sketetal muscle

Epimysium

The outermost layer of collagen tissue surrounding an entire muscle organ, separating it from adjacent tissues.

Perimysium

The middle connective tissue layer that surrounds muscle fascicles (bundles of muscle fibers) and houses blood vessels and nerves.

Endomysium

The innermost connective tissue layer surrounding individual muscle cells (fibers); it contains capillaries, nerve endings, and stem cells.

Myosatellite Cells

Stem cells located within the endomysium that function in muscle tissue repair.

Tendon

A thick, bundle-like cord of collagen fibers that anchors a muscle directly to a bone.

Aponeurosis

A broad, flat sheet of collagen tissue that attaches muscle to bone or other tissues.

Myoblasts

Embryonic precursor cells that fuse together during development to create single, multinucleated skeletal muscle fibers.

Sarcolemma

The plasma membrane enclosing a muscle cell. Changes in its electrical potential trigger muscle contraction.

Sarcoplasm

The specialized cytoplasm contained inside a muscle fiber.

Transverse Tubules (T Tubules)

Narrow tubes continuous with the sarcolemma that project deep into the cell to rapidly conduct electrical signals into the interior.

Sarcoplasmic Reticulum

specialized network of smooth endoplasmic reticulum surrounding myofibrils that stores and releases calcium ions (\text{Ca}^{2+}).

Terminal Cisternae

Expanded regions of the sarcoplasmic reticulum situated on either side of a T tubule.

Myofibril

cylindrical, longitudinal bundle of contractile proteins packed inside a muscle fiber.

Thin Filament

contractile protein filament made primarily of actin.

Sarcomere

The repeating functional unit of contraction along a myofibril, bounded on both ends by Z lines.

A Band

The dark central region of a sarcomere spanning the entire length of thick filaments.

I Band

The light region of a sarcomere containing thin filaments only.

M Line

The exact center of the sarcomere inside the A band where supporting proteins anchor thick filaments.

H Band

The area around the M line that contains thick filaments only.

Zone of Overlap

The portion of the A band where thick and thin filaments overlap and form cross-bridges.

Z Line

Protein structures marking the outer boundaries of a sarcomere at the center of each I band.

F-Actin

A twisted strand of individual globular G-actin subunits, each containing a binding site for myosin.

Nebulin

A structural protein that extends along the length of F-actin to anchor and hold it together.

Tropomyosin

A double-stranded protein that covers myosin-binding sites on actin when the muscle is relaxed.

Troponin

A three-protein complex bound to actin and tropomyosin that binds calcium ions (\text{Ca}^{2+}) to shift tropomyosin away from active sites.

Myosin

The main protein component of thick filaments, featuring a tail and a globular head that binds to actin to form a cross-bridge.

Titin

An elastic protein that connects thick filaments to the Z line, preventing overstretching and maintaining alignment.

Sliding Filament Theory

The concept that muscle contraction occurs when thin filaments slide past thick filaments toward the M line, narrowing the I and H bands while leaving filament lengths and A band width unchanged.

Neuromuscular Junction (NMJ

The intercellular junction where a motor neuron communicates with a muscle fiber.

Acetylcholine (ACh)

The neurotransmitter released by motor neurons that binds receptors on the muscle cell to trigger depolarization.

Motor End Plate

The specialized region of the sarcolemma at the NMJ containing receptors for acetylcholine.

Acetylcholinesterase (AChE)

An enzyme in the synaptic cleft that breaks down acetylcholine to stop muscle stimulation.

Excitation-Contraction Coupling

The sequence of events linking an electrical signal (action potential) on the sarcolemma to mechanical tension via calcium release.

Excitation-Contraction Coupling

The sequence of events linking an electrical signal (action potential) on the sarcolemma to mechanical tension via calcium release.

Cross-Bridge Formation

The step where an activated myosin head carrying ADP and phosphate binds to an exposed active site on actin.

Power Stroke

The step where the myosin head pivots toward the M line, pulling the thin filament along and releasing ADP and phosphate.

Cross-Bridge Detachment

The step where a new ATP molecule attaches to the myosin head, causing it to let go of actin.

Myosin Reactivation

The step where ATP is hydrolyzed into ADP and phosphate, re-cocking the myosin head into its energized resting position.

Rigor Mortis

State of body stiffening after death caused by ATP depletion; calcium leaks to expose active sites, but myosin cannot detach from actin without ATP.

Length-Tension Relationship

The principle that maximal muscle force is produced at an optimal resting sarcomere length (2.0\text{--}2.25\,\mu\text{m}) due to maximal cross-bridge interaction.

Muscle Twitch

A single cycle of contraction and relaxation in a muscle fiber caused by one action potential

Latent Period

The brief delay between the stimulus arriving at the muscle and the onset of tension, during which excitation-contraction coupling occurs.

Contraction Phase

The time period during a twitch when tension increases as cross-bridge interactions occur

Relaxation Phase

The time period during a twitch when tension decreases as calcium is pumped back into the sarcoplasmic reticulum.

Wave Summation

The addition of successive twitch forces caused by stimuli arriving before the muscle finishes relaxing

Incomplete Tetanus

A state of rapid muscle stimulation that produces near-maximum tension with brief, partial relaxation periods between stimuli.

Complete (Fused) Tetanus

A state of maximum sustained muscle tension produced by high-frequency stimulation that completely eliminates the relaxation phase.

Isotonic Contraction

A muscle contraction where tension remains constant while muscle length changes.

Concentric Contraction

An isotonic contraction where the muscle shortens as force exceeds load.

Eccentric Contraction

An isotonic contraction where the muscle lengthens while developing tension.

Isometric Contraction

A muscle contraction where tension increases, but the total length of the muscle remains unchanged.

Creatine Phosphate (CP)

A high-energy molecule synthesized in resting muscle cells used to rapidly regenerate ATP during immediate activity.

Creatine Kinase (CK)

The enzyme that transfers a phosphate group from creatine phosphate to ADP to regenerate ATP.

Aerobic Metabolism

Oxygen-dependent cellular respiration inside mitochondria that provides the primary ATP source during rest and moderate exertion.

Anaerobic Glycolysis

Oxygen-independent pathway in cytoplasm that breaks down glucose into pyruvate and lactate for fast ATP generation during peak exertion.

Muscle Fatigue

A reduced ability to produce muscle tension caused by energy reserve depletion, pH changes, or ion imbalances.

Cori Cycle

The metabolic pathway where lactate produced by muscles travels through blood to the liver, where it is converted back into glucose.

Tetanus Disease

An infection caused by Clostridium tetani whose toxin blocks spinal cord inhibition of motor neurons, producing painful spastic paralysis

Botulism

Poisoning caused by toxin from Clostridium botulinum that blocks presynaptic release of acetylcholine at the NMJ, causing flaccid paralysis.

Triad

A structural grouping consisting of one central T tubule flanked by two terminal cisternae.