Human Anatomy and Physiology: Musculoskeletal System

Comprehensive vocabulary flashcards covering the human musculoskeletal system, including bone classifications, cellular histology, skeletal divisions, joint articulations, connective tissues, and muscular functional groups.

Skeleton (Etymology)

Originates from the Greek word σκελετός (skeletos), meaning "dried up body."

Musculoskeletal System

An anatomical organ system encompassing the skeletal structures and associated muscles that function together to provide movement, support, and physical integrity.

Red Bone Marrow

Vascular bone tissue responsible for hematopoiesis (the production of red blood cells, white blood cells, and platelets); abundant in infants and found in the spongy bone of adult flat and irregular bones.

Yellow Bone Marrow

Adipose-rich marrow residing in the medullary cavity of adult long bones that stores fat and progressively replaces red bone marrow with increasing age.

Ossification

The biological process by which new bone tissue is formed during embryonic development, skeletal growth, and bone repair.

Calcitonin

A thyroid hormone that decreases blood calcium levels and inhibits bone breakdown.

Parathyroid Hormone (PTH)

A hormone secreted by the parathyroid glands that increases blood calcium levels by stimulating bone breakdown and remodeling, increasing calcium reabsorption in the kidneys, and enhancing calcium absorption in the intestines.

Compact (Cortical) Bone

The dense, solid outer layer of bone tissue composed of repeating structural units called osteons (Haversian systems), accounting for approximately $80\%$ of total bone mass.

Spongy (Cancellous) Bone

The porous, lightweight internal bone tissue composed of an open honeycomb lattice of bony struts called trabeculae, which houses red or yellow bone marrow.

Osteogenic Cell

An undifferentiated bone stem cell that undergoes division to develop into an osteoblast.

Osteoblast

A bone-building cell that synthesizes and secretes new extracellular bone matrix during bone growth, repair, and remodeling.

Osteocyte

A mature bone cell trapped within a tiny fluid-filled space called a lacuna that maintains daily bone tissue metabolism.

Osteoclast

A multinucleated bone-resorbing cell of a distinct cellular lineage that breaks down and dissolves old or damaged bone mineral and matrix.

Bone Matrix Composition

The extracellular substance of bone tissue consisting of $50\%$ mineral (calcium phosphate / hydroxyapatite for hardness), $25\%$ collagen fibers (for tensile strength and flexibility), and $25\%$ water.

Long Bones

Cylinder-like bones that are longer than they are wide, consisting of a central shaft (diaphysis) and two expanded ends (epiphyses) that function as levers to support weight and facilitate movement (e.g., femur, tibia, humerus).

Short Bones

Roughly cube-shaped bones of approximately equal length, width, and thickness that provide joint stability and support while allowing limited movement (e.g., carpals and tarsals).

Flat Bones

Thin, flat, and often curved bones that provide extensive protective coverage for internal organs and broad surfaces for muscle attachment (e.g., sternum, ribs, scapulae).

Irregular Bones

Bones with complex, irregular shapes that do not fit into other morphological classifications, serving specialized protective and support functions (e.g., vertebrae, facial bones).

Sesamoid Bones

Small, rounded bones embedded within tendons that protect tendons from excessive stress, wear, and mechanical friction (e.g., patella).

Diaphysis

The central tubular shaft of a long bone that runs between the proximal and distal ends, enclosing the yellow-marrow-filled medullary cavity in adults.

Epiphysis

The enlarged proximal or distal extremity of a long bone, filled internally with spongy bone and covered on its articular surface by hyaline cartilage.

Periosteum

A double-layered membrane covering the external surface of bone, consisting of an outer fibrous layer and an inner cellular layer rich in osteogenic cells, blood vessels, and nerves.

Endosteum

A delicate single-cell layer lining the internal medullary cavity and the surfaces of trabeculae, containing osteogenic cells, osteoblasts, osteoclasts, and osteocytes.

Axial Skeleton

The central axis of the human skeleton consisting of $80$ bones in adults, including the skull (cranium and facial bones), vertebral column, thoracic cage, hyoid bone, and auditory ossicles.

Appendicular Skeleton

The skeletal division consisting of $126$ bones, comprising the upper and lower extremities and the pectoral and pelvic girdles that anchor the limbs to the axial skeleton.

True Ribs

The first $7$ pairs of ribs (ribs $1\text{--}7$) that attach anteriorly directly to the sternum through individual strips of costal cartilage.

False Ribs

Rib pairs $8\text{--}10$ whose costal cartilages do not attach directly to the sternum, but instead merge with the cartilage of the $7\text{th}$ rib.

Floating Ribs

The bottom two pairs of ribs (ribs $11\text{ and }12$) that have no anterior attachment to either the sternum or costal cartilage.

