Seeley's Anatomy & Physiology - Chapter 09: Muscular System Histology and Physiology

Vocabulary practice flashcards covering muscle tissue types, gross anatomy, sarcomere architecture, excitation-contraction coupling, cross-bridge cycling, mechanics, energetics, and muscle tissue comparisons from Chapter 09.

Contractility

The ability of a muscle to shorten forcefully.

Excitability

The capacity of a muscle to respond to an electrical stimulus, usually originating from the nervous system.

Extensibility

The ability of a muscle to be stretched beyond its normal resting length and still be capable of contracting.

Elasticity

The ability of a muscle to recoil back to its original resting length after being stretched.

Epimysium

A dense layer of connective tissue that encircles the entire skeletal muscle and merges externally with muscular fascia.

Muscular fascia

A connective tissue layer positioned between adjacent muscles as well as between muscles and the overlying skin.

Perimysium

A loose connective tissue layer that groups muscle fibers together into bundles called fascicles and provides pathways for blood vessels and nerves.

Muscle fascicle

A discrete bundle of skeletal muscle fibers bound together by the perimysium within the muscle organ.

Endomysium

A delicate sheath of loose connective tissue that surrounds and isolates individual muscle fibers within each fascicle.

Hypertrophy

An enlargement of skeletal muscle mass resulting from an increase in muscle fiber diameter, myofibril count, and nuclei from fused satellite cells, rather than an increase in total fiber number.

Sarcolemma

The specialized plasma membrane surrounding a muscle fiber, enclosing its sarcoplasm and cellular organelles.

Transverse tubules (T tubules)

Tubular invaginations of the sarcolemma extending into the interior of the muscle fiber to transmit electrical action potentials deep into the cell.

Sarcoplasmic reticulum (SR)

A modified, specialized smooth endoplasmic reticulum in muscle fibers that actively stores and releases calcium ions ($\text{Ca}^{2+}$).

Terminal cisternae

Enlarged sac-like segments of the sarcoplasmic reticulum positioned immediately adjacent to transverse tubules.

Triad

A functional three-part complex within skeletal muscle fibers consisting of a central transverse tubule (T tubule) flanked by two terminal cisternae of the sarcoplasmic reticulum.

Myofibril

A cylindrical bundle of contractile protein filaments (myofilaments) running longitudinally within an individual muscle fiber.

Sarcomere

The basic structural and functional contractile unit of a muscle fiber, bounded on each end by a Z disk.

Z disk

A filamentous protein network forming the boundary of a sarcomere and serving as the anchor point for actin (thin) myofilaments.

I band

The light-staining band of the sarcomere that contains only actin (thin) myofilaments and is bisected by a Z disk.

A band

The dark-staining central region of a sarcomere encompassing the entire length of the myosin (thick) myofilaments, including zones where actin and myosin overlap.

H zone

The central lighter region of the A band where thick myosin filaments do not overlap with thin actin filaments in a resting sarcomere.

M line

A dark transverse line situated in the exact center of the H zone composed of delicate protein filaments that anchor myosin myofilaments.

Titin

A huge elastic protein filament that anchors thick filaments to the Z disk, preserves filament alignment, provides extensibility, and aids in restoring resting sarcomere length.

Tropomyosin

An elongated, filamentous protein that winds along the groove of the F actin double helix, physically blocking active sites on actin in relaxed muscle.

Troponin

A three-subunit protein complex that binds to actin, tropomyosin, and calcium ions ($\text{Ca}^{2+}$), acting as the regulatory switch for muscle contraction.

Cross-bridge

The mechanical connection established when an energized myosin head attaches directly to an exposed active binding site on a G actin monomer.

Neuromuscular junction (Synapse)

The point of functional contact where an axon terminal of a motor neuron meets the motor end-plate of a skeletal muscle fiber.

Resting membrane potential

The electrical charge difference across an unstimulated plasma membrane, measuring approximately $-85\,\text{mV}$ in skeletal muscle cells, produced by unequal ion distributions and maintained by the $\text{Na}^+\text{--}\text{K}^+$ pump.

Depolarization

The phase of an action potential characterized by the rapid influx of $\text{Na}^+$ through voltage-gated $\text{Na}^+$ channels, shifting the membrane potential from approximately $-85\,\text{mV}$ to $+20\,\text{mV}$.

