Chapter 1: Structure and Function of Exercising Muscle

A set of 120 vocabulary flashcards reviewing Chapter 1 on muscle anatomy, sliding filament theory, fiber types, motor unit recruitment, and force generation.

Smooth muscle

An involuntary muscle tissue found in the walls of hollow organs.

Cardiac muscle

An involuntary muscle tissue located exclusively in the heart.

Skeletal muscle

A voluntary muscle tissue attached to the skeleton that controls movement.

Epimysium

The outer connective tissue layer surrounding an entire skeletal muscle.

Fasciculi

Bundles of individual muscle fibers within a skeletal muscle, surrounded by perimysium.

Perimysium

The connective tissue sheath surrounding each bundle of muscle fibers (fasciculus).

Muscle fiber

An individual, elongated muscle cell composed of myofibrils divided into sarcomeres.

Endomysium

The delicate connective tissue layer surrounding each individual muscle fiber.

Myofibril

A cylindrical contractile element inside a muscle fiber, occurring in numbers ranging from hundreds to thousands per fiber.

Sarcomere

The basic contractile element of skeletal muscle, arranged end to end along the full length of a myofibril.

Plasmalemma

The cell membrane of a muscle fiber that fuses with the tendon, conducts action potentials, maintains pH, and transports nutrients.

Satellite cells

Cells involved in muscle growth, development, and response to injury, immobilization, and training.

Sarcoplasm

The cytoplasm of a muscle cell, containing unique features such as glycogen storage and myoglobin.

Glycogen (in sarcoplasm)

The stored form of glucose present within the sarcoplasm of a muscle cell.

Myoglobin

An oxygen-binding protein located in the sarcoplasm of muscle cells.

Transverse tubules (T-tubules)

Extensions of the plasmalemma that run deep into the muscle fiber to carry action potentials.

Sarcoplasmic reticulum (SR)

A membrane structure within the muscle fiber that serves as the storage site for $Ca^{2+}$.

Terminal cisternae

Enlarged regions of the sarcoplasmic reticulum adjacent to T-tubules involved in $Ca^{2+}$ release.

Mitochondria (in muscle fiber)

Organelles located within the sarcoplasm responsible for cellular energy production.

Striations

The distinctive striped appearance of skeletal muscle formed by repeating dark and light band patterns in sarcomeres.

A-bands

The dark stripes in a sarcomere that contain both actin (thin) and myosin (thick) protein filaments.

I-bands

The light stripes in a sarcomere that contain only thin actin filaments.

H-zone

The central region of the A-band that contains only thick myosin filaments.

M-line

The structural line situated in the exact middle of the H-zone.

Z-disk

The boundary structure defining the outer edges of a sarcomere to which actin filaments anchor.

Actin (thin filaments)

Light-appearing contractile protein filaments anchored at the Z-disk, composed of actin, tropomyosin, and troponin.

Myosin (thick filaments)

Dark-appearing contractile protein filaments made of two intertwined strands with globular heads, stabilized by titin.

Myosin globular heads

Protrusions extending $360^\text{o}$ around the thick filament axis that interact with actin filaments during contraction.

Titin

A structural protein that stabilizes myosin filaments along their axis and keeps actin filaments equally spaced.

Tropomyosin

A regulatory protein on the thin filament that covers the myosin-binding site on actin when the muscle is at rest.

Troponin

A regulatory protein anchored to actin that binds $Ca^{2+}$ and moves tropomyosin away from the active sites.

Nebulin

An anchoring protein located along thin filaments in the sarcomere structure.

Myosin-binding site

The active site on a G-actin molecule where the myosin globular head attaches during contraction.

$\alpha$-Motor neuron

A nerve cell that innervates muscle fibers to initiate muscle contraction.

Motor unit

A single ̑\alpha-motor neuron and all the specific muscle fibers it innervates.

Neuromuscular junction

The site of communication between an ̑\alpha-motor neuron and a muscle fiber, consisting of a synapse.

Synapse

The junction across which nerve impulses pass from an ̑\alpha-motor neuron to a muscle fiber.

