Chapter 6: Neuronal Signaling and the Structure of the Nervous System

Comprehensive vocabulary flashcards covering the structural organization, cellular types, electrical properties, synaptic transmission, and anatomical divisions of the nervous system.

Central Nervous System (CNS)

The major division of the nervous system consisting of the brain and the spinal cord.

Peripheral Nervous System (PNS)

The division of the nervous system that transmits signals between the central nervous system and receptors or effectors in all other parts of the body.

Glial Cells

Non-neuronal cells in the nervous system that do not generate electrical signals, but perform critical supportive and maintenance functions for neurons.

Dendrites

Highly branched neuronal processes that receive incoming electrical and chemical signals from other neurons.

Cell Body (Soma)

The part of the neuron that contains the nucleus and organelles, functioning to integrate incoming electrical information.

Axon

A long nerve fiber extension that carries outgoing electrical impulses away from the cell body toward target cells.

Initial Segment

The region of the axon directly adjacent to the cell body where action potentials are typically initiated.

Myelin Sheath

An insulating sheath composed of $20\text{ to }200\text{ layers}$ of modified plasma membrane that wraps axons to speed up electrical conduction.

Oligodendrocytes

Glial cells that produce and maintain myelin sheaths around axons within the central nervous system ($\text{CNS}$).

Schwann Cells

Glial cells that produce and wrap myelin sheaths around individual axons within the peripheral nervous system ($\text{PNS}$).

Nodes of Ranvier

Unmyelinated gaps along an axon between adjacent sections of myelin where ion flow occurs to regenerate action potentials.

Saltatory Conduction

The rapid propagation of action potentials along a myelinated axon where current jumps from one node of Ranvier to the next.

Kinesin

A motor protein that mediates anterograde axonal transport, carrying secretory vesicles from the cell body along microtubules toward the axon terminal.

Dynein

A motor protein that mediates retrograde axonal transport, carrying recycled membrane vesicles from the axon terminal along microtubules back toward the cell body.

Afferent Neurons

Neurons that convey sensory signals from peripheral receptors toward the central nervous system.

Efferent Neurons

Neurons that transmit outgoing signals away from the central nervous system to effector targets such as muscles or glands.

Interneurons

Neurons situated entirely within the central nervous system that connect and integrate signals between afferent and efferent pathways.

Nerve

A cordlike bundle containing afferent and efferent axons, blood vessels, and surrounding connective tissue within the peripheral nervous system.

Astrocytes

Glial cells in the $\text{CNS}$ that regulate the extracellular environment of neurons and help form the blood-brain barrier.

Microglia

Specialized macrophage-like glial cells in the $\text{CNS}$ that perform immune and phagocytic defense functions.

Ependymal Cells

Epithelial-like glial cells lining the ventricles of the brain that produce and circulate cerebrospinal fluid ($\text{CSF}$).

Electrical Potential

The potential to do work that arises when positive and negative electrical charges are separated across a boundary, measured as voltage or potential difference.

Resting Membrane Potential

The steady electrical voltage across an unstimulated plasma membrane, typically ranging between $-40\text{ mV}$ and $-90\text{ mV}$ due to an excess of negative charge inside the cell.

$$\text{Na}^+/\text{K}^+\text{-ATPase Pump}$$

A primary active transport carrier that hydrolyzes $\text{ATP}$ to pump $3\text{ Na}^+$ out of the cell and $2\text{ K}^+$ into the cell, creating and preserving ionic concentration gradients.

Depolarization

A shift in membrane potential from the resting value toward zero, making the intracellular environment less negative.

Overshoot

The phase of an action potential in which the intracellular potential becomes positive relative to the outside, rising above $0\text{ mV}$ up to approximately $+30\text{ mV}$.

Repolarization

The phase during which the membrane potential returns back down toward the resting potential following a depolarizing stimulus.

Hyperpolarization

A change in membrane potential that makes the inside of the cell more negative than its normal resting membrane potential (approaching $-90\text{ mV}$).

Graded Potentials

Membrane potential fluctuations that are confined to a localized area, vary proportionally in magnitude with stimulus intensity, and decay with distance.

Action Potential

A rapid, transient alteration of the membrane potential lasting $1\text{ to }4\text{ milliseconds}$ that operates on an all-or-none basis once threshold is reached.

Excitability

The specialized ability possessed by neurons and muscle cells to generate and propagate action potentials.

Tetrodoxin

A biological toxin produced by certain animals that binds to and blocks voltage-gated sodium channels, preventing action potential generation.

