Musculoskeletal System: Histology, Skeletal Architecture, Muscle Contraction, and Energetics

Comprehensive vocabulary flashcards covering skeletal system histology, bone classification and anatomy, muscle tissue characteristics, excitation-contraction coupling, cross-bridge cycling, and muscle energetics.

Motor Unit

A single somatic motor neuron and all the skeletal muscle fibers that it innervates and controls.

Motor Unit Recruitment

The progressive activation of additional motor units, typically from smallest to largest, to generate greater muscular contractile force.

Deltoid Motor Units

Large motor units containing hundreds to thousands of muscle cells per motor neuron, designed for powerful but less precise movements.

Extrinsic Eye Muscle Motor Units

Small motor units consisting of as few as 3 muscle cells per motor neuron, providing fine, high-precision motor control.

Direct Phosphorylation (Creatine Phosphate Pathway)

A single-step anaerobic reaction where creatine phosphate transfers a high-energy phosphate group to $ADP$, generating 1 $ATP$ per molecule for approximately 15 seconds of initial high-intensity activity.

Anaerobic Respiration (Glycolysis and Lactic Acid Formation)

An oxygen-independent pathway that breaks down glucose into pyruvic acid and converts it to lactic acid, yielding $2\,ATP$ per glucose to support high-intensity contractions lasting 30 to 60 seconds.

Aerobic Respiration (Oxidative Phosphorylation)

A mitochondrial, oxygen-dependent metabolic pathway that oxidizes glucose, pyruvic acid, free fatty acids, or amino acids to produce roughly $36\,ATP$ per glucose molecule along with $CO_2$ and $H_2O$ for hours of sustained exertion.

Endurance Exercise Adaptations

Skeletal muscle adaptations including an increase in mitochondrial density, greater capillary supply, elevated myoglobin content, and enhanced oxygen utilization.

Resistance Exercise Adaptations

Muscle-fiber hypertrophy resulting from the increased synthesis of contractile proteins (myofibrils), yielding a greater capacity to generate muscular force.

Neuromuscular Junction (NMJ)

The specialized synapse where the axon terminal of a somatic motor neuron communicates with the motor endplate of a skeletal muscle fiber.

Motor Endplate

The specialized, folded region of the muscle fiber sarcolemma at the neuromuscular junction containing a high density of cholinergic receptors (chemically-gated cation channels).

End-Plate Potential (EPP)

A local graded depolarization of the motor endplate produced when acetylcholine opens ligand-gated channels, allowing substantial $Na^+$ influx and minor $K^+$ efflux.

Dihydropyridine (DHP) Receptor

A voltage-sensitive protein situated in the transverse tubule (T-tubule) membrane that changes conformation upon depolarization to mechanically open adjacent ryanodine channels.

Ryanodine Receptor (RyR)

A calcium release channel embedded in the sarcoplasmic reticulum membrane that is mechanically coupled to the DHP receptor and releases $Ca^{2+}$ into the sarcoplasm when opened.

Cross-Bridge

The physical molecular attachment formed when an energized, cocked myosin head binds to an accessible myosin-binding site on an actin thin filament.

Power Stroke

The mechanical step of contraction during which the myosin head pivots and flexes, pulling the thin filament toward the center of the sarcomere while releasing bound $ADP$ and inorganic phosphate ($P_i$).

Acetylcholinesterase (AChE)

An enzyme located in the synaptic cleft and basal lamina that degrades acetylcholine into choline and acetate, terminating excitation of the muscle fiber.

Sarcoplasmic Reticulum Ca2+-ATPase Pump

An ATP-driven active transport pump that sequesters cytoplasmic $Ca^{2+}$ back into the sarcoplasmic reticulum to trigger muscle relaxation.

Muscle Twitch

A single complete physiological cycle consisting of excitation, excitation-contraction coupling, tension development (contraction), and relaxation in response to a single stimulus.

