Muscular System – Key Terms and Concepts (Seeley’s Essentials of Anatomy and Physiology, 9th Ed.)

Vocabulary flashcards covering key muscular system terms, structures, and physiological concepts from the lecture notes.

Sarcomere

The basic contractile unit of a skeletal muscle, extending from one Z disc to the next and containing overlapping actin (thin) and myosin (thick) filaments.

Myofibril

A long, rod-like bundle inside a muscle fiber that contains many sarcomeres arranged in series.

Fascicle

A bundle of muscle fibers surrounded by the perimysium.

Epimysium

Connective tissue sheath that surrounds the entire muscle.

Perimysium

Connective tissue that surrounds a fascicle within a muscle.

Endomysium

Connective tissue that surrounds an individual muscle fiber.

Skeletal muscle

Voluntary, striated muscle attached to bone; makes up about 40% of body weight.

Striations

Transverse bands observed in skeletal muscle due to the organized arrangement of actin and myosin.

Actin

Thin filament protein that forms the backbone of thin filaments and binds myosin during contraction.

Myosin

Thick filament motor protein that forms cross-bridges with actin during contraction.

Z disk

Anchor point for actin filaments at the ends of a sarcomere.

M line

Central stabilize of myosin filaments within the sarcomere.

Troponin

Regulatory protein that binds calcium and moves tropomyosin to expose actin binding sites.

Tropomyosin

Regulatory protein that blocks myosin-binding sites on actin until calcium-troponin complex moves it aside.

Neuromuscular junction (NMJ)

The synapse where a motor neuron communicates with a skeletal muscle fiber to initiate contraction.

Acetylcholine (ACh)

Neurotransmitter released at the NMJ that binds receptors on the muscle fiber to trigger a depolarizing current.

Synaptic cleft

The gap between the presynaptic terminal and the postsynaptic membrane.

Presynaptic terminal

End of the motor neuron that releases neurotransmitters (e.g., ACh) into the synaptic cleft.

Postsynaptic membrane

Motor endplate on the muscle fiber that contains ACh receptors.

Action potential

A rapid depolarization and repolarization of a cell’s membrane that propagates along the membrane.

Resting membrane potential

The baseline negative electrical charge inside the cell relative to the outside.

Na+ channels

Voltage-gated channels that open to allow Na+ influx during depolarization.

Ca2+ ions

Calcium ions released from the sarcoplasmic reticulum to trigger contraction.

Sarcoplasmic reticulum (SR)

Intracellular calcium storage organelle that releases Ca2+ during excitation–contraction coupling.

T-tubules

Invaginations of the sarcolemma that transmit action potentials into the muscle fiber interior.

Cross-bridge

Myosin head bound to actin during contraction, forming the myosin–actin linkage.

Power stroke

Pivoting of the myosin head pulling actin toward the center of the sarcomere, with release of ADP and Pi.

ATP

Adenosine triphosphate; energy source for muscle contraction; binds to myosin to detach cross-bridges.

Creatine phosphate

High-energy phosphate reservoir that rapidly regenerates ATP for short bursts of activity.

Glycolysis

Anaerobic breakdown of glucose to produce ATP and pyruvate (or lactic acid if no oxygen).

Aerobic respiration

Glucose breakdown in the presence of oxygen producing CO2, water, and large amounts of ATP (mitochondria).

Anaerobic respiration

ATP production without oxygen, primarily via glycolysis, resulting in pyruvate’s conversion to lactic acid.

Lactic acid

Byproduct of anaerobic glycolysis that can contribute to muscle fatigue.

Muscle fatigue

Temporary decrease in muscle performance due to ATP depletion, acidosis, ROS, inflammation, etc.

Isotonic contraction

Muscle contraction with changing length while the tension remains constant.

Concentric contraction

Isotonic contraction in which the muscle shortens while generating force.

Eccentric contraction

Isotonic contraction in which the muscle lengthens while producing force.

Isometric contraction

Muscle contraction with increasing tension but no change in length.

Muscle tone

Constant, partial contraction of many motor units that maintains posture.

