Chapter 14: Cellular Movement - Motility and Contractility

Comprehensive vocabulary flashcards covering microtubule- and microfilament-based cellular motility, motor proteins (kinesins, dyneins, myosins), striated and smooth muscle contraction mechanisms, and nonmuscle motility modes from Chapter 14 of Becker's World of the Cell.

Motility

The cellular movement involving movement of a cell or organism through the environment, movement of the environment past or through a cell, or movement of components within the cell.

Contractility

A specialized form of motility used specifically to describe the shortening of muscle cells.

Anterograde axonal transport

ATP-dependent transport of vesicles and proteins away from the centrosome toward the plus ends of microtubules at nerve endings, driven by kinesin-1.

Retrograde axonal transport

The movement of cargo along microtubules toward the minus ends (toward the centrosome and cell body), mediated by cytoplasmic dynein.

Fast axonal transport

The packaging and rapid movement of proteins in vesicles from the cell body to the nerve terminal along microtubules at rates of approximately $2\,\mu\text{m/sec}$ ($\sim 8\,\text{min/m}$).

Kinesin-1 (Classic kinesin)

A dimeric motor protein composed of two heavy chains and two light chains that moves cargo toward the plus ends of microtubules.

Kinesin-3

A monomeric kinesin motor protein responsible for the movement of synaptic vesicles in neurons.

Kinesin-5

A bipolar, tetrameric motor protein that drives the bidirectional sliding of microtubules during anaphase of mitosis.

Kinesin-13 (Catastrophins)

A dimeric kinesin protein that lacks motor transport activity and instead fosters depolymerization and destabilization of microtubule plus ends.

Kinesin-14

A specialized kinesin motor protein that moves toward the minus ends of microtubules and regulates spindle dynamics in meiosis and mitosis.

Axonemal dynein

A motor protein localized along doublet microtubules that activates microtubule sliding to produce the bending motion of cilia and flagella.

Axoneme

The central structural core of cilia and flagella characterized by a "9 + 2" microtubule architecture, containing 9 outer doublet microtubules and a central pair.

Basal body

A centriole-derived cylindrical structure anchored at the base of cilia and flagella consisting of a ring of nine triplet microtubules without a central pair.

Myosin II

A two-headed, microfilament-associated motor protein that slides actin filaments in muscle contraction, cytokinesis, and cell migration.

Myosin V

A two-headed actin-associated motor protein that mediates vesicle positioning and organelle trafficking along microfilaments.

Myosin VI

An actin-associated motor protein involved in endocytosis that uniquely moves toward the minus ends of microfilaments.

Sarcomere

The fundamental repeating contractile unit of a myofibril in striated muscle, delimited at each boundary by a Z line.

A band

The dark band of a sarcomere that corresponds to the entire length of the thick filaments (myosin).

I band

The light band of a sarcomere that contains only thin actin filaments and narrows as thin filaments slide into the A band during contraction.

H zone

The lighter central region within the A band of a relaxed sarcomere that contains thick filaments but lacks overlapping thin filaments.

M line

The central structural line in the middle of the H zone containing myomesin that cross-links adjacent thick filaments.

Z line

The dark structural boundary protein disc at each end of a sarcomere where actin thin filaments are anchored.

Titin

A massive elastic structural protein ($2500\,\text{kDa}$) that connects thick myosin filaments to the Z line to maintain their centering.

Nebulin

A $700\,\text{kDa}$ protein that links actin thin filaments to the Z line and stabilizes thin filament length in sarcomeres.

Myomesin

A $185\,\text{kDa}$ myosin-binding structural protein localized at the M line that links adjacent thick filaments together.

CapZ

A $68\,\text{kDa}$ actin-capping protein that attaches thin filaments to the Z line by binding to their plus ends.

Tropomodulin

An actin-capping protein that binds and caps the pointed (minus) ends of thin filaments located near the center of the sarcomere.

Troponin

A $78\,\text{kDa}$ complex of three polypeptides (TnT, TnC, and TnI) that coordinates with tropomyosin to form a calcium-sensitive switch controlling contraction.

Tropomyosin

A $64\,\text{kDa}$ rod-like protein that binds along the length of actin thin filaments and blocks myosin-binding sites when calcium levels are low.

Sliding-filament model

A model proposed in 1954 explaining that muscle contraction occurs as thin filaments slide past thick filaments into the A band without any change in the length of either filament type.

Power stroke

The step in the cross-bridge cycle where myosin undergoes a conformational change that pulls the actin filament toward the center of the sarcomere, triggered by the release of inorganic phosphate ($P_i$).

Neuromuscular junction

The site where the axon terminal of a motor neuron contacts a muscle cell to convey an action potential.

Motor end plate

The specialized folded region of the muscle cell sarcolemma beneath axon terminals where acetylcholine receptors are clustered.

Transverse (T) tubule system

A series of membrane invaginations through which sarcolemma depolarization spreads into the cell interior to stimulate the sarcoplasmic reticulum.

Sarcoplasmic reticulum (SR)

A specialized intracellular membranous organelle that sequesters, stores, and rapidly releases $Ca^{2+}$ ions to regulate muscle contraction and relaxation.

Ryanodine receptors

Calcium-release channel proteins located in the terminal cisternae of the sarcoplasmic reticulum that open when activated by adjacent T-tubule voltage-gated calcium channels.

Intercalated discs

Specialized end-to-end junctions connecting cardiac muscle cells that contain abundant gap junctions, allowing waves of depolarization to pass rapidly between cells.

Sinoatrial (SA) node

The intrinsic pacemaker of the heart that spontaneously initiates the electrical conduction wave for synchronized cardiac contraction.

Dense bodies

Plaque-like protein anchoring structures in smooth muscle cells to which bundles of actin and myosin filaments are tethered, functioning analogously to Z lines.

Myosin light-chain kinase (MLCK)

An enzyme activated by the $Ca^{2+}$-calmodulin complex that phosphorylates regulatory myosin light chains to stimulate contraction in smooth muscle.

Myosin light-chain phosphatase

An enzyme that dephosphorylates myosin light chains when intracellular $Ca^{2+}$ levels drop, causing smooth muscle relaxation.

Lamellipodium

A broad, flattened sheet-like protrusion at the leading edge of a crawling cell, driven by the forward polymerization of branched actin filaments.

Filopodium

A slender, finger-like pointing protrusion supported by parallel bundles of actin filaments that extends from migrating cells.

Focal adhesions

Integrin-mediated multiprotein contact sites that attach actin microfilaments inside the cell to extracellular matrix components on the outside.

Chemotaxis

The directional migration of a cell along a chemical gradient of diffusible cues, mediated by cell-surface G protein-linked receptors.

Pseudopodium

A large, blunt-ended dynamic cellular protrusion extended during amoeboid movement.

Gelation

The transition of advancing cytosol into a non-fluid, congealed gel state as it reaches the tip of an extending pseudopodium.

Solation

The transition of congealed actin gel into a fluid, streaming state at the trailing rear of an amoeboid cell.

Cytoplasmic streaming (Cyclosis)

The actomyosin-dependent circular movement of internal fluid cytosol around a large central vacuole against stationary cortical actin filaments.