Vertebral Column

The flexible backbone consisting of $33$ total vertebrae divided into $7$ cervical ($\text{C1--C7}$), $12$ thoracic ($\text{T1--T12}$), $5$ lumbar ($\text{L1--L5}$), $5$ fused sacral (sacrum), and $4$ fused coccygeal (coccyx) segments.

Fibrous Joint (Synarthrosis)

An immovable articulation where adjacent bones are held together by dense fibrous connective tissue with no intervening joint cavity (e.g., skull sutures).

Cartilaginous Joint (Amphiarthrosis)

A slightly movable joint where articulating bones are connected by cartilage without a synovial cavity (e.g., intervertebral discs and the pubic symphysis).

Synovial Joint (Diarthrosis)

A freely movable joint characterized by an articular capsule containing a fluid-filled synovial cavity and articular cartilage-covered bone ends (e.g., knee, shoulder, elbow).

Ball and Socket Joint

A multiaxial synovial joint where a globular bone head fits into a cup-shaped depression, permitting movement in multiple planes including rotation and circumduction (e.g., shoulder and hip joints).

Hinge Joint

A uniaxial synovial joint allowing movement in only one plane (flexion and extension), where a convex surface fits into a concave depression (e.g., elbow and knee joints).

Pivot Joint

A synovial joint in which a rounded or pointed bone surface articulates with a ring formed by bone and a ligament, allowing rotational movement around a central axis (e.g., the atlantoaxial joint between C1 and C2).

Ligament

A band of dense, collagenous connective tissue that attaches bone to bone, provides mechanical stability, prevents joint dislocation, and exhibits viscoelastic properties.

Tendon

A tough band of dense regular connective tissue with parallel arrays of collagen fibers that anchors muscle to bone and transmits muscle contraction force.

Hyaline Cartilage

The most widespread cartilage type, featuring a glassy, smooth extracellular matrix that provides low-friction gliding surfaces in articular joints, costal ribs, the nose, and the trachea.

Fibrocartilage

A durable, tensile-resistant cartilage packed with thick collagen bundles designed to absorb high compressive stress, found in intervertebral discs, knee menisci, and the pubic symphysis.

Elastic Cartilage

A resilient, flexible cartilage rich in elastic fibers that provides shape and recoil, found in the external ear (pinna), epiglottis, and Eustachian tube.

Osteoarthritis

A degenerative joint disorder characterized by the gradual wearing down and thinning of articular cartilage, narrowing of the joint space, bone-on-bone contact, and the formation of bone spurs (osteophytes).

Skeletal Muscle Tissue

Voluntary, striated muscle tissue composed of long, cylindrical fibers containing multiple peripherally located nuclei, attached to bones by tendons.

Cardiac Muscle Tissue

Involuntary, striated muscle tissue found exclusively in the heart wall, characterized by branching fibers, single central nuclei, and intercalated discs.

Smooth Muscle Tissue

Involuntary, non-striated muscle tissue consisting of spindle-shaped cells with a single centrally located nucleus, found in the walls of hollow organs and blood vessels.

Epimysium

The outermost sheath of dense connective tissue that encloses the entire external surface of an individual skeletal muscle.

Perimysium

The middle connective tissue layer that groups individual skeletal muscle fibers into bundles called fascicles.

Endomysium

The delicate connective tissue layer that surrounds each individual skeletal muscle fiber (muscle cell).

Sarcomere

The basic functional contractile unit of a myofibril bounded by adjacent Z lines, composed of organized thick (myosin) and thin (actin) myofilaments.

Prime Mover (Agonist)

The principal muscle responsible for generating the major contractile force to produce a specific joint movement (e.g., biceps brachii in elbow flexion).

Antagonist Muscle

A muscle that opposes the action of the prime mover and must relax to permit smooth movement when the agonist contracts (e.g., triceps brachii in elbow flexion).

Synergist Muscle

A muscle that aids the prime mover by delivering additional contractile force or guiding movement trajectory (e.g., brachioradialis assisting biceps brachii in elbow flexion).

Fixator Muscle

A stabilizing muscle that immobilizes a joint or the origin of a prime mover to prevent unwanted movement during action (e.g., deltoid stabilizing the shoulder during arm flexion).

Frontalis

A cranial muscle of facial expression innervated by the facial nerve ($\text{CN VII}$) that elevates the eyebrows and horizontally wrinkles the forehead.

Orbicularis Oculi

A circular sphincter muscle surrounding the orbit innervated by the facial nerve ($\text{CN VII}$) that firmly closes the eyelids during blinking and squinting.

Zygomaticus Major

A muscle of facial expression originating on the zygomatic bone and inserting into the corner of the mouth, innervated by $\text{CN VII}$, which draws the angle of the mouth upward and laterally to produce a smile.

Corrugator Supercilii

A facial muscle that draws the eyebrows medially and downward, producing vertical wrinkles on the forehead when frowning.