Human Anatomy and Physiology: Skeletal and Muscular Systems

Comprehensive practice flashcards reviewing skeletal and muscular system anatomy, bone histology, hormone regulation of calcium, and the sliding filament theory of muscle contraction.

Axial Skeleton

The division of the skeletal system comprising $80$ bones organized along the central longitudinal axis of the body, including the skull, auditory ossicles, hyoid bone, vertebral column, sternum, and ribs.

Appendicular Skeleton

The division of the skeletal system consisting of $126$ bones that includes the upper and lower limbs and the pectoral and pelvic girdles which attach the limbs to the axial skeleton.

Osteoblast

A bone-building cell that synthesizes and secretes bone extracellular matrix, takes up minerals from the blood, and initiates calcification during bone growth and repair.

Osteocyte

A mature bone cell trapped within a lacuna that maintains daily bone metabolism and exchange of nutrients and wastes with blood.

Osteoclast

A large, multinucleated cell derived from fused monocytes that breaks down and resorbs bone extracellular matrix, releasing calcium and phosphate minerals into the bloodstream.

Compact Bone

Dense bone tissue composed of repeating structural units called osteons (Haversian systems) that makes up the bulk of the diaphysis of long bones to provide protection, support, and resistance to external stresses.

Spongy Bone

Trabecular bone tissue that lacks osteons and consists of an irregular latticework of thin bone columns called trabeculae; it houses red bone marrow where hematopoiesis occurs and predominates in short, flat, and irregular bones as well as the epiphyses of long bones.

Osteon

The fundamental repeating microscopic structural unit of compact bone, consisting of concentric lamellae arranged around a central canal containing blood vessels and nerves.

Canaliculi

Tiny microscopic canals radiating in all directions from lacunae that connect adjacent lacunae to each other and to central canals, allowing osteocytes to communicate and exchange nutrients.

Diaphysis

The shaft or central body of a long bone, predominantly made of compact bone and containing a central medullary cavity.

Epiphysis

The distal and proximal extremities or articular ends of a long bone, composed mainly of spongy bone surrounded by a thin layer of compact bone.

Metaphysis

The transitional region in a mature long bone located between the diaphysis and epiphysis; during growth in a growing bone, it contains the epiphyseal plate where the bone elongates.

Periosteum

A tough sheath of dense irregular connective tissue surrounding the bone surface wherever it is not covered by articular cartilage; contains bone-forming osteogenic cells that allow growth in diameter (width) and serves as an attachment point for tendons and ligaments.

Endosteum

A thin connective tissue membrane that lines the inner surface of the medullary cavity and contains a single layer of bone-forming cells.

Hematopoiesis

The physiological process of blood cell production that takes place within red bone marrow located in spongy bone tissue.

Parathyroid Hormone (PTH)

A hormone that increases blood calcium levels by stimulating osteoclast activity to break down bone extracellular matrix and release $\text{Ca}^{2+}$ into the bloodstream.

Calcitonin

A hormone secreted by the thyroid gland that inhibits osteoclast activity, slowing down bone resorption and decreasing the amount of calcium released into the blood.

Coronal Suture

The immovable joint uniting the anterior frontal bone with the two superior-lateral parietal bones.

Sagittal Suture

The midline immovable cranial joint connecting the right and left parietal bones along the superior aspect of the skull.

Lambdoid Suture

The transverse immovable joint located posteriorly on the skull that unites the two parietal bones with the occipital bone.

Squamous Suture

The curved cranial joint uniting the parietal bone with the temporal bone on the lateral aspect of the skull.

Foramen Magnum

The large aperture located in the inferior portion of the occipital bone through which the medulla oblongata of the brain connects with the spinal cord.

Zygomatic Arch

The anatomical bridge or cheekbone formed by the union of the temporal process of the zygomatic bone and the zygomatic process of the temporal bone.

Sella Turcica

A saddle-shaped bony structure on the superior surface of the body of the sphenoid bone containing the hypophyseal fossa, which accommodates the pituitary gland.

Crista Galli

A triangular, cock's comb-shaped superior midline ridge of the ethmoid bone that serves as an anterior attachment point for the falx cerebri membrane covering the brain.

Cribriform Plate

A horizontal, sieve-like plate of the ethmoid bone that forms the roof of the nasal cavity and contains olfactory foramina transmitting olfactory nerves to the brain.