Functions of Bone

The primary bodily functions of the skeleton, encompassing structural support, organ protection, movement leverage, electrolyte/acid-base balance (storing calcium and phosphate), and hematopoiesis.

Typical Adult Bone Count

206 distinct bones, resulting from the progressive fusion during growth of the larger number of separate skeletal elements present in infants.

Diaphysis

The hollow, tubular shaft forming the long axis of a long bone, consisting of a thick collar of compact bone surrounding the medullary cavity.

Epiphysis

The expanded proximal or distal end of a long bone, largely filled with spongy bone containing red marrow and covered externally by articular cartilage.

Metaphysis

The transitional zone between the diaphysis and epiphysis in a long bone that houses the epiphyseal plate (growth plate) in growing bones or the epiphyseal line in adults.

Medullary Cavity

The hollow central space within the diaphysis of an adult long bone, filled predominantly with adipose-rich yellow bone marrow.

Periosteum

A tough external sheath consisting of an outer fibrous layer of dense irregular connective tissue and an inner osteogenic layer containing bone-forming cells and blood vessels.

Endosteum

A delicate internal connective tissue membrane lining the medullary cavity, trabeculae, and central canals, containing osteoprogenitor cells, osteoblasts, and osteoclasts.

Red Bone Marrow

Hematopoietic connective tissue located within the pores and trabecular spaces of spongy bone that is responsible for producing red blood cells, white blood cells, and platelets.

Yellow Bone Marrow

Lipid-rich connective tissue composed primarily of adipocytes, found occupying the medullary cavities of adult long bones as an energy reserve.

Bone Matrix Composition

A composite material consisting of an organic phase (primarily collagen fibers providing tensile strength and flexibility) and an inorganic phase (mainly calcium-phosphate crystals providing hardness and compression resistance).

Osteoprogenitor (Osteogenic) Cells

Mesenchymal stem cells found in the inner periosteum and endosteum that divide to replenish their population and differentiate into osteoblasts.

Osteoblasts

Bone-forming cells that synthesize and secrete the unmineralized organic matrix known as osteoid (collagen and calcium-binding proteins).

Osteocytes

Mature bone cells trapped within lacunae that maintain the mineralized extracellular matrix, detect mechanical stress and strain, and coordinate bone remodeling.

Osteoclasts

Large, multinucleated cells derived from the fusion of hematopoietic monocyte-macrophage stem cell lines that resorb and dissolve mineralized bone matrix.

Osteon (Haversian System)

The structural and functional microscopic unit of compact bone, consisting of concentric lamellae of calcified matrix surrounding a central Haversian canal.

Canaliculi

Microscopic hair-like canals radiating across lamellae that interconnect adjacent lacunae and the central canal, enabling osteocyte cytoplasmic extensions to exchange nutrients and waste.

Perforating (Volkmann) Canals

Transverse or oblique channels in compact bone that conduct blood vessels and nerves from the periosteum and medullary cavity into the central Haversian canals.

Trabeculae

The branching lattice of delicate struts and plates constituting spongy (cancellous) bone, oriented along stress lines and lacking true osteons.

Sesamoid Bone

A small, rounded bone that develops and embeds within a tendon where it crosses a joint, exemplified by the patella.

Sutural (Wormian) Bones

Small, irregularly shaped islands of bone that form along and within the sutures of the human cranium.

Skeletal Muscle Tissue

A voluntary, striated muscle tissue containing long, multinucleated, unbranched cylindrical fibers attached to the skeleton or deep fascia.

Cardiac Muscle Tissue

An involuntary, striated muscle tissue containing branched, mononucleated cells joined by intercalated discs that contract autorhythmically.

Smooth Muscle Tissue

An involuntary, non-striated muscle tissue comprising small, spindle-shaped, mononucleated cells located in the walls of visceral organs and blood vessels.

Excitability

The muscle property characterized by the ability to respond to chemical and electrical stimuli by altering membrane electrical potential.

Conductivity

The muscle property that allows an electrical change (action potential) to propagate across the plasma membrane.

