Cable Properties of Nerves and Synaptic Transmission

A set of vocabulary flashcards covering nerve cable properties, conduction parameters, synaptic transmission mechanisms, integration processes, and neurotransmitter systems.

Axoplasm

The cytoplasm found within the interior of an axon.

Passive Spread

The process by which local current spreads passively to adjacent membrane patches when a patch of axon membrane is depolarized.

Membrane Resistance ($R_m$)

The ohmic resistance to current crossing the membrane, reflecting the difficulty of ion leakage across the lipid bilayer via ion channels; higher $R_m$ reduces ion leakage and allows depolarization to travel further down the axon.

Membrane Capacitance ($C_m$)

The capacity of the lipid bilayer to function as a capacitor by separating and storing electrical charges across the membrane.

Internal Axoplasmic Resistance ($R_i$)

The electrical resistance through the internal axoplasm of a nerve fiber.

$A\alpha$ Nerve Fibers

Large somatic motor nerve fibers with diameters of $12\text{--}22\,\mu m$ and conduction velocities of $70\text{--}120\,m/s$.

$A\delta$ Nerve Fibers

Nerve fibers transmitting sharp pain with diameters of $1\text{--}6\,\mu m$ and conduction velocities of $12\text{--}30\,m/s$.

$C$ Nerve Fibers

Unmyelinated nerve fibers transmitting dull, aching pain with diameters of $0.5\text{--}1.2\,\mu m$ and conduction velocities of $0.2\text{--}2\,m/s$.

Time Constant ($\tau$)

A cable property given by $\tau = R_m C_m$, indicating how fast a membrane depolarizes, measured as the time required for the membrane potential to reach $63\%$ of its final value following current injection.

Length Constant ($\lambda$)

A cable property reflecting how far current spreads down a nerve fiber, defined as the distance over which the potential decays to $63\%$ of its final value.

Nodes of Ranvier

Periodic gaps in the myelin wrapping of peripheral nerves measuring $2\text{--}3\,\mu m$ wide, spaced $1\text{--}3\,mm$ apart, and highly enriched with $Na^+$ channels.

Saltatory Conduction

The mode of nerve impulse propagation in myelinated axons where the electrical current appears to jump from node to node.

Multiple Sclerosis

A neurological condition in which the immune system attacks myelinating cells of the central nervous system (CNS).

Electrical Synapse

A specialized cell-to-cell junction that enables direct, bidirectional, and extremely fast ionic flow between excitable cells via gap junctions composed of connexin proteins.

Chemical Synapse

The most common type of synapse in the nervous system, where information is transmitted unidirectionally across a $20\text{--}40\,nm$ synaptic cleft via chemical neurotransmitters.

Terminal Bouton (Discrete Synapse)

A classic chemical synapse arrangement where an axon terminal ends and connects directly onto another neuron.

En Passant (Diffuse Chemical Synapse)

A synaptic configuration where neurotransmitters are released from swellings along the length of an axon as it passes by a target neuron, rather than at a terminal ending.

Synaptotagmin

A cytosolic sensor protein that binds incoming $Ca^{2+}$ to trigger the fusion and exocytosis of docked synaptic vesicles at the presynaptic membrane.

Synaptic Cleft

The narrow extracellular space measuring approximately $20\text{--}40\,nm$ between the presynaptic and postsynaptic membranes through which neurotransmitters diffuse.

Excitatory Postsynaptic Potential (EPSP)

A postsynaptic depolarization caused by opening $Na^+$ and $K^+$ channels, driving the membrane potential toward an intermediate reversal potential between $K^+$ ($-45\,mV$) and $Na^+$ ($66\,mV$).

Inhibitory Postsynaptic Potential (IPSP)

A postsynaptic hyperpolarization or stabilization caused by opening $Cl^-$ channels, shifting the membrane potential toward the $Cl^-$ equilibrium potential ($-90\,mV$) and away from threshold.

Spatial Summation

The integration process where multiple presynaptic inputs fire simultaneously onto a single postsynaptic neuron to combine their electrical effects.

Temporal Summation

The integration process where a single presynaptic neuron fires multiple action potentials in rapid succession down the same axon, summing their postsynaptic potentials.

Synaptic Facilitation

A short-term form of synaptic plasticity that enhances transmission strength over a duration of $10\text{--}100\,ms$.

Long-Term Potentiation (LTP)

A persistent strengthening of synapses based on recent patterns of activity that underlies memory formation, characterized by increased presynaptic neurotransmitter release and enhanced postsynaptic receptor sensitivity.

Ionotropic Receptors

Ligand-gated ion channel receptors that produce rapid, direct postsynaptic electrical responses upon neurotransmitter binding.