Repolarization

The restoration of resting membrane potential as voltage-gated $\text{Na}^+$ channels inactivate and voltage-gated $\text{K}^+$ channels open, allowing $\text{K}^+$ efflux from approximately $+20\,\text{mV}$ back toward $-85\,\text{mV}$.

Excitation-contraction coupling

The physiological mechanism by which an electrical action potential on the sarcolemma leads to intracellular $\text{Ca}^{2+}$ release from the sarcoplasmic reticulum and subsequent mechanical sliding of myofilaments.

Power stroke

The pivoting motion of the myosin head at its flexible hinge region following phosphate release, which pulls the actin myofilament inward toward the center of the sarcomere.

Recovery stroke

The realignment of the myosin head back to its pre-power stroke, high-energy cocked configuration following ATP hydrolysis.

Muscle twitch

The rapid contraction and relaxation cycle of a single muscle fiber occurring in response to a single electrical action potential in its motor neuron.

Lag phase (Latent phase)

The short time interval between the initial application of an electrical stimulus and the onset of detectable muscle shortening or tension development.

Motor unit

A single motor neuron together with all the individual skeletal muscle fibers innervated and controlled by its terminal axon branches.

Treppe

The stepped increase in twitch force observed when a muscle is subjected to repeated identical stimuli following complete relaxation between contractions, driven by residual sarcoplasmic $\text{Ca}^{2+}$ and increased enzymatic efficiency.

Wave summation

The addition of contraction forces that occurs when high-frequency action potentials arrive before the muscle fiber has completely relaxed from preceding twitches.

Incomplete tetanus

A state of sustained, undulating muscle contraction resulting from rapid stimulation where the muscle fibers partially relax between consecutive stimuli.

Complete tetanus

A steady, maximal, fusion contraction occurring at stimulation frequencies so high that no relaxation phase takes place between successive stimuli.

Active tension

The mechanical force developed by a contracting muscle fiber as cross-bridges pull actin past myosin, governed by the length-tension relationship.

Multiple motor-unit recruitment

The orderly activation of increasing numbers of motor units within a muscle in response to increasing stimulus strength, producing graded whole-muscle contractile force.

Threshold stimulus

The minimum stimulus voltage or intensity required to evoke an action potential in a motor neuron and produce a twitch in a single motor unit.

Maximal stimulus

The stimulus strength that activates the motor neurons of all motor units within a whole muscle, generating maximal contractile force.

Slow-twitch oxidative (Type I / SO) fiber

A small-diameter, fatigue-resistant skeletal muscle fiber equipped with high myoglobin, dense capillary beds, numerous mitochondria, and slow myosin ATPase, specialized for posture and prolonged endurance.

Fast-twitch glycolytic (Type IIb / FG) fiber

A large-diameter, fatigue-prone muscle fiber that exhibits low myoglobin, few capillaries and mitochondria, high glycogen stores, and fast myosin ATPase, adapted for rapid and powerful bursts of short duration.

Excess postexercise oxygen consumption (EPOC)

The elevated volume of oxygen intake required following exercise to clear lactate, replenish oxygen bound to myoglobin, and rebuild creatine phosphate and ATP stores to restore cellular homeostasis.

Physiological contracture

An extreme state of muscle fatigue caused by severe intracellular ATP depletion, in which cross-bridges cannot detach and calcium pumps cannot operate, leaving the muscle locked in contraction.

Caveolae

Small, shallow pocket-like indentations of the smooth muscle sarcolemma that concentrate extracellular $\text{Ca}^{2+}$ and functionally substitute for transverse tubules.

Dense bodies

Protein structures scattered throughout the sarcoplasm and attached to the sarcolemma in smooth muscle cells that anchor thin actin filaments and noncontractile intermediate filaments.

Calmodulin

The calcium-binding regulatory protein in smooth muscle cells that, when saturated with $\text{Ca}^{2+}$, binds and activates myosin light-chain kinase.

Myosin kinase (Myosin light-chain kinase)

An enzyme activated by the $\text{Ca}^{2+}\text{--}\text{calmodulin}$ complex in smooth muscle that transfers a phosphate from ATP onto myosin heads to initiate cross-bridge cycling.

Myosin phosphatase

An intracellular enzyme in smooth muscle cells that dephosphorylates myosin heads, causing cross-bridge detachment and muscle relaxation.

Intercalated disks

Specialized transverse junctions connecting adjacent cardiac muscle cells that incorporate gap junctions for electrical conduction and desmosomes for mechanical cohesion.