Membrane Potentials, Action Potentials, and Muscle Contraction Flashcards

Comprehensive vocabulary flashcards covering membrane potential mechanics, action potential propagation, nerve anatomy, skeletal muscle ultrastructure, and muscle contraction physiology based on lecture notes.

Resting Membrane Potential

The difference between the electric potential in the intracellular and extracellular matrices of a cell when it is not excited, exhibiting values such as $-90\,mV$ in skeletal muscle, $-55\,mV$ in smooth muscle, $-80\,mV$ in cardiac muscle, and $-65\,mV$ in neurons.

Diffusible Ions

Ions capable of passing through the cell membrane, specifically sodium ($Na^+$), potassium ($K^+$), calcium ($Ca^{2+}$), and chloride ($Cl^-$).

Non-diffusible Ions

Ions that cannot pass through the cell membrane, primarily proteins carrying negative charges that contribute significantly to intracellular electronegativity.

Sodium-Potassium Pump

An electrogenic pump ($Na^+-K^+$ ATPase) that uses energy to expel $3$ molecules of sodium in exchange for $2$ molecules of potassium, maintaining concentration gradients across the cell membrane.

Passive Channels

Pores located along the excitable cell membrane that allow the diffusion of molecules down their concentration gradients without energy expenditure.

Active Channels

Specialized membrane ion channels that open to allow ion transport in response to changes in membrane potential (potential-gated), binding of a chemical messenger (ligand-gated), or mechanical stimulation.

Electrochemical Equilibrium

The point at which the electrical gradient opposing ion movement balances the concentration gradient, stopping net diffusion (e.g., $-94\,mV$ for potassium efflux and $+61\,mV$ for sodium influx).

Action Potential

A rapid electrical signal that propagates along the surface of an excitable cell's membrane due to sequential sodium and potassium ion movements through specific ion channels.

Depolarization Stage

The phase of an action potential during which voltage-gated sodium channels open, allowing rapid influx of sodium ions and causing the inner membrane potential to become positive.

Repolarization Stage

The phase of an action potential during which sodium channels become refractory/close and voltage-gated potassium channels open, allowing potassium efflux to return the membrane voltage toward its resting level.

Hyperpolarization Stage

The phase of an action potential in which voltage-gated potassium channels remain open briefly after reaching the resting level, driving the membrane potential more negative than its resting state.

Axoplasm

The viscid intracellular fluid filling the central core of an axon.

Node of Ranvier

A small, uninsulated unmyelinated gap of about $2\,\mu m$ to $3\,\mu m$ in length occurring every $1\,mm$ to $3\,mm$ along a myelinated nerve fiber where ion exchange occurs.

Saltatory Conduction

The propagation mechanism in myelinated nerve fibers where action potentials jump from node to node, increasing nerve transmission velocity $5\text{--}50\text{ fold}$ and conserving axon energy.

Sarcolemma

The thin membrane enclosing a skeletal muscle fiber, composed of a true plasma membrane and an outer coat of polysaccharide material containing thin collagen fibrils.

Myofibrils

Contractile elements within a muscle fiber composed of approximately $1500$ adjacent myosin filaments and $3000$ actin filaments.

I Bands

Light bands in myofibrils containing only actin filaments, which are isotropic to polarized light.

A Bands

Dark bands in myofibrils containing myosin filaments and overlapping ends of actin filaments, which are anisotropic to polarized light.

Sarcomere

The structural unit of a myofibril or muscle fiber located between two successive Z disks.

Titin

A large, springy protein molecule with a molecular weight of approximately $3 \times 10^6$ that maintains the side-by-side alignment of myosin and actin filaments.

Sarcoplasm

The intracellular fluid between myofibrils containing large amounts of magnesium, potassium, phosphate, protein enzymes, and abundant parallel mitochondria.

Sarcoplasmic Reticulum

A specialized endoplasmic reticulum surrounding myofibrils in skeletal muscle that regulates calcium storage, release, and reuptake.

Neuromuscular Junction

The specialized junction where a branch of a large myelinated motor nerve fiber contacts a skeletal muscle fiber near its midpoint.

End Plate Potential

A local potential increase of $50\,mV$ to $75\,mV$ created at the motor end plate when acetylcholine opens ligand-gated cation channels, triggering local sodium influx.

Isometric Contractions

Muscle contractions where tension increases without shortening the muscle length, occurring when the external load exceeds the force developed by the muscle.

Isotonic Contractions

Muscle contractions where the muscle shortens while maintaining constant tension, occurring when the developed force exceeds the external load.

Slow Fibers (Type 1, Red Muscle)

Muscle fibers characterized by smaller size, dense capillary networks, abundant mitochondria, and high myoglobin content to support continuous aerobic/oxidative metabolism.

Fast Fibers (Type 2, White Muscle)

Muscle fibers characterized by large diameter for high strength, extensive sarcoplasmic reticulum, rich glycolytic enzymes, fewer mitochondria, and low myoglobin content.

Tetanic Contraction

A sustained, maximal muscle contraction caused by high-frequency stimulation (typically $50\text{--}100\,Hz$) where calcium accumulates continuously in the sarcoplasm without muscle relaxation.

Unfused Tetanus

A form of tetanic contraction resulting from intermediate stimulation frequency where the muscle partially relaxes between consecutive action potentials, producing fluctuating force.

Fused Tetanus

A form of tetanic contraction resulting from high-frequency stimulation where all individual twitches merge completely into a smooth, steady, maximum-tension force plateau.