Metabotropic Receptors

G-protein coupled receptors that mediate slower-onset, sustained downstream cellular effects following ligand binding.

Acetylcholine (ACh)

The major neurotransmitter at the neuromuscular junction (NMJ) and the chemical transmitter released by preganglionic neurons in both the sympathetic and parasympathetic nervous systems.

Catecholamines

A class of neurotransmitters derived from the amino acid tyrosine, consisting sequentially of dopamine, norepinephrine, and epinephrine.

Glutamate

The primary excitatory neurotransmitter in the central nervous system, acting on ionotropic receptors such as NMDA and AMPA.

GABA

The primary inhibitory neurotransmitter in the central nervous system, which acts via ionotropic $GABA_A$ receptors ($\uparrow Cl^-$ influx) and metabotropic $GABA_B$ receptors ($\uparrow K^+$ efflux).

$5\text{-}HT_3$ Receptor

The only ionotropic receptor subtype within the serotonin (5-HT) receptor family, functioning as a ligand-gated channel that mediates excitatory postsynaptic potentials.

Axoplasm

The specialized intracellular cytoplasm within an axon that provides a conductive fluid pathway for longitudinal ionic current flow along the interior of the nerve fiber.

Electrotonic Conduction (Passive Current Spread)

The passive, non-regenerative spread of local electrical current along an axon through the axoplasm internally and extracellular fluid externally, decaying exponentially over time and distance unless threshold is attained.

Membrane Resistance ($R_m$)

The ohmic electrical resistance to ionic current flow across the cell membrane via transmembrane ion channels; a higher $R_m$ conserves internal charge by reducing ionic leak.

Membrane Capacitance ($C_m$)

The capacity of the hydrophobic lipid bilayer to separate and store electrical charges between the conductive axoplasm and the extracellular fluid.

External Solution Resistance ($R_o$)

The longitudinal electrical resistance encountered by ionic current flowing through the extracellular fluid surrounding the axonal membrane.

Internal Axoplasmic Resistance ($R_i$)

The longitudinal ohmic resistance encountered by ionic currents moving through the internal core of the axon cytoplasm.

Conduction Velocity

The speed at which an action potential propagates along a nerve fiber, increasing proportionally with larger fiber diameter and myelination.

Capacitive Current ($i_c$)

Electrical current that flows across the membrane capacitor during charging or discharging, defined by $i_c = C \frac{dV}{dt}$.

Total Membrane Conductance ($G$)

The overall ease with which ions cross a membrane patch of area $A$, defined by $G = G_m A$, where $G_m$ is specific membrane conductance per unit area.

Specific Resistivity ($\rho_i$)

The intrinsic resistivity of the internal cytoplasmic electrolyte solution, measured across two opposing faces of a $1\,\text{cm}^3$ cube of axoplasm.

Time Constant ($\tau$)

A parameter defined by $\tau = R_m C_m$ that quantifies the temporal rate of membrane charging and discharging; represents the time required to reach $63\%$ of the steady-state potential or decay by $63\%$.

Length Constant ($\lambda$)

A parameter defined by $\lambda = \sqrt{\frac{a R_m}{2 \rho_i}}$ that measures the spatial reach of electrotonic current down a fiber; represents the distance at which voltage falls by $63\%$ (to $1/e$ or $\approx 37\%$ of $V_0$).

Kirchhoff's Current Law

A physical principle stating that the algebraic sum of all electrical currents entering and leaving any circuit node must equal zero ($i(x) = i(x + \Delta x) + i_m$).

Cable Equation

A linear partial differential equation ($\lambda^2 \frac{\partial^2 V'}{\partial x^2} - \tau \frac{\partial V'}{\partial t} = V'$) modeling voltage changes across space and time in passive neuronal processes.

Steady-State Cable Behavior

The condition where membrane charging is complete ($\frac{\partial V}{\partial t} = 0$), yielding an exponential voltage decay with physical distance: $V(x) = (V_0 - V_r) e^{-x/\lambda}$.

Myelin Sheath

A multi-lamellar concentric lipid wrapping around axons that dramatically elevates specific membrane resistance ($R_m$) and reduces specific membrane capacitance ($C_m$).

Neurofilaments

Individual intermediate filaments within the neuronal axoplasm that provide structural support and establish axonal diameter/caliber.

Nodes of Ranvier

Periodic unmyelinated gaps of $2 - 3\,\mu\text{m}$ along myelinated axons, spaced $1 - 3\,\text{mm}$ apart, highly enriched in voltage-gated $\text{Na}^+$ channels.

