Muscle Structure, Physiology, and Mechanics Flashcards

A collection of 100 flashcards based on lecture notes regarding muscle anatomy, sarcomere organization, neuromuscular transmission, contraction mechanics, and pathologies.

Excitable

The characteristic of a muscle cell where a motor neuron stimulates it by releasing ACH to generate a change in membrane potential, developing action potentials.

Contractile

The ability of a muscle to forcibly shorten due to changes in membrane potential.

Extensible

The capacity of a muscle to stretch beyond its normal resting point.

Elastic

The ability of a stretched muscle to retract back to its normal or resting point.

Epimysium

A dense, fibrous, irregular, and very tough connective tissue that forms the outermost layer covering the muscle tissue.

Fascicles

Bundles of muscle fibers located inside the muscle belly.

Perimysium

A dense, fibrous, irregular connective tissue that covers each grouping of fascicles and is a direct continuation of the epimysium.

Muscle Fiber (Cell)

The individual cells that make up fascicle bundles.

Endomysium

A layer made of areolar connective tissue that covers the plasma membrane (sarcolemma) of individual muscle fibers.

Tendon

A rope-like connective tissue rich in collagen that connects muscle to bone.

Aponeurosis

A sheet-like connective tissue that connects muscle to bone.

Origin

The part of the bone that does not move during a muscle contraction.

Insertion

The part of the bone that moves during a muscle contraction.

Direct Connection

An attachment where the epimysium is connected to the bone via the periosteum or perichondrium.

Indirect Connection

An attachment where tendons, which are resilient to friction, connect the muscle to the bone while conserving space.

Sarcoplasmic Reticulum

A derivative of the endoplasmic reticulum that serves as the calcium storage factory in muscle fibers.

Sarcolemma

The plasma membrane that surrounds each individual muscle fiber.

Sarcomere

The functional unit of a myofibril, defined as the distance from one Z-disc to another Z-disc.

Z-disk

A zigzag protein structure composed of $\alpha$-actinin that attaches myofibrils to one another across the muscle fiber.

Thick Filament

A red structure within the sarcomere composed primarily of myosin filaments.

Titin

A springy protein that anchors the thick filament to the Z-disc and connects it to the M-line to maintain side-by-side filament alignment.

M-line

The midpoint of the sarcomere responsible for connecting titin and stabilizing the thick filament; composed of myomesin, C-proteins, and creatine kinase.

A-band

The anisotropic dark band representing the full length of the thick filament, including overlapping actin filaments.

I-band

The isotropic light band consisting only of actin filaments, spanning the distance between the ends of thick filaments in adjacent sarcomeres.

H-zone

The distance between the ends of thin filaments within the same sarcomere.

G-actin

The globular monomer of actin filaments, with each molecule having one molecule of ADP attached.

F-actin

The filamentous polymer formed by multiple G-actin monomers resulting in a supramolecular helix.

Tropomyosin

A protein that surrounds and blocks the active sites of actin in a resting position to prevent myosin heads from binding.

Troponin C

The specific subunit of troponin where $Ca^{2+}$ binds to initiate the contraction process.

Troponin T

The subunit of troponin that serves as the binding site for tropomyosin.

Troponin I

The inhibitory subunit of troponin which serves as the site where actin filaments bind.

Myosin Tail

An $\alpha$-helix structure composed of two heavy chains that connects to the thick filament.

Myosin Head

A globular polypeptide structure that binds to actin active sites and contains myosin ATPase to cleave ATP.

Myosin ATPase

An enzyme located in the myosin head that cleaves ATP into ADP and Pi to provide energy for sliding myofilaments.

Regulatory Light Chain (RLC)

A polypeptide chain in the myosin neck that can be phosphorylated to change the activity of the myosin.

Type IIx Muscle Fiber

Fast glycolytic fibers characterized by fast contractility, intermediate diameter, very low fatigue resistance, and high power production.

Dystrophin

A protein that links actin to the sarcolemma through a protein complex, which then links to the extracellular matrix.

