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Resting Membrane Potential

The difference in electrical voltage between the inside and outside of a neuron, where the outside of the plasma membrane is more positively charged than the inside.

Action Potential (AP)

A temporary reversal in resting membrane potential where voltage rapidly rises (depolarization) and falls (repolarization), propagating down an axon.

Axon Hillock

The region of a neuron where incoming stimuli accumulate and an action potential is generated if the threshold overcomes the resting potential.

Saltatory Conduction

The mode of electrical propagation in myelinated neurons where the action potential jumps from one node of Ranvier to the next, significantly speeding up signal conduction.

Neuromuscular Junction (NMJ)

A specialized synapse, also called a motor end plate, where a nerve fiber meets a target skeletal muscle fiber.

Synaptic Cleft

The narrow extracellular space separating the presynaptic axon terminal and the postsynaptic muscle fiber membrane.

Acetylcholine (ACh)

The neurotransmitter stored in synaptic vesicles of the motor axon terminal that is released into the synaptic cleft to stimulate muscle cell depolarization.

Acetylcholinesterase (AChE)

The enzyme released into the synaptic cleft that breaks down acetylcholine to terminate stimulation and allow the muscle to relax.

Power Stroke

The phase of muscle contraction where the myosin head releases $\text{ADP}$ and $\text{P}_i$ and flexes into a bent position, tugging the actin thin filament toward the center of the sarcomere.

Recovery Stroke

The resetting phase of the cross-bridge cycle where myosin binds a new $\text{ATP}$, detaches from actin, and hydrolyzes $\text{ATP}$ to recock its head into position.

Botulinum Toxin

A toxin produced by Clostridium botulinum that inhibits acetylcholine release at the NMJ, causing flaccid paralysis that sets in at approximately two weeks, with partial recovery at three months and complete recovery at six months.

Tetanus Toxin

A toxin produced by Clostridium tetani, second only to Botulinum toxin D in lethality, that blocks inhibitory neurotransmitter release and causes rigid/spastic paralysis.

Latrotoxin

A toxin found in black widow spider venom that induces massive release of acetylcholine from presynaptic neurons, causing pain, muscle contraction, and potential paralysis.

Tetradotoxin

A toxin found in pufferfish that blocks neuronal ion channels and prevents membrane depolarization.

Rigor Mortis

The hardening of muscles and stiffening of the body beginning $3$ to $4\text{ hours}$ after death, peaking at approximately $12\text{ hours}$, and diminishing over $48$ to $60\text{ hours}$ due to calcium leakage and lack of $\text{ATP}$ to break cross-bridges.

Sarcolemma

The plasma membrane surrounding a skeletal muscle fiber (myocyte).

Sarcoplasm

The cytoplasm of a muscle fiber.

Sarcoplasmic Reticulum (SR)

The specialized smooth endoplasmic reticulum in a muscle fiber that stores calcium ions ($\text{Ca}^{2+}$).

Transverse Tubules (T Tubules)

Tubular invaginations of the sarcolemma that conduct electrical action potentials deep into the interior of the muscle fiber.

Terminal Cisternae

Enlarged regions of the sarcoplasmic reticulum that store calcium ions ($\text{Ca}^{2+}$) and release them upon T-tubule signaling.

Triad

A three-part structure in skeletal muscle consisting of a central transverse tubule flanked on each side by a terminal cisterna of the sarcoplasmic reticulum.

Sarcomere

The smallest contractile unit of a muscle fiber, comprised of alternating dark and light bands of myofilaments.

Thick Filament

A myofilament composed of two woven chains of myosin proteins with protruding heads that interact with actin.

Thin Filament

A myofilament composed of chains of actin associated with regulatory proteins tropomyosin and troponin.

Tropomyosin

A regulatory protein that physically covers the active myosin-binding sites on actin when a muscle is relaxed.

Troponin

A small calcium-binding protein attached to tropomyosin that shifts tropomyosin away from actin active sites when bound by $\text{Ca}^{2+}$.

Origin

The usually stationary, proximal attachment point of a skeletal muscle to a bone.

Insertion

The distal attachment point of a skeletal muscle attached to the moving bone, which is pulled toward the origin during contraction.

Prime Mover (Agonist)

The principal muscle primarily responsible for producing a specific movement at a joint.

Antagonist

A muscle that opposes or reverses the action of a prime mover.

Synergist

A muscle that assists the prime mover and helps stabilize supporting muscles around a joint.

Contractility

The functional property of muscle tissue that enables it to pull on attachment points and shorten with force.

Extensibility

The functional property of muscle tissue that enables it to stretch or extend.

Elasticity

The functional property of muscle tissue that allows it to return to its original resting length when relaxed.

Skeletal Muscle Tissue

Striated, multinucleated muscle tissue under voluntary control that attaches to bones.

Cardiac Muscle Tissue

Striated, branched, involuntary muscle tissue found in the heart, possessing 1 to 2 nuclei per fiber and intercalated discs connecting cells.

Intercalated Disc

A specialized junction connecting adjacent cardiac muscle cells that allows wave-like electrical and mechanical contractions across the heart.

Smooth Muscle Tissue

Non-striated, involuntary muscle tissue with a single nucleus per cell, found surrounding internal organs such as the intestines.

Circular Muscle

A muscle architecture arrangement (also called a sphincter) where relaxation enlarges the opening and contraction shrinks and closes the opening.