Saltatory Conduction

The rapid, discontinuous propagation of action potentials along myelinated axons, appearing to jump from one node of Ranvier to the next via rapid electrotonic internodal spread.

$A\alpha$ Fibers

Large, myelinated somatic motor axons with diameters of $12 - 22\,\mu\text{m}$ and rapid conduction velocities of $70 - 120\,\text{m/s}$.

$A\delta$ Fibers

Small, lightly myelinated axons with diameters of $1 - 6\,\mu\text{m}$ and velocities of $12 - 30\,\text{m/s}$, primarily transmitting sharp, acute, localized pain.

$C$ Fibers

Small, unmyelinated nerve fibers with diameters of $0.5 - 1.2\,\mu\text{m}$ and slow conduction velocities of $0.2 - 2\,\text{m/s}$, mediating dull, aching, diffuse pain.

Multiple Sclerosis (MS)

An autoimmune disorder where the immune system destroys CNS myelin and oligodendrocytes, reducing $R_m$ and increasing $C_m$, causing conduction slowing or complete failure.

Electrical Synapse

A specialized direct junction between adjacent excitable cells formed by gap junctions (connexin proteins) allowing instantaneous, bidirectional ionic and metabolic flux.

Chemical Synapse

A strictly unidirectional junction where presynaptic depolarization triggers neurotransmitter release into a $20 - 40\,\text{nm}$ synaptic cleft to bind postsynaptic receptors after a brief synaptic delay.

Terminal Bouton (Discrete Synapse)

The classic specialized chemical junction formed where a swelling at the terminal end of an axon synapses directly onto a postsynaptic target cell.

Synapses En Passant (Diffuse Chemical Synapses)

A configuration where an axon forms multiple periodic swellings (varicosities) along its length, releasing neurotransmitters diffusely into surrounding tissue as it passes.

Dendritic Spine

A small actin-rich micro-protrusion along a dendritic shaft that isolates and processes incoming synaptic inputs.

Axosomatic Synapse

A synaptic contact where an axon terminal synapses directly onto the neuronal cell body (soma).

Axodendritic Synapse

The most common anatomical arrangement where an axon terminal establishes contact with a dendrite or dendritic spine.

Axoaxonal Synapse

A specialized synapse made directly onto another axon terminal or initial segment, commonly modulating presynaptic inhibition or facilitation.

Dendrodendritic Synapse

A direct functional communicative contact formed between two dendritic shafts.

Axoextracellular Synapse

A synapse wherein an axon discharges chemical transmitters non-specifically into the broad extracellular space.

Axosecretory Synapse

A neurosecretory junction where an axon terminates near vascular capillary beds to release signaling substances directly into the bloodstream.

One-to-One Synaptic Transfer

A synaptic arrangement where an individual presynaptic action potential routinely triggers a single postsynaptic action potential (e.g., neuromuscular junction).

One-to-Many Synaptic Transfer

A rare amplifying synaptic arrangement where a single presynaptic action potential triggers a burst or volley of multiple postsynaptic action potentials.

Many-to-One Synaptic Transfer

The predominant CNS pattern where multiple independent presynaptic inputs must fire concurrently or in rapid succession to summate and elicit a postsynaptic response.

Synaptotagmin

The specialized calcium-sensor protein on synaptic vesicles that, upon binding cytosolic $\text{Ca}^{2+}$, initiates vesicle membrane fusion and exocytosis.

Enzymatic Degradation

A synaptic signal termination mechanism in which specialized enzymes within the cleft hydrolyze neurotransmitters (e.g., acetylcholinesterase cleaving acetylcholine).

Active Reuptake

A termination mechanism where secondary active transporters on the presynaptic terminal or surrounding glial processes pump neurotransmitters out of the cleft.

Selective Serotonin Reuptake Inhibitors (SSRIs)

Antidepressant medications that inhibit presynaptic serotonin transporters, prolonging serotonin presence in the synaptic cleft to enhance postsynaptic signaling.

Excitatory Postsynaptic Potential (EPSP)

A transient depolarization of the postsynaptic membrane caused by mixed permeability to $\text{Na}^+$ and $\text{K}^+$, driving the potential toward an intermediate reversal potential.

Inhibitory Postsynaptic Potential (IPSP)

A transient hyperpolarization or voltage stabilization of the postsynaptic membrane typically mediated by selective $\text{Cl}^-$ influx toward $E_{\text{Cl}} = -90\,\text{mV}$.

Spatial Summation

The additive integration of simultaneous postsynaptic potentials produced by multiple different presynaptic terminals synapsing at separate locations on a single postsynaptic neuron.

