Specialized smooth ER that stores and releases Ca²⁺ ions to trigger contraction.

T-Tubule

Deep invagination of sarcolemma that conducts action potentials into muscle fiber interior.

Triad

Complex consisting of one T-tubule flanked by two terminal cisterns of SR.

Concentric Contraction

Type of contraction where the muscle shortens to produce work (e.g., lifting phase of curl).

Eccentric Contraction

Contraction that generates force while muscle lengthens, acting as a brake (e.g., lowering phase).

Isometric Contraction

Muscle contracts without changing length (e.g., holding a plank).

Sliding Filament Mechanism

Process in which myosin heads pull actin filaments toward sarcomere center, shortening muscle.

Motor Neuron

Nerve cell that innervates skeletal muscle fibers.

Neuromuscular Junction

Site where a motor neuron’s terminal bouton meets a muscle fiber.

Terminal Bouton (Axon Terminal)

Enlarged ending of an axon storing neurotransmitter vesicles.

Synaptic Cleft

Small gap between terminal bouton and sarcolemma at a neuromuscular junction.

Acetylcholine (ACh)

Neurotransmitter released at neuromuscular junctions to excite muscle fibers.

Motor Unit

A single motor neuron and all the muscle fibers it innervates.

Slow Oxidative Fiber

“Red” muscle fiber that contracts slowly, resists fatigue, and generates ATP aerobically.

Fast Glycolytic Fiber

“White” muscle fiber that contracts rapidly, fatigues quickly, and produces ATP anaerobically.

Fast Oxidative Fiber

Intermediate fiber that contracts quickly and relies mainly on aerobic metabolism.

Origin

Less movable bone attachment point of a skeletal muscle.

Insertion

More movable bone attachment point of a skeletal muscle.

Caveolae

Sarcolemmal infoldings in smooth muscle that function similarly to T-tubules.

Fascicle

A bundle of skeletal muscle fibers wrapped by connective tissue.

Circular muscle

Muscle whose fascicles form concentric rings around an opening; acts as a sphincter (e.g., orbicularis oculi).

Convergent muscle

Muscle with broad origin and fascicles that converge toward a single tendon of insertion (e.g., pectoralis major).

Parallel muscle

Muscle whose fascicles run parallel to the long axis; includes fusiform (biceps brachii) and straplike (sartorius).

Pennate muscle

Muscle with short fascicles attaching obliquely to a long tendon; can be uni-, bi-, or multipennate (e.g., deltoid).

Unipennate

Pennate muscle with fascicles on one side of the tendon (e.g., extensor digitorum longus).

Bipennate

Pennate muscle with fascicles on both sides of the tendon (e.g., rectus femoris).

Multipennate

Pennate muscle with fascicles attaching to one large central tendon from many directions (e.g., deltoid).

Agonist (prime mover)

Muscle chiefly responsible for producing a particular movement.

Antagonist

Muscle that opposes or reverses the action of an agonist.

Synergist

Muscle that aids a prime mover by adding force or reducing undesirable movements.

Muscle compartment

Group of muscles in a limb surrounded by deep fascia that share similar functions, blood supply, and innervation.

Anterior arm compartment

Contains elbow flexors (biceps brachii, brachialis); innervated by musculocutaneous nerve.

Posterior arm compartment

Contains elbow extensors (triceps brachii); innervated by radial nerve.

Anterior forearm compartment

Houses wrist/digital flexors and pronators; innervated by median or ulnar nerve.

Posterior forearm compartment

Houses wrist/digital extensors; innervated by radial nerve.

Anterior thigh compartment

Contains hip flexors/knee extensors; innervated by femoral nerve.

Posterior thigh compartment

Contains hip extensors/knee flexors; innervated by tibial branch of sciatic nerve.

Medial thigh compartment

Contains thigh adductors; innervated by obturator nerve.

Anterior leg compartment

Contains ankle dorsiflexors/digital extensors; innervated by deep fibular nerve.

Posterior leg compartment

Contains ankle plantar flexors/digital flexors; innervated by tibial nerve.

Lateral leg compartment

Contains evertors/plantar flexors of foot; innervated by superficial fibular nerve.

Deltoid

Multipennate shoulder muscle; prime mover of arm abduction.

Pectoralis major

Convergent chest muscle; prime mover of arm flexion and adduction.

Latissimus dorsi

Broad back muscle; prime mover of arm extension and adduction.

Rotator cuff

Group of four muscles (supraspinatus, infraspinatus, teres minor, subscapularis) stabilizing the glenohumeral joint.

Biceps brachii

Anterior arm muscle; flexes elbow and supinates forearm.

Triceps brachii

Posterior arm muscle; prime mover of elbow extension.

Brachialis

Deep to biceps; strong elbow flexor.

Brachioradialis

Forearm muscle on lateral radius; flexes elbow.

Pronator teres

Superficial anterior forearm muscle; pronates forearm.

Pronator quadratus

Deep anterior forearm muscle; primary forearm pronator.

Supinator

Deep posterior forearm muscle; assists in forearm supination.

Flexor carpi radialis

Anterior forearm muscle; flexes and abducts hand at wrist.

Flexor carpi ulnaris

Anterior forearm muscle; flexes and adducts hand at wrist.

Palmaris longus

Anterior forearm muscle; tenses palmar fascia.

Flexor digitorum superficialis

Anterior forearm muscle; flexes wrist and middle phalanges of fingers 2–5.

Flexor digitorum profundus

Deep anterior forearm muscle; flexes distal interphalangeal joints.

Flexor pollicis longus

Deep anterior forearm muscle; flexes thumb.

Extensor digitorum

Superficial posterior forearm muscle; prime mover of finger extension.

Extensor carpi radialis longus

Posterior forearm muscle; extends and abducts wrist.

Extensor carpi ulnaris

Posterior forearm muscle; extends and adducts wrist.

Abductor pollicis longus

Deep posterior forearm muscle; abducts and extends thumb.

Extensor pollicis brevis/longus

Deep posterior muscles; extend thumb joints.

Extensor indicis

Deep posterior forearm muscle; extends index finger.

Flexor retinaculum

Fibrous band on anterior wrist anchoring flexor tendons.

Extensor retinaculum

Fibrous band on posterior wrist anchoring extensor tendons.

Thenar muscles

Intrinsic hand muscles at thumb base (abductor, flexor, opponens pollicis, adductor pollicis).

Hypothenar muscles

Intrinsic hand muscles at little-finger base (abductor, flexor, opponens digiti minimi).

Midpalmar muscles

Intrinsic hand group including lumbricals and palmar/dorsal interossei for fine finger movements.

Orbicularis oris

Circular muscle around the mouth; closes and protrudes lips.