Paranasal Sinuses

Mucous membrane-lined cavities located within the frontal, sphenoid, ethmoid, and maxillary bones that lighten the skull, warm and moisten incoming air, and act as resonating chambers for speech.

Mandible

The lower jawbone; the largest, strongest, and only movable bone of the adult face, featuring a body, ramus, condylar process, and coronoid process.

Intervertebral Disc

A fibrocartilaginous cushion located between adjacent vertebral bodies from the second cervical vertebra ($C_2$) down to the sacrum, composed of an outer ring of fibrocartilage and a soft elastic interior to permit movement and absorb vertical compression.

Atlas (C1)

The first cervical vertebra, which lacks a vertebral body and spinous process; its superior articular facets articulate with the occipital condyles to permit the nodding 'yes' head motion.

Axis (C2)

The second cervical vertebra, distinguished by a superior vertical peg called the dens that articulates with the atlas to allow rotational side-to-side ('no') head movement.

Dens

A prominent tooth-shaped upright projection rising from the superior surface of the body of the axis ($C_2$) that acts as a pivot around which the atlas rotates.

Vertebra Prominens (C7)

The seventh cervical vertebra, featuring a single, long, non-bifurcated spinous process that projects posteriorly and can be palpated at the base of the neck.

Sternum

The flat, dagger-like anterior chest bone composed of three fused parts: the superior manubrium, the middle body, and the inferior xiphoid process.

True Ribs

Rib pairs $1$ through $7$, which connect directly to the sternum through their own individual bars of hyaline costal cartilage.

False Ribs

Rib pairs $8$ through $12$, whose costal cartilages either connect indirectly to the sternum via the cartilage of rib $7$ (ribs $8\text{--}10$) or lack any anterior sternal connection entirely (floating ribs $11\text{--}12$).

Floating Ribs

Rib pairs $11$ and $12$, a subset of false ribs that have no anterior attachment to the sternum or costal cartilage.

Pectoral Girdle

The skeletal complex comprising the clavicle and scapula that attaches the bones of the upper limb to the axial skeleton without directly articulating with the vertebral column.

Clavicle

An S-shaped bone that transmits mechanical forces from the upper limb to the trunk; its medial end articulates with the manubrium of the sternum, and its lateral end articulates with the acromion of the scapula.

Glenoid Cavity

A shallow depression on the lateral angle of the scapula that accepts the rounded head of the humerus to form the glenohumeral (shoulder) joint.

Acromion

A flattened, prominent lateral projection continuing from the scapular spine that articulates with the clavicle and forms the highest point of the shoulder.

Coracoid Process

A hook-like anterior projection on the superior border of the scapula that serves as an attachment site for muscles and ligaments.

Deltoid Tuberosity

A roughened, V-shaped area located on the lateral surface of the humerus shaft where the deltoid muscle tendon attaches.

Olecranon

A prominent, curved proximal projection of the ulna that forms the point of the elbow and lodges within the olecranon fossa of the humerus during forearm extension.

Pelvic Girdle

A skeletal structure consisting of two hip bones (ossa coxae) that articulate posteriorly with the sacrum and anteriorly with each other at the pubic symphysis, connecting the lower limbs to the axial skeleton.

Acetabulum

The deep lateral hemispherical socket of the hip bone formed by the union of the ilium, ischium, and pubis that articulates with the head of the femur.

Obturator Foramen

The largest foramen in the human skeleton, bounded by the pubis and ischium of the hip bone, largely covered by a fibrous membrane.

True Pelvis

The inferior portion of the pelvic cavity situated below the pelvic brim that encloses the pelvic organs and serves as the bony birth canal in females.

False Pelvis

The superior, expanded portion of the pelvis situated above the pelvic brim, bounded laterally by the iliac blades, that houses lower abdominal organs.

Linea Aspera

A prominent longitudinal double-edged ridge running down the posterior surface of the femoral shaft that serves as an attachment site for several thigh muscles.

Calcaneus

The largest and strongest tarsal bone, forming the heel of the foot, which transmits body weight to the ground and provides attachment for the Achilles tendon.

Talus

The most superior tarsal bone, which articulates with the medial malleolus of the tibia and lateral malleolus of the fibula to form the true ankle joint.

