Muscular System, Biomechanics, and Neuromuscular Function

Comprehensive vocabulary flashcards covering skeletal, cardiac, and smooth muscle tissues, neuromuscular transmission, sarcomere mechanics, anatomical muscle nomenclature, joint classifications, lever systems, and movement mechanics.

Stapedius

The smallest muscle in the human body, located in the ear; it measures $6\,\text{mm}$ long, holds the inner ear together, and connects to the eardrum.

Masseter

The strongest muscle in the human body by weight, located in the jaw; it enables the teeth to close with a force of up to $55\,\text{pounds}$ on the incisors or $200\,\text{pounds}$ on the molars.

Gluteus maximus

The largest muscle in the human body, situated in the buttocks.

Heart

The hardest-working muscle in the body, which continuously pumps $5\,\text{quarts}$ of blood per minute and approximately $2{,}000\,\text{gallons}$ daily.

Sarcolemma

The specialized plasma cell membrane surrounding an individual muscle fiber (myofiber).

Sarcoplasm

The cytoplasm of an individual muscle cell (myofiber).

Sarcoplasmic reticulum

The specialized smooth endoplasmic reticulum of a muscle cell that surrounds myofibrils and stores and releases calcium ions ($\text{Ca}^{2+}$).

Myofibrils

Long, rod-like contractile organelles running parallel within a muscle cell, composed of bundles of thick (myosin) and thin (actin) myofilaments.

Skeletal muscle

Voluntary, striated muscle tissue composed of single, long, cylindrical, multinucleate cells; there are approximately $650$ skeletal muscles in the human body, primarily attached to bones across joints via tendons.

Cardiac muscle

Involuntary, striated muscle tissue found exclusively in the walls of the heart, composed of branching chains of uninucleate cells connected by intercalated discs.

Intercalated disks

Specialized microscopic junctional zones in cardiac muscle where adjacent cells interlock to create tight mechanical and electrical bonds, allowing the heart to withstand high pressure.

Smooth muscle

Also called visceral muscle, this is the weakest muscle type; it is involuntary and non-striated, composed of single, spindle-shaped (fusiform) uninucleate cells that line hollow organs like blood vessels, the gastrointestinal tract, and the bladder.

Tendons

Tough bands of dense regular connective tissue composed of collagen fibers that anchor skeletal muscles to bones.

Origin

The specific skeletal muscle attachment site that connects to an immovable or stationary bone.

Insertion

The skeletal muscle attachment site that connects to a movable bone, which is pulled toward the origin during contraction.

Motor unit

A functional neuromuscular structure composed of a single motor neuron and all the skeletal muscle fibers (myofibers) it innervates.

Acetylcholine (ACh)

The specific neurotransmitter stored in synaptic vesicles and released via exocytosis into the synaptic cleft at the neuromuscular junction.

Acetylcholinesterase

The enzyme located in the synaptic cleft that rapidly degrades acetylcholine, thereby terminating muscle cell stimulation.

Depolarization

A local reversal in the resting electrical polarity of the sarcolemma occurring when acetylcholine opening channels allows a greater influx of sodium ions ($\text{Na}^+$) than efflux of potassium ions ($\text{K}^+$).

Action potential

An unstoppable, self-propagating electrical impulse that spreads across the entire surface of the sarcolemma and travels into T tubules to trigger contraction.

Sodium-potassium pump

The active transport mechanism that restores the original resting ion distribution across the sarcolemma by pumping $\text{Na}^+$ out and $\text{K}^+$ back into the cell.

Troponin

A globular regulatory protein complex bound to actin that binds calcium ions ($\text{Ca}^{2+}$), causing a conformational shift in tropomyosin.

Tropomyosin

A filamentous regulatory protein that wraps around actin filaments, blocking myosin-binding sites in a relaxed muscle until displaced by troponin.

Power stroke

The mechanical step of cross-bridge cycling where ADP and inorganic phosphate ($\text{P}_i$) are released from the myosin head, prompting it to pivot and slide the thin filament toward the center of the sarcomere.

Sarcomere

The microscopic structural and functional contractile unit of striated muscle, defined as the segment between two consecutive Z discs; the term literally translates to 'fleshy part'.

Titin

Elastic protein filaments that run through the core of thick filaments and anchor them directly to Z discs, aiding in resting tension and structural integrity.

A band

The dark band of a sarcomere extending the entire length of the thick (myosin) filaments.

I band

The light band of a myofibril that spans parts of two adjacent sarcomeres and contains only thin (actin) filaments.

Irritability (responsiveness)

The fundamental functional property of a muscle fiber to receive and respond to chemical or electrical stimuli.

Contractility

The unique functional property of muscle tissue enabling it to forcibly shorten when adequately stimulated.

Extensibility

The functional property that allows muscle cells and fibers to lengthen or stretch beyond their resting length.

Elasticity

The functional property enabling muscle tissue to recoil and resume its original resting length after being stretched.

Temporalis

A fan-shaped masticatory muscle located over the temporal bone on the lateral aspect of the skull.

Occipitalis

A cranial muscle situated over the posterior aspect of the cranium that draws the scalp backward.

Zygomaticus major

A superficial facial muscle extending obliquely from the zygomatic bone to the corner of the mouth, used to smile.

