Cellular Electrophysiology, Synaptic Transmission, and Muscle Physiology

Vocabulary flashcards covering cellular electrophysiology, passive cable properties, action potential dynamics, synaptic mechanisms, neurotransmitters, and skeletal muscle excitation-contraction coupling.

Nernst Potential ($V_{eq}$)

The membrane potential at which electrical forces across a cell membrane precisely balance the chemical concentration gradient for a given ion, resulting in zero net ionic flux.

Driving Force ($DF$)

The net electrochemical push or pull acting on an ion ($E_m - E_X$) that determines the direction and magnitude of ion movement across the cell membrane.

Ionic Current ($I_X$)

The actual movement of charged ions across the cell membrane, calculated by the equation $I_X = g_X(E_m - E_X)$, requiring both open channels ($g_X > 0$) and a non-zero driving force.

Goldman-Hodgkin-Katz (GHK) Equation

A mathematical formula used to calculate the resting membrane potential ($E_m$) based on intracellular and extracellular concentration gradients and the relative membrane permeabilities of $K^+$, $Na^+$, and $Cl^-$.

Chord Conductance ($g_i$)

A measurement of how easily an ion flows across a cell membrane at any given voltage, defined mathematically as the slope of the straight chord connecting an operating point on an $I\text{-}V$ curve to the ion's reversal potential.

Reversal Potential

The specific membrane potential at which the net ionic current reverses direction from inward to outward (crossing the voltage axis where net current is zero).

Threshold Potential

The critical voltage level (~$-60\,\text{mV}$ in nerves) at which an applied depolarizing stimulus triggers an action potential 50% of the time.

Absolute Refractory Period

A $1\text{--}2\,\text{ms}$ cooldown interval immediately following excitation where voltage-gated $Na^+$ channel inactivation gates snap shut, making it impossible to trigger a second action potential.

Relative Refractory Period

A $2\text{--}4\,\text{ms}$ interval following the absolute refractory period during which a second action potential can be elicited, but requires a stronger stimulus than in a resting cell.

Tetrodotoxin (TTX)

A neurotoxin derived from pufferfish that selectively binds to and blocks voltage-gated $Na^+$ channels, abolishing $Na^+$ conductance and action potential generation.

Tetraethylammonium (TEA)

A pharmacological agent that blocks voltage-gated $K^+$ channels, preventing outward $K^+$ current and halting membrane repolarization.

Unitary Current ($i$)

The single-channel current carried by an individual open ion channel under physiological conditions, contributing to total current via $I = N P_o i$.

Patch Clamping

An experimental electrophysiological technique used to study single ion channels by isolating a tiny membrane patch, controlling voltage, and recording microscopic currents.

Time Constant ($\tau$)

A cable property defined by $\tau = R_m C_m$, indicating how fast a membrane depolarizes by measuring the time needed for the potential to reach 63% of its final value.

Length Constant ($\lambda$)

A cable property reflecting how far current spreads passively down a nerve fiber, defined as the distance at which a graded voltage change decays to 63% of its initial value.

Nodes of Ranvier

Periodic unmyelinated gaps ($2\text{--}3\,\mu\text{m}$ wide, spaced $1\text{--}3\,\text{mm}$ apart) along an axon that are richly populated with voltage-gated $Na^+$ channels.

Saltatory Conduction

The mode of rapid nerve impulse propagation in myelinated axons where the electrical depolarizing current appears to jump from one Node of Ranvier to the next.

Electrical Synapse

A specialized bidirectional cellular junction that allows direct, rapid ionic current flow between adjacent excitable cells via gap junction channels composed of connexin proteins.

Chemical Synapse

A unidirectional junction between cells where an action potential triggers the release of neurotransmitter molecules across a $20\text{--}40\,\text{nm}$ synaptic cleft to bind postsynaptic receptors.

Synaptotagmin

A calcium-sensing protein on synaptic vesicles that binds cytosolic $Ca^{2+}$ to trigger the fusion of docked vesicles with the presynaptic terminal membrane.

Excitatory Postsynaptic Potential (EPSP)

A local postsynaptic membrane depolarization caused by the opening of channels permeable to both $Na^+$ and $K^+$, driving membrane potential toward an intermediate reversal potential.

Inhibitory Postsynaptic Potential (IPSP)

A local postsynaptic membrane hyperpolarization or stabilization caused by the opening of $Cl^-$ channels, shifting the potential toward $E_{Cl}$ (~$-90\,\text{mV}$) away from threshold.

Spatial Summation

The algebraic integration of postsynaptic potentials generated simultaneously by multiple separate presynaptic inputs terminating across different regions of a single postsynaptic neuron.

Temporal Summation

The additive integration of postsynaptic potentials produced when a single presynaptic neuron fires multiple action potentials in rapid succession down the same axon.

Long-Term Potentiation (LTP)

A persistent, activity-dependent strengthening of synaptic efficacy involved in learning and memory, mediated by increased presynaptic transmitter release and enhanced postsynaptic sensitivity.

Ionotropic Receptor

A ligand-gated ion channel receptor that directly opens an integral ion pore upon neurotransmitter binding to produce rapid postsynaptic electrical responses.

Metabotropic Receptor

A G-protein coupled receptor (GPCR) that initiates intracellular signaling cascades upon neurotransmitter binding, yielding slower but longer-lasting physiological effects.

Motor Unit

The fundamental functional unit of motor control, comprising a single somatic motor neuron and all the muscle fibers it innervates.

End-Plate Potential (EPP)

A large, graded postsynaptic depolarization generated at the muscle motor end plate via acetylcholine binding to nicotinic receptors, which spreads passively to trigger an action potential.

Miniature End-Plate Potential (MEPP)

A small, spontaneous subthreshold depolarization (~$0.4\,\text{mV}$) produced at the neuromuscular junction by the exocytosis of a single vesicle (quantum) of acetylcholine.

Botulinus Toxin (BOTOX)

A bacterial toxin produced by Clostridium botulinum that blocks acetylcholine release from presynaptic motor nerve terminals, resulting in flaccid muscle paralysis.

Sarcomere

The repeating functional and contractile unit of a myofibril, delineated on both ends by Z-disks and containing interdigitating thick and thin filaments.

Tropomyosin

A filamentous protein that lies along the groove of the actin filament and physically blocks myosin-binding sites on actin when a muscle is at rest.

Troponin C (TnC)

The calcium-binding subunit of the troponin complex that cooperatively binds $Ca^{2+}$, triggering a conformational shift in tropomyosin to expose actin's myosin-binding sites.

Titin

A massive elastic cytoskeletal protein extending from the M-line to the Z-disk that centers thick filaments within the sarcomere and changes length during contraction.

Nebulin

An inelastic cytoskeletal protein extending along thin filaments that sets actin filament length during assembly and aligns actin monomers within the sarcomere.

Triad

A specialized skeletal muscle membrane junction consisting of a single central transverse tubule (T-tubule) closely flanked by two terminal cisternae of the sarcoplasmic reticulum.

Dihydropyridine (DHP) Receptor

A voltage-sensing L-type calcium channel located in the T-tubule membrane that changes conformation upon depolarization to activate adjacent sarcoplasmic reticulum ryanodine receptors.

Ryanodine Receptor (RyR)

A calcium-release channel situated on the sarcoplasmic reticulum membrane that releases stored $Ca^{2+}$ into the sarcoplasm when triggered by activated DHP receptors.