Cable Properties of Nerves, Synaptic Transmission, and Neurotransmitters

Vocabulary flashcards covering the cable properties of nerve axons, synaptic transmission mechanisms, postsynaptic integration, and major neurotransmitter classes from the lecture notes.

Axoplasm

The internal cytoplasm of an axon.

Passive Spread

The mechanism by which local current spreads passively to adjacent membrane patches following depolarization, decaying exponentially over time and distance unless the depolarization reaches threshold.

Membrane Resistance ($R_m$)

The ohmic resistance to current flow across the cell membrane, reflecting the difficulty of ions leaking across the lipid bilayer via ion channels; an increase in $R_m$ reduces ion leakage and allows current to travel further down the axon.

Membrane Capacitance ($C_m$)

The ability of the lipid bilayer to act as a capacitor by separating and storing electrical charges across the membrane, normalized per unit area ($C_m = \frac{C}{A}$) to allow comparison of membranes regardless of size.

Axoplasmic Specific Resistance ($\rho$)

The electrical resistance measured between two opposite faces of a $1\,\text{cm}^3$ cube of cytoplasm, which determines internal axoplasmic resistance via the relation $R_i = \frac{\rho \cdot d}{A}$.

Time Constant ($\tau$)

A measure of how fast the membrane depolarizes ($\tau = R_m C_m$), defined as the time required following current injection for the membrane potential to reach $63\%$ of its final value.

Length Constant ($\lambda$)

A parameter characterizing how far current spreads passively down a nerve fiber, defined as the distance over which the membrane potential decays to $63\%$ of its value.

Fiber Type $A\alpha$

A large-diameter somatic motor nerve fiber with a diameter of $12\text{--}22\,\mu\text{m}$ and a high conduction velocity of $70\text{--}120\,\text{m/s}$.

Fiber Type $A\delta$

A myelinated nerve fiber involved in transmitting sharp pain, exhibiting a diameter of $1\text{--}6\,\mu\text{m}$ and a conduction velocity of $12\text{--}30\,\text{m/s}$.

Fiber Type $C$

A small, unmyelinated nerve fiber involved in transmitting aching pain, characterized by a diameter of $0.5\text{--}1.2\,\mu\text{m}$ and a conduction velocity of $0.2\text{--}2\,\text{m/s}$.

Myelination

An insulating wrapping around axons that accelerates action potential propagation by increasing membrane resistance ($R_m$), increasing the length constant ($\lambda$), and decreasing membrane capacitance ($C_m$) due to increased membrane thickness.

Nodes of Ranvier

Periodic gaps in the myelin sheath along peripheral nerves that are $2\text{--}3\,\mu\text{m}$ wide, spaced $1\text{--}3\,\text{mm}$ apart, and densely populated with voltage-gated $\text{Na}^+$ channels.

Saltatory Conduction

The rapid form of action potential propagation along myelinated axons in which the electrical current appears to jump from one Node of Ranvier to the next.

Multiple Sclerosis

A condition in which the immune system attacks myelinating cells of the central nervous system, leading to unmyelinated axon segments that suffer from rapid voltage drop-off.

Electrical Synapse

A bidirectional, extremely rapid cell junction allowing direct ionic current flow between excitable cells via gap junctions composed of connexin proteins.

Chemical Synapse

The most common and unidirectional type of neuronal junction, where neurotransmitters are released from a presynaptic terminal, diffuse across a $20\text{--}40\,\text{nm}$ synaptic cleft, and bind to postsynaptic receptors.

Terminal Bouton

A classic discrete chemical synapse where an axon terminal ends and forms a direct connection onto another neuron.

Synapse en Passant

A diffuse chemical synapse configuration where neurotransmitter is released from swellings along the length of an axon as it passes another neuron, rather than from a terminal end.

Synaptotagmin

A calcium-sensing protein on synaptic vesicles that binds incoming cytosolic $\text{Ca}^{2+}$ to trigger vesicle docking and fusion with the presynaptic membrane.

Excitatory Postsynaptic Potential (EPSP)

A graded depolarization of the postsynaptic membrane produced by the opening of $\text{Na}^+$ and $\text{K}^+$ channels, driving the membrane potential toward an intermediate reversal potential.

Inhibitory Postsynaptic Potential (IPSP)

A graded hyperpolarization or stabilization of the postsynaptic membrane produced by opening $\text{Cl}^-$ channels, which drives the potential toward the $\text{Cl}^-$ reversal potential of $-90\,\text{mV}$ and away from threshold.

Spatial Summation

The integration of postsynaptic potentials resulting from multiple distinct presynaptic neurons firing action potentials at a single postsynaptic neuron simultaneously.

Temporal Summation

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

Facilitation

A short-term form of synaptic plasticity characterized by increased synaptic strength lasting for $10\text{--}100\,\text{ms}$.

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 heightened postsynaptic sensitivity.

Ionotropic Receptors

Ligand-gated ion channels that directly open upon neurotransmitter binding to produce rapid and immediate postsynaptic electrical responses.

Metabotropic Receptors

G-protein coupled receptors that initiate intracellular signaling cascades, yielding slower onsets and more sustained cellular effects compared to ionotropic receptors.

Acetylcholine (ACh)

A major excitatory neurotransmitter that serves as the exclusive transmitter at the neuromuscular junction (NMJ) and is released by pre-ganglionic neurons in both the sympathetic and parasympathetic nervous systems.

Catecholamines

A group of neurotransmitters derived from tyrosine through the sequential pathway: Tyrosine $\rightarrow$ L-DOPA $\rightarrow$ Dopamine $\rightarrow$ Norepinephrine $\rightarrow$ Epinephrine.

NMDA Receptor

An ionotropic glutamate receptor that increases $\text{Ca}^{2+}$ influx in response to glutamate binding to produce excitatory postsynaptic potentials.

AMPA Receptor

An ionotropic glutamate receptor that increases $\text{Na}^+$ and $\text{K}^+$ flux in response to glutamate binding to generate excitatory postsynaptic potentials.

GABA Receptors

Receptors for the primary inhibitory neurotransmitter in the CNS, consisting of ionotropic $GABA_A$ receptors that increase $\text{Cl}^-$ influx and metabotropic $GABA_B$ receptors that activate GPCRs to induce $\text{K}^+$ efflux.

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

A monoamine neurotransmitter derived from tryptophan with at least $14$ subtypes across the $5\text{-HT}_1$ to $5\text{-HT}_7$ families, mostly metabotropic except for the ionotropic $5\text{-HT}_3$ receptor which mediates excitatory responses.