Biol 251: Lecture 10 - Neuronal Conduction and Synaptic Transmission

Comprehensive vocabulary flashcards covering action potential propagation, saltatory conduction, refractory periods, synaptic structures, and postsynaptic potentials from Biology 251 Lecture 10.

Electrotonic current (EC)

The rapid electrical current flow along the interior of the axon membrane that spreads ahead of an action potential but decays over distance unless insulated by myelin.

Myelin sheath

A lipid-rich insulating layer wrapping around certain axons that prevents ion leak and increases the conduction velocity of electrical signals by 15 to 250 times compared to unmyelinated axons.

Saltatory conduction

The rapid form of nerve impulse conduction in myelinated axons where action potentials appear to leap from one node of Ranvier to the next.

Internode

The myelin-covered segment of an axon between adjacent nodes of Ranvier, typically with a length about 100 times the axon diameter, ranging from $200\,\mu\text{m}$ to $2\,\text{mm}$.

Multiple Sclerosis (MS)

An autoimmune disorder in which the immune system attacks the central nervous system, causing demyelination that leads to current leakage and slowed or impaired nerve conduction.

Absolute refractory period

The phase during and immediately following an action potential during which no new action potential can be triggered, set by the time required for voltage-gated $Na^+$ channels to reactivate from their inactivated state to their resting closed state.

Relative refractory period

The phase following the absolute refractory period where voltage-gated $Na^+$ channels have returned to their resting state but voltage-gated $K^+$ channels remain open, requiring a larger-than-normal depolarizing stimulus to fire an action potential.

Three conformational states of voltage-gated $Na^+$ channels

The functional forms of the voltage-gated $Na^+$ channel: the resting state (activation gate closed, inactivation gate open), activated state (both gates open), and inactivated state (activation gate open, inactivation gate closed).

Ligand-gated ion channels (neuronal localization)

Ion channels situated primarily on dendrites and the cell body that open upon binding specific chemical messengers to generate local (graded) potentials.

Voltage-gated $Ca^{2+}$ channels (neuronal localization)

Ion channels concentrated at the axon terminal that open in response to membrane depolarization, permitting calcium influx that triggers neurotransmitter vesicle exocytosis.

Electrical synapse

A neuronal junction mediated by gap junction protein pores that allow direct, bidirectional, and fast ionic current flow between adjacent cells; typically excitatory.

Chemical synapse

A unidirectional junction that converts an incoming presynaptic electrical signal into a chemical neurotransmitter signal released across the synaptic cleft to generate postsynaptic potentials.

Synaptic calcium concentration gradient

The steep concentration difference of calcium across the axon terminal membrane, having an intracellular resting concentration of approximately $[Ca^{2+}]_i \sim 10^{-4}\,\text{mM}$ and an extracellular concentration of approximately $[Ca^{2+}]_o \sim 1\,\text{mM}$.

Acetylcholinesterase (AChE)

A specific hydrolytic enzyme present within the synaptic cleft that terminates synaptic signaling by enzymatic degradation of acetylcholine.

Neurotransmitter reuptake

A synaptic clearance process in which released neurotransmitter molecules are actively transported back into the presynaptic neuron or absorbed by surrounding cells such as astrocytes.

Myasthenia gravis

An autoimmune disorder characterized by antibodies attacking acetylcholine receptors (AChR) on the postsynaptic membrane of neuromuscular junctions, impairing transmission and producing muscle weakness.

Ionotropic receptor

A receptor protein that functions directly as a ligand-gated ion channel, producing immediate, short-duration changes in membrane potential when bound by a neurotransmitter.

Metabotropic receptor

A neurotransmitter receptor that lacks an integral ion channel and instead activates a G-protein and intracellular second messenger signaling cascade to modulate ion channels or cellular enzymes.

Excitatory postsynaptic potential (EPSP)

A graded, depolarizing shift in postsynaptic membrane potential produced when ligand-gated channels open and allow positive ions (such as $Na^+$ or $Ca^{2+}$) to enter the cell, bringing it closer to threshold.

Inhibitory postsynaptic potential (IPSP)

A graded, hyperpolarizing shift in postsynaptic membrane potential produced by the opening of ligand-gated channels that allow $Cl^-$ influx or $K^+$ efflux, moving the membrane potential farther away from threshold.

Summation

The additive integration of multiple postsynaptic graded potentials (EPSPs and IPSPs) at the axon's trigger zone (axon hillock) to determine whether membrane potential reaches threshold to generate an action potential.