Temporal Summation

The integration of consecutive postsynaptic potentials generated when a single presynaptic axon discharges multiple action potentials in rapid succession.

Synaptic Plasticity

The activity-dependent ability of synapses to modify their physiological strength, transmission efficacy, structure, and molecular composition over time.

Synaptic Facilitation

An ultrashort enhancement of synaptic transmission lasting $10 - 100\,\text{ms}$, caused by residual presynaptic $\text{Ca}^{2+}$ accumulation increasing transmitter release probability.

Synaptic Augmentation

A short-term activity-dependent increase in synaptic efficacy persisting over several seconds due to accumulated presynaptic calcium.

Post-Tetanic Potentiation (PTP)

A sustained enhancement of synaptic transmission persisting from seconds to several minutes following high-frequency tetanic stimulation.

Synaptic Fatigue (Synaptic Depression)

A decline in postsynaptic response amplitude during sustained high-frequency stimulation, driven primarily by depletion of the readily releasable pool (RRP) of vesicles.

Long-Term Potentiation (LTP)

A persistent, long-lasting enhancement of synaptic transmission induced by patterned stimulation, serving as a primary cellular mechanism for learning and memory.

Classical Neurotransmitters

Small-molecule signaling chemicals synthesized locally in axon terminals, packaged into clear synaptic vesicles, and cleared rapidly via enzymatic breakdown or reuptake.

Neuropeptides

Polypeptide transmitters synthesized in the soma, processed through the ER and Golgi apparatus, and transported via fast axonal transport to terminals, often co-secreted with classical transmitters.

Neuromodulators

Chemical messengers that act presynaptically or postsynaptically to alter neuronal excitability, transmitter release probability, or signaling gain without directly mediating fast synaptic currents.

Neurohormones

Signaling molecules released directly into the systemic bloodstream by neurosecretory neurons to act on distant physiological targets.

Nitric Oxide ($\text{NO}$)

An unconventional gaseous messenger synthesized on demand that diffuses across membranes to activate intracellular enzymes such as soluble guanylyl cyclase.

Purinergic Transmitters

Purine-based signaling molecules (ATP and adenosine) that are co-secreted with classical transmitters to mediate fast excitation and modulation in tissues like smooth muscle.

Ionotropic Receptors

Ligand-gated ion channels that open directly and rapidly upon binding a specific neurotransmitter, generating rapid postsynaptic potentials within milliseconds.

Metabotropic Receptors

Seven-transmembrane G-protein coupled receptors (GPCRs) that activate heterotrimeric G-proteins and secondary messenger cascades, eliciting slower and longer-lasting cellular effects.

Acetylcholine (ACh)

The primary excitatory transmitter at the mammalian neuromuscular junction, synthesized and released by all autonomic preganglionic neurons and all parasympathetic postganglionic neurons.

Nicotinic ACh Receptors

Ionotropic, non-selective cation channels whose activation drives rapid $\text{Na}^+$ influx and $\text{K}^+$ efflux to produce depolarizing EPSPs.

Muscarinic ACh Receptors ($M_1 - M_5$)

Metabotropic GPCRs; $M_1, M_3, M_5$ couple to $\text{G}_q$ to activate PLC, $M_2$ couples to $\text{G}_i$ to open $\text{K}^+$ channels, and $M_4$ couples to $\text{G}_i$ to inhibit adenylyl cyclase.

Catecholamines

A family of monoamine transmitters derived enzymatically from L-Tyrosine, consisting of dopamine, norepinephrine, and epinephrine.

Tyrosine Hydroxylase

The rate-limiting enzyme in catecholamine biosynthesis that converts L-Tyrosine into $L\text{-DOPA}$.

Phenylethanolamine N-methyltransferase (PNMT)

The terminal enzyme in catecholamine biosynthesis that methylates norepinephrine to synthesize epinephrine (adrenaline).

Dopamine Receptors ($D_1$ and $D_2$)

$D_1$ receptors couple to $\text{G}_s$ to stimulate adenylyl cyclase and PKA (causing depolarization), whereas $D_2$ couples to $\text{G}_i$ to inhibit adenylyl cyclase and open $\text{K}^+$/$\text{Cl}^-$ channels (hyperpolarization).

Parkinson's Disease

A neurodegenerative pathology marked by the loss of dopaminergic neurons in the substantia nigra pars compacta, treated clinically with the dopamine precursor $L\text{-DOPA}$.

Serotonin ($5\text{-HT}$)

A monoamine neurotransmitter synthesized from tryptophan (via tryptophan 5-hydroxylase and aromatic amino acid decarboxylase) that regulates mood, sleep, and enteric functions.