Muscle Physiology Practice Flashcards

Comprehensive vocabulary flashcards covering muscle anatomy, classification, the mechanism of contraction (sliding-filament theory), energy metabolism, and muscle fiber types.

Functional Classification of Muscle

The categorization of muscles into voluntary and involuntary types.

Structural Classification of Muscle

The categorization of muscles into striated and smooth types.

Skeletal Muscle

Striated voluntary muscle that is generally attached to the skeleton.

Smooth Muscle

Smooth involuntary muscle found primarily in tubular organs that lacks sarcomeres.

Cardiac Muscle

Striated involuntary muscle found in the wall of the heart, characterized by intercalated disks.

Epimysium

Connective tissue that ensheathes the entire skeletal muscle.

Fasicles (or fascicles)

Subunits or bundles of muscle fibers within a skeletal muscle.

Perimysium

Connective tissue that surrounds individual fascicles.

Endomysium

Connective tissue that ensheathes individual muscle fibers (muscle cells).

Myofibrils

Fibrils contained within muscle cells that consist of long protein molecules called myofilaments.

Sarcolemma

The muscle cell plasma membrane.

Sarcoplasm

The muscle cell cytoplasm containing mitochondria and enzymes.

Sarcoplasmic reticulum

A structure enveloping each myofibril whose primary function is to store $Ca^{2+}$ ions.

T tubules

Transverse extensions of the sarcolemma that conduct electrical impulses from the cell surface to the sarcoplasmic reticulum.

Terminal cisterns

Enlarged ends of the sarcoplasmic reticulum that make contact with T tubules.

Triad

The structure formed at the A, I junction by the contact of a T tubule and two terminal cisterns.

A (anisotropic) band

The region of the myofibril constituted by thick filaments.

I (isotropic) band

The region of the myofibril constituted by thin filaments.

H band

A lighter region within the A band where there is no overlap between thick and thin filaments.

M line

A dark line within the H band representing proteins that connect adjacent thick filaments.

Z line

A dark line running through each I band that connects adjacent thin filaments.

Sarcomere

The unit from one Z line to the next, serving as the functional or contractile unit of a muscle.

Myosin

The protein that composes thick myofilaments, consisting of a tail and two heads.

Cross-bridges

The myosin heads that project from the core of the thick filament and contain actin-binding and ATPase sites.

Actin

Spherical molecules that form thin filaments in the shape of a double helix.

Tropomyosin

A long protein filament that entwines actin and blocks myosin attachment sites in a relaxed muscle.

Troponin T (TnT)

The subunit of troponin that binds to tropomyosin.

Troponin I (TnI)

The subunit of troponin that binds to actin to hold the troponin-tropomyosin complex in place.

Troponin C (TnC)

The subunit of troponin that binds to $Ca^{2+}$ ions to produce a conformational change.

Resting Membrane Potential (RMP) of Muscle

The electrical charge of a skeletal muscle fiber at rest, which is $-90\,mV$.

Excitation-contraction coupling

The process where depolarization of the muscle fiber initiates contraction via electrical signals.

Dihydropyridine receptors (DHPRs)

Voltage-dependent calcium channels in the T-tubule membrane that sense voltage changes.

Ryanodine receptors (RyRs)

Receptors in the sarcoplasmic reticulum that open to release $Ca^{2+}$ into the sarcoplasm.

Power stroke

The rotation of the myosin head that pulls the thin filament toward the center of the sarcomere after $P_i$ is released.

Sliding-Filament Theory

The explanation that muscle force is produced by thick and thin filaments sliding past one another without changing their own lengths.

Creatine phosphate (phosphagen system)

An alactic anaerobic energy source that directly phosphorylates ADP to ATP and sustains contraction for $\approx 5-8$ seconds.

Lactic anaerobic metabolism

Energy generation through glycolysis that produces lactate and can sustain contraction for up to $1\,minute$.

Oxidative phosphorylation

A slow but efficient system of cellular respiration in mitochondria used for prolonged activity.

Rigor

A state of extreme rigidity caused by the depletion of ATP and creatine phosphate; called rigor mortis after death.

Isometric Contractions

Contractions where muscle develops force equal to the load, resulting in no shortening or visible movement.

Isotonic Contractions

Contractions in which a muscle generates constant force to move a load, resulting in a change in length.

Concentric isotonic contraction

A contraction where the muscle shortens because the generated force overcomes the resistance.

Eccentric isotonic contraction

A contraction where the muscle lengthens because the generated force is insufficient to overcome the external load.

Motor unit

A motor neuron and all the associated skeletal muscle fibers it stimulates.

Threshold stimulus

The minimal stimulus required to result in a muscle twitch.

Latent period

The phase of a twitch lasting $\approx 2\,msec$ between stimulation and force generation, required for $Ca^{2+}$ release.

Type I (Slow Oxidative) fibers

Small, red, fatigue-resistant fibers loaded with mitochondria and myoglobin, dominant in postural muscles.

Type IIa (Fast oxidative) fibers

Red, fatigue-resistant fibers with high ATP-splitting rates and high contraction velocity.

Type IIb (Fast Glycolytic) fibers

Large, white fibers rich in glycogen that fatigue easily and are used for rapid movements.