Sternocleidomastoid

A prominent two-headed neck muscle extending from the sternum and clavicle up to the mastoid process of the temporal bone.

Frontalis

The cranial muscle covering the frontal bone of the forehead that raises the eyebrows and wrinkles the forehead horizontally.

Orbicularis oculi

The circular sphincter muscle that surrounds the eye socket and closes the eyelids.

Orbicularis oris

The circular sphincter muscle surrounding the mouth that closes and protrudes the lips.

Pectoralis major

A large, fan-shaped muscle covering the superior portion of the anterior chest wall.

Pectoralis minor

A flat, thin muscle lying deep to the pectoralis major on the anterior chest.

Serratus anterior

A saw-toothed muscle situated on the lateral surface of the ribs beneath the pectoralis major.

External abdominal oblique

The outermost lateral abdominal muscle whose fibers run downward and inward toward the midline.

Rectus abdominis

The medial, superficial anterior abdominal muscle running vertically from the pubis to the rib cage.

Transversus abdominis

The deepest muscle layer of the anterior and lateral abdominal wall, featuring fibers arranged horizontally.

Trapezius

A large diamond- or kite-shaped superficial muscle covering the posterior neck and superior back.

Latissimus dorsi

A broad, triangular muscle covering the lower posterior thorax and lumbar region.

Triceps brachii

A three-headed muscle located on the posterior compartment of the arm that extends the elbow joint.

Biceps brachii

A two-headed muscle situated on the anterior upper arm that flexes the elbow and supinates the forearm.

Deltoid

A thick, triangular muscle that caps the shoulder joint, serving as the prime mover of arm abduction.

Brachialis

An anterior arm muscle situated deep to the biceps brachii that acts as a major flexor of the forearm at the elbow.

Brachioradialis

A superficial muscle located on the lateral forearm that assists in elbow flexion.

Gastrocnemius

The superficial, two-bellied posterior calf muscle that plantarfexes the foot and flexes the knee.

Tibialis anterior

A superficial muscle lying on the anterior lateral side of the tibia that dorsiflexes and inverts the foot.

Soleus

A broad, flat muscle of the posterior calf located deep to the gastrocnemius that plantarflexes the foot.

Fulcrum (F)

The fixed point or hinge about which a lever pivots; in the human body, this role is performed by a joint.

Lever Arm

The rigid bar or rod that turns about a fulcrum; in the human body, this is formed by bones.

Resistance (R)

The load or weight being moved by a lever; in the human body, this is the weight of the body part or an external load.

Effort (M)

The force applied to move a resistance; in the human body, this force is provided by muscle contractions.

First class lever

A lever system where the fulcrum is located between the effort and the resistance ($\text{Effort} - \text{Fulcrum} - \text{Resistance}$); an anatomical example is nodding the head.

Second class lever

A lever system where the resistance is located between the fulcrum and the effort ($\text{Fulcrum} - \text{Resistance} - \text{Effort}$); an anatomical example is standing on tiptoes.

Third class lever

A lever system where the effort is applied between the fulcrum and the resistance ($\text{Fulcrum} - \text{Effort} - \text{Resistance}$); an anatomical example is flexing the forearm at the elbow.

Cartilaginous joint

A structural class of joints where bones are united directly by cartilage, such as intervertebral discs.

Fibrous joint

A structural class of joints where bones are joined by dense fibrous connective tissue without a joint cavity, such as cranial sutures.

Synovial joint

The most common structural joint type, characterized by articulating bone ends separated by a joint cavity enclosed in a fibrous capsule and containing synovial fluid.

Plane joint

A nonaxial synovial joint with flat articular surfaces that allows bones to glide past one another, such as intercarpal joints.

Hinge joint

A uniaxial synovial joint where the cylindrical end of one bone fits into the trough of another, allowing flexion and extension, such as the humeroulnar joint.

Pivot joint

A uniaxial synovial joint where the rounded end of one bone fits into a sleeve or shallow cavity of another to permit rotation, such as the radioulnar joint.

Condylar joint

A biaxial synovial joint where an egg-shaped articular surface fits into an oval cavity, such as metacarpophalangeal and metatarsophalangeal joints.

Saddle joint

A biaxial synovial joint in which each articulating surface possesses both concave and convex regions, such as the carpometacarpal joint.

Ball-and-socket joint

A multiaxial synovial joint where a spherical head of one bone fits into a cup-shaped depression of another, permitting motion in all planes, such as the coxofemoral joint.

Flexion

A movement that decreases the angle between articulating bones, bringing them closer together, typical of hinge and ball-and-socket joints.

Extension

A movement that increases the angle between articulating bones, moving them farther apart; the direct opposite of flexion.

Rotation

A movement that turns a bone around its own longitudinal axis, typical of pivot and ball-and-socket joints.

Abduction

A movement that carries a limb away from the midline or median plane of the body.

Adduction

A movement that draws a limb toward the midline or median plane of the body.

Circumduction

A complex movement in which the proximal end of a limb remains stationary while the distal end moves in a circle; a combination of flexion, extension, abduction, and adduction.

Manual muscle testing (MMT)

A physical examination procedure utilized to evaluate muscle weakness, atrophy, and/or paralysis; the standard MMT8 variation evaluates $8$ specific muscle groups.