Axon hillock

The region of the motor neuron cell body where the axon originates and action potentials are generated.

Axon terminal

The endpoint of an ̑\alpha-motor neuron axon where acetylcholine (ACh) is stored and released.

Dendrites

Branched extensions of an ̑\alpha-motor neuron cell body that receive incoming signals.

Motor end plates

Specialized regions of the muscle fiber plasmalemma located at the neuromuscular junction containing ACh receptors.

Excitation-contraction coupling

The sequence of events by which an action potential on the plasmalemma leads to the sliding of myofilaments.

Acetylcholine (ACh)

The neurotransmitter released from the axon terminal into the synaptic cleft to initiate muscle excitation.

ACh receptors

Proteins located on the plasmalemma of the muscle fiber that bind acetylcholine released from the axon terminal.

Synaptic cleft

The narrow gap between the axon terminal of an ̑\alpha-motor neuron and the plasmalemma of a muscle fiber.

Storage vesicles

Structures within the axon terminal that contain the neurotransmitter acetylcholine (ACh).

$Ca^{2+}$ release trigger

An action potential traveling down the T-tubules reaching the sarcoplasmic reticulum, which responds to electrical charge.

Troponin-$Ca^{2+}$ complex

The structure formed when $Ca^{2+}$ binds to troponin, causing a conformational change that moves tropomyosin.

Resting tropomyosin position

The position of tropomyosin blocking the myosin-binding site on actin in the absence of $Ca^{2+}$.

Sliding filament theory

The model describing how muscle contraction occurs by thick and thin filaments sliding past one another to shorten the sarcomere.

Power stroke

The action where the myosin head tilts back from $90^\text{o}$ to $45^\text{o}$, pulling the thin actin filament toward the M-line.

Sarcomere contracted state

The condition where myosin heads pull actin toward the center, shortening the sarcomere, H-zone, and I-band while A-band remains constant.

Myosin cross-bridge ready state angle

The $45^\text{o}$ angle at which the myosin cross-bridge is tightly bound to the actin filament prior to ATP binding.

Myosin head angle during binding

The $90^\text{o}$ angle assumed by the myosin head when it binds to a new actin molecule after hydrolyzing ATP.

ATP binding function (in contraction)

The step in the contraction cycle where ATP binds to myosin, allowing the myosin head to release from the actin filament.

Inorganic phosphate ($P_i$) release function

The step in the cross-bridge cycle that initiates the power stroke, tilting the myosin head back to $45^\text{o}$.

ADP release function

The event following the power stroke where the myosin head releases ADP and returns to its tightly bound ready state.

Adenosine triphosphate (ATP)

The essential energy molecule required for both muscle contraction (power stroke) and muscle relaxation ($Ca^{2+}$ pumping).

Myosin ATPase

An enzyme located on the myosin head that splits ATP into ADP and $P_i$ to release energy.

ATP hydrolysis equation

The chemical breakdown of ATP represented as $\text{ATP} \rightarrow \text{ADP} + P_i + \text{energy}$.

Active $Ca^{2+}$ pumping

The ATP-dependent process by which $Ca^{2+}$ is pumped back into the sarcoplasmic reticulum when action potentials cease.

Muscle relaxation trigger

The cessation of action potentials, which stops electrical stimulation of the SR and triggers $Ca^{2+}$ reuptake.

Type I muscle fibers

Slow-twitch muscle fibers that make up $\sim 50\%$ of an average muscle and have high aerobic endurance.

Type I peak tension time

The time required for Type I fibers to reach peak tension, which is $110\,ms$.

Type II muscle fibers

Fast-twitch muscle fibers that reach peak tension rapidly and are subdivided into Type IIa and Type IIx.

Type II peak tension time

The time required for Type II fibers to reach peak tension, which is $50\,ms$.

Type IIa muscle fibers

Fast-twitch fibers comprising $\sim 25\%$ of an average muscle, characterized as fast oxidative/glycolytic.

Type IIx muscle fibers

Fast-twitch fibers comprising $\sim 25\%$ of an average muscle, characterized as fast glycolytic and used for short, explosive efforts.