Convergent Muscle

A muscle design whose origin is wider than its point of insertion (e.g., pectoralis major).

Parallel Muscle

A muscle architecture whose fibers run parallel to each other (also known as strap muscles), representing the majority of skeletal muscles in the human body.

Pennate Muscle

A muscle architecture characterized by numerous muscle fibers per unit blending into a central tendon running the length of the muscle; very strong but tires easily.

Fusiform Muscle

A spindle-shaped muscle architecture where the central muscle belly is wider than the origin and insertion (e.g., biceps brachii).

Masseter

The main muscle used for chewing, which elevates the mandible to close the mouth.

Temporalis

A muscle of mastication that retracts the mandible.

Sternocleidomastoid

The prime mover of neck flexion, which also rotates and flexes the head.

Extrinsic Eye Muscles

A group of six muscles located within the eye socket that control eye movement and cannot be viewed on the visible eyeball.

Pelvic Girdle

The skeletal and muscular structure of the lower body designed to stabilize and support body weight, having a reduced range of motion compared to the shoulder.

Iliopsoas Group

A major muscle group consisting of the psoas major, psoas minor, and iliacus that originates on the pelvic girdle and inserts on the femur to facilitate movement.

Gluteal Muscles

A group consisting of the gluteus maximus, gluteus medius, and gluteus minimus, representing some of the largest and most powerful muscles in the human body.

Neuromuscular Disease

Any disorder that compromises synaptic transmission between a motor neuron and a muscle cell, typically arising from genetic mutations or autoimmune reactions.

Myasthenia Gravis (MG)

An autoimmune disorder characterized by autoantibodies directed against acetylcholine receptors ($\text{AChR}$) in 80% of cases or postsynaptic muscle-specific kinase ($\text{MuSK}$) in 0–10% of cases, leading to impaired neuromuscular transmission.

Neonatal Myasthenia Gravis

A transient autoimmune disorder affecting 1 in 8 infants born to mothers diagnosed with MG, caused by transplacental transfer of $\text{AChR}$ antibodies and typically resolving within about three months.

Congenital Myasthenic Syndromes (CMS)

A group of disorders functionally analogous to myasthenia gravis but distinguished fundamentally by their hereditary, genetic origin rather than autoimmune mechanisms.

Duchenne Muscular Dystrophy (DMD)

An X-linked genetic disorder caused by the absence of the structural protein dystrophin at the neuromuscular junction; affects 1 in 3,600–6,000 males, causing muscle degeneration, calf hypertrophy, elevated creatine kinase, and frequently death by age 30.

Gowers Sign

A characteristic clinical sign of proximal muscle weakness where a patient uses their hands to push on their legs to walk themselves up into an erect standing position.

Amyotrophic Lateral Sclerosis (ALS)

A progressive motor neuron disease (Lou Gehrig’s disease) caused by degeneration of upper and lower motor neurons responsible for voluntary movement, characterized by weakness and muscle atrophy while sparing bowel, bladder, and ocular motility.

Cerebral Palsy

A broad clinical term denoting loss or impairment of motor function secondary to brain damage (typically prenatal), impacting balance, reflexes, muscle tone, coordination, and voluntary control.

Ligament

A fibrous connective tissue structure that connects bone to bone, susceptible to stretch or tear injuries known as sprains.

Tendon

A tough band of fibrous connective tissue connecting muscle to bone, susceptible to stretch or tear injuries known as strains.

Sprain

A musculoskeletal injury characterized by the stretching or tearing of ligaments that stabilize joints.

Strain

A musculoskeletal injury characterized by the stretching or tearing of a muscle or tendon.

Diaphragm

The primary dome-shaped muscle of respiration whose alternating contraction and relaxation drive volume changes in the thoracic cavity to induce inhalation and exhalation.

Valsalva Maneuver

A physiologic maneuver in which holding one's breath fixes the diaphragm and stabilizes peritoneal cavity pressure while contracting abdominal muscles, aiding defecation, urination, and childbirth.

Intercostal Muscles

Three sets of thoracic muscles positioned between adjacent ribs that assist in respiration by modifying the dimensions and volume of the rib cage.

Pelvic Diaphragm

The muscular sheet closing the pelvic floor, composed of the levator ani and the coccygeus muscles.

Levator Ani

The principal supportive muscular component of the pelvic floor that holds the pelvic organs, subdivided into the pubococcygeus and iliococcygeus muscles.

Rotator Cuff

A musculotendinous cuff formed by four muscles (supraspinatus, infraspinatus, teres minor, subscapularis) that stabilizes the humeral head within the glenoid cavity while facilitating arm elevation and rotation.

Supraspinatus

A rotator cuff muscle that stabilizes the humeral head in the socket, maintains upper arm stability, and assists with arm abduction/elevation.

Infraspinatus

The primary rotator cuff muscle responsible for lateral rotation and extension of the shoulder joint.

Teres Minor

The smallest muscle of the rotator cuff group, assisting primarily with lateral rotation of the arm away from the body.

Subscapularis

An anterior rotator cuff muscle that secures the humerus to the scapula and functions to rotate the arm, hold it horizontally, and lower it.

Bursitis

Inflammation or irritation of a bursa—a fluid-filled sac serving as a protective cushion around joints and tendons such as the rotator cuff—caused by repetitive movement or infection.

Tendinitis

Inflammation or irritation of a tendon caused by overuse or repetitive motion, frequently presenting near joints as conditions such as tennis elbow or swimmer's shoulder.