3 (Final): Muscular System- Histology and Excitation-Contraction

These vocabulary flashcards cover the histology, organelles, proteins, and the excitation-contraction coupling process of the muscular system based on the lecture notes.

Myocytes

The specialized cells of muscle tissue that are tightly packed together and surrounded by a small degree of extracellular matrix.

Muscle fibers

The term used to describe skeletal and smooth muscle cells due to their elongated, fibrous shape.

Myofilaments

Elaborate networks of actin and myosin protein filaments that perform microscopic contraction by doing the actual pulling.

Striations

The striped or banded appearance of skeletal and cardiac muscle tissue caused by the precise alignment of myofilaments into sarcomeres.

Excitability

Also called responsiveness; the ability of a cell to receive and respond to a stimulus by changing its membrane potential.

Contractility

The unique ability of muscle tissue to shorten forcibly when adequately stimulated.

Extensibility

The ability of muscle cells to be stretched, sometimes even beyond their resting length, when in a relaxed state.

Elasticity

The ability of a muscle cell to recoil and resume its resting length after being stretched.

Intercalated discs

Unique thickened cellular junctions in cardiac muscle that contain desmosomes for strength and gap junctions for synchronized electrical conduction.

Dense bodies

Structures in smooth muscle cells, analogous to Z-discs, where myofilaments are anchored diagonally to the cell's cytoskeleton.

Myoblasts

Stem cells that differentiate from mesenchymal cells and fuse together to form multi-nucleated skeletal muscle fibers.

Satellite cells

Quiescent muscle stem cells located outside the sarcolemma that can proliferate and fuse to fibers for growth, maturation, and regeneration.

Sarcolemma

The plasma membrane of a muscle fiber.

Sarcoplasm

The cytoplasm of a muscle fiber, containing specialized components like glycosomes and myoglobin.

Glycosomes

Granules found in the sarcoplasm that store glycogen, the preferred fuel source for creating $ATP$.

Myoglobin

A red pigment in muscle fibers that stores oxygen and releases it to the mitochondria for $ATP$ production.

Myofibrils

Contractile organelles that account for approximately $80\%$ of a muscle fiber's cellular volume and consist of linked sarcomeres.

Sarcomere

The basic functional contractile unit of a myofibril, bounded at each end by Z-discs.

Z-discs

Protein-rich structures that mark the boundaries of a sarcomere, bind sarcomeres to one another, and anchor thin myofilaments.

Transverse tubules (T-tubules)

Infoldings of the sarcolemma that encircle every sarcomere, allowing nerve signals to penetrate to the deepest regions of the muscle fiber.

Sarcoplasmic reticulum (SR)

A specialized network of membranous tubules surrounding each myofibril that regulates intracellular levels of $Ca^{2+}$.

Terminal cisternae

Enlargements of the sarcoplasmic reticulum that lie adjacent to T-tubules and release stored $Ca^{2+}$ when stimulated by an action potential.

Triad

A grouping of three units consisting of one T-tubule situated between a pair of terminal cisternae.

Myosin

The protein that makes up thick myofilaments; it features 'heads' for binding to actin and 'tails' tethered to the M-line by titin.

Actin

The primary protein of thin myofilaments, arranged in a double helix with specific binding sites for myosin heads.

Tropomyosin

A regulatory protein that covers the binding sites on actin when the muscle is at rest, inhibiting cross-bridge formation.

Troponin

A regulatory protein that binds to $Ca^{2+}$ and subsequently signals tropomyosin to uncover the active sites on actin.

Dystrophin

A structural protein that links thin myofilaments to the sarcolemma and extracellular matrix to stabilize the membrane during high force generation.

Titin

A structural protein that anchors thick myofilaments to the M-line and helps the muscle resist excessive stretching and recoil back to shape.

A-Band

The dark, middle portion of a sarcomere representing the full length of the thick myofilaments; its length remains constant during contraction.

H-Zone

The narrow region in the center of the A-band where only thick myofilaments exist; it disappears as the sarcomere shortens.

I-Band

The lighter area of the sarcomere containing only thin myofilaments; it shortens as the fibers overlap more during contraction.

Neuromuscular junction (NMJ)

The synapse or point of communication between a somatic motor neuron's axon terminal and a muscle fiber.

Acetylcholine (ACh)

The specific neurotransmitter released by somatic motor neurons to trigger muscle depolarization.

Motor end plate

The specialized portion of the sarcolemma at the neuromuscular junction that contains receptors for Acetylcholine.

Acetylcholinesterase

The enzyme that breaks down Acetylcholine in the synaptic cleft to prevent continuous muscle action potentials.

Cross-bridge

The temporary physical connection formed when a myosin head binds to an exposed active site on an actin filament.

Power stroke

The step in the contraction cycle where the myosin head bends, pulling the thin filament toward the M-line and releasing $ADP$ and $P_i$.

Sliding Filament Model

The theory that muscle contraction occurs as thin filaments slide past thick filaments, increasing overlap without changing the actual length of the proteins.

Endomysium

The areolar connective tissue membrane that surrounds each individual muscle fiber.

Perimysium

The dense irregular connective tissue that surrounds a fascicle, which is a bundle of muscle fibers.

Epimysium

The dense irregular connective tissue layer that surrounds the entire muscle organ.

Insertion

The site of muscle attachment on the bone that moves when the muscle contracts.

Origin

The site of muscle attachment on the bone that serves as the fixed anchor during movement.

Tendon

A ropelike indirect attachment made of tough collagen fibers that joins muscle to bone.

Aponeurosis

A sheetlike indirect attachment that joins muscle to bone or other structures.

Isometric contraction

A type of contraction where tension develops but the muscle length does not change and no movement occurs.

Isotonic contraction

A category of contraction where tension develops and the overall length of the muscle changes, categorized as concentric or eccentric.

Concentric contraction

An isotonic contraction where the muscle generates enough force to overcome resistance and shortens.

Eccentric contraction

An isotonic contraction where the muscle lengthens while under load; this type produces the greatest overall force.