Muscle biopsy

A diagnostic procedure where a small ($10-100\,g$) piece of muscle is removed, frozen, sliced, and examined under a microscope.

Gel electrophoresis (muscle typing)

A laboratory technique used to separate different myosin isoforms by size to identify Type I vs. Type II fibers.

Fast myosin ATPase

An ATPase isoform characteristic of Type II fibers that allows rapid contraction cycling.

Slow myosin ATPase

An ATPase isoform characteristic of Type I fibers that results in slower contraction cycling.

Sarcoplasmic reticulum in Type II fibers

A highly developed SR network in Type II fibers enabling $Ca^{2+}$ release that is $3\text{ to }5$ times faster $V_o$.

Type I motor unit size

A motor unit composed of a smaller motor neuron innervating $\le 300$ muscle fibers.

Type II motor unit size

A motor unit composed of a larger motor neuron innervating $\ge 300$ muscle fibers.

Slow-twitch (ST)

An alternative name under System 2 for Type I muscle fibers.

Slow oxidative (SO)

An alternative classification name under System 3 for Type I muscle fibers based on metabolic activity.

Fast-twitch a (FTA)

An alternative name under System 2 for Type IIa muscle fibers.

Fast oxidative/glycolytic (FOG)

An alternative classification name under System 3 for Type IIa muscle fibers.

Fast-twitch x (FTX)

An alternative name under System 2 for Type IIx muscle fibers.

Fast glycolytic (FG)

An alternative classification name under System 3 for Type IIx muscle fibers.

Oxidative capacity

The relative ability of a muscle fiber to produce ATP aerobically, rated high in Type I, moderately high in Type IIa, and low in Type IIx.

Glycolytic capacity

The relative ability of a muscle fiber to produce ATP anaerobically, rated low in Type I, high in Type IIa, and highest in Type IIx.

Fatigue resistance

The ability of a muscle fiber to sustain work over time, rated high in Type I, moderate in Type IIa, and low in Type IIx.

Motor unit strength

The force-producing capability of a motor unit, rated low in Type I, high in Type IIa, and high in Type IIx.

Contractile speed

The speed of muscle fiber shortening, classified as slow in Type I and fast in both Type IIa and Type IIx.

Peak power hierarchy

The order of maximal power output among muscle fiber types: $\text{Type IIx} > \text{Type IIa} > \text{Type I}$.

Optimal power force percentage

The relative force level at which all muscle fibers reach peak power, occurring at $\sim 20\%$ of peak force.

Limb fiber type distribution

The pattern where individual arm and leg muscle fiber ratios are generally similar within a single person.

Soleus muscle fiber composition

A specific muscle in which Type I fibers predominate in virtually everyone.

Endurance athlete fiber dominance

An athletic profile where Type I muscle fibers predominate in the primary active muscles.

Power athlete fiber dominance

An athletic profile where Type II muscle fibers predominate in the primary active muscles.

Motor neuron conduction velocity

The speed at which nerve impulses travel down an ̑\alpha-motor neuron axon, which is slower for Type I and faster for Type II.

Aerobic endurance of Type I fibers

The high capacity of Type I fibers to efficiently produce ATP from fat and carbohydrate in the presence of oxygen.

Anaerobic ATP production in Type II fibers

The primary metabolic path for Type II fibers, which produce ATP without oxygen and fatigue quickly.

Type IIa fiber exercise suitability

Activities requiring short, high-intensity endurance efforts, such as a $1\text{,}600\,m$ run.

Type IIx fiber exercise suitability

Activities requiring short, explosive maximum effort, such as a $100\,m$ sprint.

Genetic factors in fiber differentiation

The primary determinant of muscle fiber types, as ̑\alpha-motor neurons dictate fiber differentiation.

Training-induced fiber type changes

Small changes in muscle fiber type characteristics, capped at approximately $10\%$, induced by endurance or strength training.

Aging effect on muscle fibers

The age-related process in which skeletal muscles progressively lose Type II motor units.