Slow oxidative (SO) fibers

Type I muscle fibers; slow contractions, high endurance, rely on aerobic respiration.

Fast oxidative (FO) fibers

Type IIa fibers; fast contractions, mainly aerobic but can switch to glycolysis.

Fast glycolytic (FG) fibers

Type IIb fibers; fast contractions, primarily anaerobic; fatigue quickly.

Agonist (prime mover)

Muscle whose contraction produces the main movement in a given direction.

Antagonist

Muscle that opposes the action of the agonist.

Synergist

Muscle that assists the agonist by adding force or stabilizing joints.

Fixator

A synergist that immobilizes the origin to improve the action of the agonist.

Motor unit

Single motor neuron and all the muscle fibers it innervates.

Origin

The attachment site that remains relatively fixed during a movement.

Insertion

The attachment site that moves toward the origin during a movement.

Tendon

Dense connective tissue that attaches a muscle to bone.

Rotator cuff

Group of four shoulder muscles: supraspinatus, infraspinatus, teres minor, and subscapularis.

Diaphragm

Primary muscle of respiration that separates the thorax from the abdomen.

External intercostals

Muscles between the ribs that help elevate the ribs during inspiration.

Internal intercostals

Muscles between the ribs that aid expiration by drawing ribs downward.

Rectus abdominis

abdominal wall muscle that flexes the vertebral column.

External abdominal oblique

Abdominal wall muscle that compresses abdomen and rotates vertebral column.

Internal abdominal oblique

Abdominal wall muscle that compresses abdomen and supports trunk.

Transversus abdominis

Deep abdominal wall muscle that compresses abdomen.

Pelvic floor muscles

Muscles supporting pelvic viscera, including levator ani and perineal muscles.

Levator ani

Pelvic floor muscle that supports pelvic organs and maintains continence.

Sartorius

Longest human muscle; crosses thigh from hip to knee.

Quadriceps femoris

Group of four muscles on the front of the thigh extending the knee.

Hamstrings

Posterior thigh muscles (biceps femoris, semitendinosus, semimembranosus) that flex the knee.

Gastrocnemius

Calf muscle that plantarflexes the foot and assists knee flexion.

Soleus

Calf muscle beneath gastrocnemius that plantarflexes the foot; important for posture.

Tibialis anterior

Front leg muscle that dorsiflexes and inverts the foot.

Extensor digitorum longus

Forearm/leg muscle that extends the toes and dorsiflexes the foot.

Extensor hallucis longus

Extensor of the big toe; assists dorsiflexion.

Fibularis (peroneus) muscles

Or in the leg: fibularis longus and brevis, evert and plantarflex the foot.

Intercostal muscles

Muscles between the ribs; external (inspiration) and internal (forced expiration) muscles.

Linea alba

Fibrous structure running down the midline of the abdomen; aponeuroses of abdominal muscles meet here.

Dense bodies

Anchoring points for smooth muscle actin filaments, analogous to Z-discs in striated muscle.

Calveoli

Membrane invaginations in smooth muscle cells that aid calcium handling.

Autorhythmicity

Self-excitable pacemaker cells in cardiac muscle that set heart rate.

Intercalated disks

junctions between cardiac muscle cells that allow rapid electrical coupling.

Fascicle arrangement

Pattern of muscle fiber organization within a muscle (circular, convergent, parallel, triangular, rhomboidal, rectangular, pennate, fusiform).

Pennate muscle

Fascicles arranged obliquely to a tendon; includes uni-, bi-, and multipennate forms.

Skeletal muscle organization (levels)

Muscle organized into muscle fibers -> myofibrils -> actin/myosin; connective tissue layers structure the whole.

Sliding filament model

Myosin cross-bridges pull actin filaments; sarcomere shortens with bands (H and I) reducing, A band unchanged.

Nerve supply and muscle fiber stimulation

Motor neurons release ACh at NMJ, triggering an action potential in the muscle fiber.

Summary concept – energy for contraction

ATP is regenerated from immediate stores, creatine phosphate, glycolysis, and aerobic respiration to sustain contraction.