Muscular System Lecture Notes

Comprehensive vocabulary flashcards covering the anatomy, physiology, functions, and clinical conditions of the muscular system based on lecture notes.

Movement

Voluntary locomotion and environmental manipulation.

Posture

Maintenance of physical form against gravity.

Thermogenesis

Heat production via skeletal muscle activity.

Contractility

Ability of muscle to shorten forcefully, or contract.

Excitability

Capacity of muscle to respond to an electrical stimulus.

Extensibility

The capacity of a muscle to be stretched beyond its normal resting length and still be able to contract.

Elasticity

Ability of muscle to spring back to its original resting length after it has been stretched.

Skeletal Muscle

Striated and multi-nucleated muscle constituting approximately $40\%$ of body weight under conscious/voluntary control.

Smooth Muscle

Non-striated, involuntary muscle found in the walls of hollow organs, blood vessels, and the iris.

Cardiac Muscle

Striated and branched involuntary muscle found only in the heart that exhibits autorhythmicity.

Visceral Smooth Muscle

Also called Unitary Smooth Muscle; the most common type found in hollow organs, characterized by cells electrically coupled by gap junctions.

Multiunit Smooth Muscle

Smooth muscle found in large lung airways, large arteries, and arrector pili muscles that contracts only when stimulated by motor nerves or hormones.

Epimysium

The outermost tough connective tissue sheath covering the entire muscle, blending into tendons.

Perimysium

Intermediate connective tissue layer that bundles individual muscle cells together into organized groups called fascicles.

Endomysium

The innermost layer consisting of a delicate, fine connective tissue sheath wrapping every individual muscle fiber.

Sarcolemma

The specialized plasma membrane enclosing the entire muscle fiber; it transmits electrical signals.

T-tubules (Transverse Tubules)

Deep extensions of the sarcolemma that dive into the fiber to carry electrical signals rapidly to all parts of the cell.

Sarcoplasmic Reticulum (SR)

Specialized smooth endoplasmic reticulum that stores and releases calcium ions ($Ca^{2+}$).

Sarcosomes

Abundant mitochondria within the muscle fiber that fuel high energy demands.

Myofibrils

Long, parallel cylindrical bundles of contractile proteins filling the sarcoplasm; the functional units that shorten to produce movement.

Sarcomere

The smallest repeating contractile unit of a myofibril, bounded by Z-lines.

Myosin

Thick myofilament composed of "heads" that form cross-bridges and pull on thin filaments.

Actin

Globular protein that forms the track for myosin to slide along; the bulk of the thin filament.

Troponin

Regulatory protein that holds tropomyosin in place and contains binding sites for calcium ions.

Tropomyosin

Protein that blocks the active sites on the actin molecules, preventing contraction when the muscle is at rest.

I Bands

Light bands in a myofibril consisting of thin filaments (actin) attached to Z lines.

A Bands

Dark bands in a myofibril consisting of thick filaments (myosin).

Dystrophin

Structural protein that links thin filaments to the proteins of the sarcolemma.

Neuromuscular Junction (NMJ)

The specialized synapse or point of contact between a motor neuron's axon terminal and the muscle fiber's sarcolemma.

Motor End Plate

The thickened, folded region of the sarcolemma at the NMJ packed with receptors for acetylcholine (ACh).

Synaptic Cleft

The tiny gap between the neuron's axon terminal and the motor end plate where chemical signals cross.

Depolarization

Phase of the action potential where voltage-gated $Na^+$ channels open, allowing $Na^+$ to rush into the cell and reverse the internal charge to $+20\,mV$.

Repolarization

Phase of the action potential where $Na^+$ channels close and voltage-gated $K^+$ channels open, restoring the negative internal charge.

Acetylcholine (ACh)

The neurotransmitter released into the synaptic cleft that stimulates receptors to initiate an impulse in the sarcolemma.

Acetylcholinesterase

Enzyme that immediately breaks down ACh in the synaptic cleft to prevent continuous muscle contraction.

Muscle Twitch

A single, brief contraction and relaxation cycle in a muscle fiber consisting of lag, contraction, and relaxation phases.

Treppe (Staircase Effect)

When a rested muscle is repeatedly stimulated at low frequencies, each subsequent contraction produces slightly greater tension until leveling off.

Recruitment

Also known as Multiple-Motor-Unit Summation; the process of scaling force by increasing the number of active motor units.

Isotonic (Concentric)

Contraction where tension remains constant while the muscle shortens (e.g., lifting a dumbbell).

Isotonic (Eccentric)

Contraction where tension remains constant while the muscle lengthens under load (e.g., lowering a dumbbell).

Isometric

Contraction where muscle length remains static, but internal tension increases dramatically.

Muscle Tone

The constant state of partial contraction maintaining posture against gravity.

Pennate

Fascicle arrangement where short fascicles attach obliquely to a central tendon like a feather.

Origin

The point of muscle attachment that remains relatively stable when a muscle contracts.

Insertion

The point of muscle attachment that moves during contraction.

Prime Movers (Agonist)

Muscles that produce the actual muscular action.

Synergist

Muscles that act as a group to help prime movers produce a desired effect.

Antagonist

Muscles whose partial contrary movement help produce a smoother primary action.

Fixators

Specialized synergists that inhibit or prevent movements that are not required.

Class I Lever

A lever system where the fulcrum is in the middle (e.g., neck extension tilting the head back).

Class II Lever

A lever system where weight is in the middle; designed for high strength (e.g., calf raise).

Class III Lever

A lever system where the pull is in the middle; the most common lever in the body prioritizing range and speed (e.g., biceps curl).

Duchenne Muscular Dystrophy

Severe inherited muscle dystrophy caused by a mutation in the dystrophin gene, leading to progressive weakness.

Myasthenia Gravis

Autoimmune disorder causing muscle weakness and impairment of movement.

Botulinum Toxin

Neurotoxin that causes paralysis by blocking the pre-synaptic release of ACh at the NMJ.

Tetanus

Condition known as 'Lock Jaw' caused by C. tetani, leading to muscle spasms and opisthotonus.