Duchenne Muscular Dystrophy

A severe X-linked recessive disorder where no functional dystrophin is produced due to nonsense or frameshift mutations.

Becker Muscular Dystrophy

A less severe muscle disorder caused by misfolded dystrophin protein due to missense mutations, developing around age 10-20.

Gower sign

A clinical presentation where a patient uses their upper extremities to stand up, often seen in muscular dystrophy cases.

Anterior (Ventral) Grey Horn

The region of the spinal cord that contains clumps of cell bodies of somatic motor neurons.

Somatic Motor Neuron

A neuron that coordinates voluntary movement, stimulates skeletal muscle, and primarily uses acetylcholine.

Alpha Motor Neuron

The specific motor neuron that supplies skeletal muscles.

Choline Acetyl Transferase

The enzyme that catalyzes the reaction: Choline + Acetyl CoA $\rightarrow$ Acetylcholine + CoA.

Anterograde Axonal Transport

The method by which vesicles and mitochondria are moved from the cell body to the synaptic bulb.

VAChT Transporter Complex

The vesicular acetylcholine transporter complex that brings ACh into vesicles using a proton concentration gradient.

Proton Concentration Gradient (Vesicular)

The gradient created by proton pumps (using ATP) where high internal proton levels allow the ACh antiporter to function via secondary transport.

Synaptotagmin

A vesicle snare protein (V-snare) that participates in the exocytosis of acetylcholine.

Synaptobrevin

A vesicle snare protein (V-snare) that wraps around T-snares during vesicle fusion with the cell membrane.

SNAP-25

A target snare protein (T-snare) located on the cell membrane that interacts with V-snares for vesicle fusion.

Syntaxin

A target snare protein (T-snare) located on the cell membrane that binds with synaptobrevin.

Synaptic Cleft

The space into which the internal environment of synaptic vesicles is exposed during exocytosis.

Resting Membrane Potential (Neuron)

The voltage of a neuron at rest, typically $-70\,mV$, where the inside is relatively more negative than the outside.

Repolarization (Neuron)

The process of returning the neuron to $-70\,mV$ achieved through the exit of $K^+$ ions via voltage-gated channels.

Nicotinic Receptors

Pentameric ligand-gated ion channels on the muscle surface, composed of 2 $\alpha$, 1 $\beta$, 1 $\partial$, and 1 $\gamma$ subunits.

Transverse Tubules (T-tubules)

Inward extensions or invaginations of the sarcolemma that allow depolarization waves to travel into the cell fiber.

Triad

A structural group within a muscle fiber consisting of one T-tubule and two adjacent terminal cisternae of the sarcoplasmic reticulum.

Resting Potential (Muscle Fiber)

The voltage inside a muscle fiber in relation to its surroundings when inactive, typically $-90\,mV$.

Sodium/Potassium ATPase

A membrane protein that maintains resting potential by moving 2 $K^+$ ions into the cell and 3 $Na^+$ ions out while consuming ATP.

Threshold Potential

The voltage ($-55\,mV$) at which activation gates of voltage-gated sodium channels open to initiate an action potential.

Motor End Plate Potential

The initial positive shift in membrane potential caused by $Na^+$ influx through open nicotinic receptors.

Dihydrophyridine Receptors

Receptors within the T-tubule that are stimulated by depolarization and mechanically activate ryanodine receptors.

Ryanodine Receptor Type 1 (RYR-1)

A receptor embedded in the sarcoplasmic reticulum membrane that, when activated, creates a pore to release $Ca^{2+}$ ions into the sarcoplasm.

Calsequestrin

A protein that binds to calcium to maintain its high concentration within the sarcoplasmic reticulum.

Power Stroke

The component of muscle contraction where the release of inorganic phosphate (Pi) causes myosin to push the thin filament toward the M-line.

Acetylcholinesterase

The enzyme in the synaptic cleft that terminates muscle stimulation by breaking down ACh into acetate and choline.

Myasthenia Gravis

An autoimmune disease where autoantibodies block nicotinic receptors, causing progressive muscle weakness that worsens with activity.