Epimysium

The outermost layer of dense irregular connective tissue that completely encircles an entire skeletal muscle organ.

Perimysium

A sheath of dense irregular connective tissue that bundles $10$ to $100$ or more individual skeletal muscle fibers together into a fascicle.

Endomysium

A delicate interior layer of connective tissue that wraps around each individual skeletal muscle fiber (cell).

Fascicle

A distinct cylindrical bundle of skeletal muscle fibers wrapped together by perimysium.

Sarcolemma

The specialized plasma membrane that envelops an individual muscle fiber (muscle cell).

Sarcoplasm

The cytoplasm of a muscle fiber that fills the space between myofibrils and contains glycogen, myoglobin, and high numbers of mitochondria.

Sarcoplasmic Reticulum (SR)

A specialized fluid-filled system of smooth endoplasmic reticulum tubules that encircle each myofibril and store $\text{Ca}^{2+}$ ions in resting muscle fibers.

Transverse Tubule (T-Tubule)

Narrow, tunnel-like invaginations of the sarcolemma that tunnel inward from the surface toward the center of each muscle fiber to propagate muscle action potentials rapidly throughout the cell.

Sarcomere

The basic functional contractile unit of a skeletal muscle myofibril, demarcated from one Z disc to the next adjacent Z disc.

Myosin

The primary contractile motor protein constituting thick filaments, featuring projecting globular heads that bind to actin active sites to perform power strokes.

Actin

The main contractile protein composing thin filaments, containing specific active binding sites for attachment by myosin cross-bridges.

Tropomyosin

A ribbon-like regulatory protein wrapped around the actin helix that physically blocks the myosin-binding sites on actin during muscle relaxation.

Troponin

A regulatory protein complex bound to tropomyosin that contains binding sites for $\text{Ca}^{2+}$; upon binding calcium, it undergoes a conformational shift that pulls tropomyosin away from actin active sites.

A Band

The dark, central region of a sarcomere extending the entire length of the thick filaments, which does not change width during muscle contraction.

I Band

The lighter zone of a sarcomere that contains only thin filaments and is bisected by a Z disc; shortens and narrows during muscle contraction.

H Zone

The central lighter region of the A band containing only thick filaments; narrows and disappears during muscle contraction as thin filaments slide inward.

Motor Unit

A functional neuromuscular entity consisting of a single somatic motor neuron along with all the individual muscle fibers it stimulates and innervates.

Neuromuscular Junction (NMJ)

The specialized synapse formed between the terminal axon branch of a motor neuron and the adjacent motor end plate of a skeletal muscle fiber.

Motor End Plate

The specialized, highly folded region of the sarcolemma opposite the synaptic end bulb that contains abundant acetylcholine (ACh) receptors.

Acetylcholine (ACh)

The neurotransmitter released from synaptic vesicles into the synaptic cleft of the neuromuscular junction that binds to receptors to generate a muscle action potential.

Acetylcholinesterase

An enzyme present within the synaptic cleft that rapidly hydrolyzes and breaks down acetylcholine, terminating muscle action potentials until new ACh is released.

Muscle Tone

A continuous, involuntary state of partial muscle contraction resulting from alternating activation of small groups of motor units, which maintains posture, balance, and firmness.

Slow Oxidative Fibres

Fatigue-resistant skeletal muscle fibers with slower contraction speeds that produce ATP predominantly via aerobic cellular respiration for endurance and posture.

Fast Glycolytic Fibres

Large-diameter, powerful skeletal muscle fibers that fatigue quickly and generate ATP primarily by anaerobic glycolysis for rapid, high-intensity contractions.

Prime Mover (Agonist)

The principal muscle directly responsible for producing a specific intended movement or motion at a joint.

Antagonist

A muscle that opposes the action of the prime mover by stretching and yielding to allow smooth, controlled motion.

Synergist

A muscle that assists the prime mover by adding extra force or preventing unwanted movement at intermediate joints.

Rectus

A directional naming descriptor indicating that muscle fibers run straight and parallel to the body midline (e.g., rectus abdominis).

Oblique

A directional naming descriptor indicating that muscle fibers run diagonally relative to the body midline (e.g., external oblique).

Intercalated Discs

Specialized irregular transverse thickenings of the sarcolemma that connect adjacent cardiac muscle fibers, containing desmosomes for physical adherence and gap junctions for rapid electrical conduction.