The Nervous System: Neuroanatomy, Membrane Potentials, and Synaptic Transmission

Comprehensive vocabulary flashcards covering anatomical classifications of neurons, ion channel physiology, resting and action potentials, refractory dynamics, and synaptic transmission mechanisms.

Sensory (Afferent) Division

The functional division of the nervous system that carries electrical impulses from sensory receptors toward the central nervous system ($\text{CNS}$).

Motor (Efferent) Division

The functional division of the nervous system that conducts electrical impulses away from the central nervous system ($\text{CNS}$) to effector organs such as muscles and glands.

Association Neuron (Interneuron)

A neuron situated entirely within the central nervous system ($\text{CNS}$) that integrates incoming sensory information and coordinates motor output.

Multipolar Neuron

The most abundant structural class of neuron in the body and major type in the $\text{CNS}$, featuring three or more processes (one axon and multiple dendrites), such as cerebellar Purkinje cells and pyramidal cells.

Bipolar Neuron

A rare structural class of neuron possessing two processes (one axon and one dendrite) extending from opposite sides of the cell body, found in special sensory organs like the olfactory mucosa and retina.

Unipolar (Pseudounipolar) Neuron

A structural class of neuron possessing a single short process that divides into a peripheral process and a central process, found primarily in the $\text{PNS}$ within dorsal root ganglia and cranial sensory ganglia.

Cell Body (Soma)

The biosynthetic center of a neuron that contains the nucleus, nucleolus, and Nissl bodies.

Dendrites

The branching, receptive extensions of a neuron that receive chemical and electrical signals from other cells.

Axon

The elongated neuron process responsible for generating and conducting electrical nerve impulses away from the cell body toward target cells.

Axon Hillock

The conical region of the neuron cell body where the axon originates and where action potentials are initiated if threshold is reached.

Myelin Sheath

A segmented lipid-rich insulation surrounding axons—formed by Schwann cells in the $\text{PNS}$ and oligodendrocytes in the $\text{CNS}$—that significantly increases electrical impulse conduction velocity.

Nodes of Ranvier

Gaps in the myelin sheath along the axon where the axolemma is exposed, allowing saltatory conduction of action potentials.

Leakage Channels

Protein-lined, water-filled plasma membrane channels that remain continuously open across the soma, dendrites, and axon, facilitating passive ion movement ($K^+$, $Na^+$, $Cl^-$) and establishing resting membrane potential.

Chemically-Gated Channels

Ion channels located predominantly on dendrites and the soma that open or close in response to the specific binding of a chemical messenger, such as a neurotransmitter.

Voltage-Gated Channels

Excitable ion channels found on the axon hillock, axon, and axon terminals that open or close in response to changes in the membrane potential.

Ohm's Law

The relationship stating that current ($I$) is directly proportional to voltage ($V$) and inversely proportional to resistance ($R$), expressed as $I = \frac{V}{R}$.

Resting Membrane Potential (RMP)

The steady charge separation across the plasma membrane of a non-stimulated excitable cell, characterized by an inside-negative potential of approximately $-70\,mV$.

$Na^+/K^+$ ATP Pump

A primary active transport membrane enzyme that hydrolyzes $\text{ATP}$ to export three $Na^+$ ions and import two $K^+$ ions, maintaining the ion concentration gradients essential for the resting membrane potential.

Depolarization

A reduction in membrane potential where the interior of the cell becomes less negative (or positive) relative to the resting membrane potential, primarily mediated by $Na^+$ influx.

Hyperpolarization

An increase in membrane potential where the cell interior becomes more negative than the resting potential, typically driven by $K^+$ efflux or $Cl^-$ influx.

Action Potential (AP)

A brief, all-or-none reversal of membrane electrical polarity (a $100\,mV$ fluctuation from $-70\,mV$ to $+30\,mV$) conducted along an axon via sequential opening of voltage-gated $Na^+$ and $K^+$ channels.

AP Threshold

The critical level of depolarization at the axon hillock (typically $-55\,mV$, representing a $15\,mV$ shift from resting) required to initiate the positive feedback opening of voltage-gated $Na^+$ channels.

Absolute Refractory Period

The interval during an action potential when voltage-gated $Na^+$ channels are open or inactivated, making it impossible for any subsequent stimulus, regardless of strength, to initiate a second action potential.

Relative Refractory Period

The period following the absolute refractory period during which most $Na^+$ channels are closed and $K^+$ channels are open, requiring an exceptionally strong stimulus to reach threshold and generate an action potential.

Saltatory Conduction

The rapid mechanism of action potential propagation along myelinated axons in which depolarization leaps from one node of Ranvier to the next.

Chemical Synapse

A specialized functional junction between a presynaptic axon terminal and a postsynaptic membrane, across which signals are transmitted via chemical neurotransmitters traversing a $10\text{--}20\,nm$ synaptic cleft.

Synaptic Delay

The rate-limiting time interval ($0.3\text{--}5.0\,ms$) required for calcium-mediated neurotransmitter release, diffusion across the synaptic cleft, and postsynaptic receptor binding.

Excitatory Postsynaptic Potential (EPSP)

A local, graded depolarization of the postsynaptic membrane produced by the opening of chemically-gated channels permeable simultaneously to both $Na^+$ and $K^+$.

Inhibitory Postsynaptic Potential (IPSP)

A local, graded hyperpolarization of the postsynaptic membrane produced by the opening of chemically-gated channels selective for $K^+$ efflux or $Cl^-$ influx.

Temporal Summation

The additive postsynaptic effect generated when a single presynaptic terminal releases neurotransmitter in rapid succession before previous postsynaptic potentials decay.

Spatial Summation

The additive postsynaptic effect resulting from the simultaneous stimulation of multiple distinct presynaptic terminals at different locations on the same postsynaptic neuron.

Glutamate

The primary excitatory amino acid neurotransmitter in the central nervous system ($\text{CNS}$).

GABA (Gamma-Aminobutyric Acid)

The primary inhibitory amino acid neurotransmitter in the central nervous system ($\text{CNS}$).

Nitric Oxide (NO)

An unconventional gas neurotransmitter that is synthesized on demand rather than stored in vesicles, diffusing retrogradely from postsynaptic cells back to presynaptic terminals.

G-Protein Linked Receptor

A slow-acting metabotropic receptor where neurotransmitter binding triggers a G protein to activate adenylate cyclase, converting $\text{ATP}$ to cyclic $\text{AMP}$ ($\text{cAMP}$) to phosphorylate target ion channels.

Reflex Arc

A series neural circuit pathway comprising five sequential components: receptor, sensory neuron, integration center, motor neuron, and effector.