Ptosis

Droopy eyelid, which is a common early symptom of eye muscle weakness in Myasthenia Gravis.

Lambert Eaton Syndrome

An autoimmune disorder where autoantibodies bind to voltage-gated calcium channels on the pre-synantic neuron, preventing ACh release.

Botulin Toxin

A protease produced by Clostridium Botulinum that cleaves SNAP-25, synaptobrevin, or syntaxin, preventing ACh release.

Tetanus Toxin

A neurotoxin that cleaves synaptobrevin, travels to the spinal cord, and prevents the release of GABA, leading to muscle spasms.

Sarin Gas

A chemical agent that binds irreversibly to acetylcholinesterase, leading to muscle fatigue, paralysis, and respiratory failure.

Dendrotoxin

A toxin from the black mamba snake that inhibits voltage-gated potassium channels, leading to continued release of ACh and convulsions.

Bungarotoxin $\alpha$

A toxin produced by cobra snakes that blocks nicotinic receptors on the muscle surface, preventing activation.

Subthreshold Stimulus

A stimulus that does not reach the minimal threshold needed to produce a muscle contraction, resulting in no response.

Threshold Stimulus

The minimum amount of voltage needed to produce the first observable muscle contraction.

Maximal Stimulus

The voltage level that produces the most force a muscle can exert, after which increasing voltage no longer increases contraction.

Multiple Motor Unit Summation

Also known as recruitment, it is the combined effect of multiple motor units acting within a muscle at any given time.

Size Principle

The recruitment order where the smallest muscle fibers and most excitable motor neurons are activated first.

Muscle Twitch

A very brief muscle contraction in response to a single neural stimulus, consisting of latent, contraction, and relaxation phases.

Latent Phase

The very brief period during a muscle twitch where cross-bridges are formed but the fibers are not yet contracting.

Contraction Phase

The phase of a muscle twitch where myosin heads create power strokes and tension increases rapidly.

Relaxation Phase

The phase of a muscle twitch where tension decreases as potassium channels open and $Ca^{2+}$ is reuptaken into the sarcoplasmic reticulum.

Refractory Period

A short window of time during which a muscle cell cannot generate another action potential or contraction.

Unfused (Incomplete) Tetanus

A sustained and quivering contraction produced by a frequency of stimulation that allows only partial relaxation.

Fused (Complete) Tetanus

A sustained and powerful contraction produced by stimuli that are one after the other with no breaks, allowing no relaxation.

Wave Summation

Also known as temporal summation, it involves several stimuli provided one after the other to increase the strength of contraction.

Isometric Contraction

A contraction that generates tension while muscle length remains constant because the generated force is smaller than the resistance of the load.

Isotonic Contraction

A contraction where force remains constant while the muscle either shortens or lengthens.

Concentric Contraction

A form of isotonic contraction where the muscle shortens because the muscle force exceeds the load force.

Eccentric Contraction

A form of isotonic contraction where the muscle lengthens to lower a load slowly, generating high tension.

Fulcrum

The fixed point around which movement occurs in a lever system.

Dense Bodies

Smooth muscle structures made of $\alpha$-actinin that act as Z-discs and are anchored to the plasma membrane.

Attachment Plaques

Integrin proteins located at the inner cytosolic side of smooth muscle cells that anchor dense bodies to the membrane.

Unitary (Single Unit) Smooth Muscle

Visceral smooth muscle cells interconnected by gap junctions that generate rhythmic and simultaneous contractions.

Multiunit Smooth Muscle

Independent smooth muscle cells, each with their own nerve ending, providing fine control (e.g., in the iris and ciliary muscle).

Interstitial Cell of Cajal

Pacemaker cells in smooth muscle that intrinsically depolarize to create slow waves and the basic electrical rhythm.

Caveolae

Invaginations in smooth muscle membranes that contain voltage-gated calcium channels.

Calmodulin

The protein in smooth muscle that binds with intracellular calcium to form a complex that activates contraction enzymes.

Myosin Light Chain Kinase (MLCK)

The enzyme that adds a phosphate to the regulatory light chain on the myosin neck to stimulate ATPase activity.