Contractility

The ability of muscle tissue to pull on attachment points and generate mechanical force by actively shortening.

Extensibility

The capacity of muscle fibers to be stretched beyond resting length without rupturing or sustaining cellular damage.

Elasticity

The mechanical property allowing a muscle to passively recoil to its normal resting length after being stretched.

Fascicle

A discrete bundle of parallel skeletal muscle fibers demarcated and bound by a sleeve of perimysium.

Epimysium

An external layer of dense collagenous connective tissue that envelops an entire skeletal muscle organ.

Perimysium

Connective tissue septa that encase muscle fascicles, routing neurovascular structures into the muscle interior.

Endomysium

A delicate sheath of areolar connective tissue surrounding each individual muscle fiber, providing electrical insulation and capillary access.

Indirect Muscle Attachment

The most common muscular connection where the collagen fibers of the epimysium, perimysium, and endomysium coalesce into a distinct tendon that attaches to bone periosteum.

Direct (Fleshy) Muscle Attachment

An attachment where the epimysium is directly fused to the bone periosteum over a broad area, appearing as though muscle fibers emerge straight from bone.

Aponeurosis

A broad, flattened sheet of dense regular connective tissue serving as an expanded tendon to anchor wide muscles, such as abdominal wall muscles.

Convergent Muscle

A muscle morphology in which fascicles spread over a wide origin area and converge onto a single, narrow insertion tendon, as seen in the pectoralis major.

Pennate Muscle

A muscle architecture where short fascicles attach obliquely to a central tendon running the length of the muscle, maximizing fiber density and force generation at the expense of shortening distance.

Sarcolemma

The specialized plasma membrane of a muscle fiber that maintains membrane potential and conducts action potentials.

Sarcoplasm

The intracellular fluid (cytoplasm) of a muscle cell, containing abundance of glycogen granules, myoglobin proteins, and myofibrils.

Transverse Tubules (T-Tubules)

Tubular invaginations of the sarcolemma that penetrate deep into the center of the muscle fiber, conducting electrical impulses to terminal cisternae.

Sarcoplasmic Reticulum (SR)

A specialized smooth endoplasmic reticulum network surrounding each myofibril that stores high concentrations of calcium ions for muscle contraction.

Triad (Skeletal Muscle)

A three-part anatomical arrangement consisting of one central transverse tubule flanked symmetrically by two enlarged terminal cisternae of the sarcoplasmic reticulum.

Myosin Molecule

The motor protein component of thick filaments, featuring an elongated coiled-coil tail and two globular heads capable of binding actin and hydrolyzing $ATP$.

Actin Filament

A helical polymer of globular actin monomers forming thin filaments, where each monomer possesses an active site capable of binding a myosin head.

Tropomyosin

A rod-shaped regulatory protein that blocks myosin-binding sites on actin thin filaments when the muscle cell is at rest.

Troponin

A three-protein regulatory complex attached to tropomyosin that binds $Ca^{2+}$, causing a conformational shift that rolls tropomyosin away from actin active sites.

Sarcomere

The repeating structural and functional contractile unit of a striated muscle fibril, defined between two successive Z discs.

A Band

The anisotropic, dark-staining central band of a sarcomere spanning the entire length of the thick myosin filaments, including overlap zones with thin filaments.

I Band

The isotropic, light-staining region of a sarcomere comprising thin actin filaments only, centered on and bisected by a Z disc.

H Zone

The lighter central portion of the A band where thick myosin filaments do not overlap with thin actin filaments in relaxed muscle.

M Line

A narrow dark transverse protein meshwork situated in the middle of the H zone that bridges and holds thick myosin filaments in register.

Z Disc (Z Line)

A zigzag protein structure serving as the lateral boundary of a sarcomere, anchoring thin filaments and elastic titin filaments.

Sliding Filament Theory

The physiological mechanism of muscle contraction stating that thin actin filaments slide past thick myosin filaments toward the M line, narrowing the I band and H zone while the length of the filaments remains unchanged.