Absolute Refractory Period

The interval during an action potential when voltage-gated $\text{Na}^+$ channels are either open or inactivated, preventing any subsequent stimulus from triggering a second action potential.

Relative Refractory Period

The interval following the absolute refractory period during which slowly closing $\text{K}^+$ channels keep the membrane hyperpolarized; a second action potential can occur only if stimulated by a stronger-than-normal stimulus.

Electrical Synapse

A specialized junction where presynaptic and postsynaptic cells are linked by gap junctions, allowing direct local ionic current flow between cells.

Chemical Synapse

A junction between two excitable cells where the presynaptic cell releases chemical neurotransmitters across a synaptic cleft to bind receptors on the postsynaptic membrane.

SNARE Proteins

Membrane-bound proteins that interact with synaptotagmin in the presence of $\text{Ca}^{2+}$ to mediate the docking and exocytotic fusion of synaptic vesicles with the presynaptic membrane.

Excitatory Postsynaptic Potential (EPSP)

A graded depolarizing change in the postsynaptic membrane potential that brings the neuron closer to threshold.

Inhibitory Postsynaptic Potential (IPSP)

A graded hyperpolarizing change in the postsynaptic membrane potential that drives the cell further below threshold.

Temporal Summation

The additive membrane potential change that occurs when a single presynaptic axon is stimulated repeatedly in rapid succession before the prior postsynaptic potentials have decayed.

Spatial Summation

The additive membrane potential change produced when two or more distinct presynaptic inputs terminate on the same postsynaptic cell and fire simultaneously.

Axo-Axonic Synapse

A synapse formed by an axon terminal of one neuron contacting the axon terminal of another neuron, modulating $\text{Ca}^{2+}$ influx and neurotransmitter release.

Neuromodulators

Molecules that fine-tune synaptic transmission by modifying metabolic cascades and receptor sensitivity via slower intracellular second-messenger pathways.

Acetylcholine (ACh)

A prominent neurotransmitter in both the $\text{CNS}$ and $\text{PNS}$; neurons utilizing this transmitter degenerate in individuals suffering from Alzheimer's disease.

Muscarinic Receptors

A class of acetylcholine receptors coupled to G proteins, found primarily on autonomic parasympathetic effector organs.

Nicotinic Receptors

Ligand-gated ion channel acetylcholine receptors found on skeletal muscle fibers and at autonomic ganglia.

Catecholamines

A family of biogenic amines synthesized from amino acids that includes dopamine, norepinephrine, and epinephrine.

Dopamine

A biogenic amine neurotransmitter that plays key roles in reward-motivated behaviors and motor control pathways.

Serotonin

A biogenic amine neurotransmitter synthesized from amino acids that functions in regulating emotions and sleep cycles.

Glutamate

The principal excitatory amino acid neurotransmitter in the central nervous system.

GABA

Gamma-aminobutyric acid, the primary inhibitory amino acid neurotransmitter within the brain.

Endogenous Opioids

Neuropeptides consisting of two or more amino acids (such as beta-endorphins) that bind opiate receptors to modulate pain and pleasure.

Nitric Oxide

A short-lived gas produced enzymatically that diffuses freely across membranes to act as a neuromodulator and vasodilator without exocytotic vesicle packaging.

Cerebral Cortex

The outer convoluted shell of gray matter in the cerebrum that participates in sensory perception, skilled voluntary movements, reasoning, and memory.

Basal Nuclei

Subcortical gray matter structures within the cerebrum responsible for controlling posture, movement coordination, and behavioral patterns.

Thalamus

A large nuclear mass in the diencephalon that acts as a major synaptic relay station and processing center for sensory inputs bound for the cerebral cortex.

Hypothalamus

The diencephalic master regulator of homeostatic functions, connected by a stalk to the pituitary gland.

Epithalamus

A diencephalic division containing the pineal gland that functions in regulating circadian and biological rhythms.

Limbic System

An interconnected anatomical forebrain network—including the hippocampus, septal nuclei, olfactory bulbs, thalamus, and hypothalamus—involved in emotion, learning, and behavior.

Cerebellum

A hindbrain center essential for coordinating voluntary movements, controlling posture and balance, and storing motor memories.

Reticular Formation

A network of cell bodies and axon bundles running through the brainstem essential for life, regulating cardiovascular and respiratory reflexes, sleep, wakefulness, and attention.

Brainstem

The stalk of the brain composed of the midbrain, pons, and medulla oblongata through which ascending and descending fiber tracts travel.

Dorsal Horn

The posterior region of spinal gray matter where sensory afferent fibers enter via dorsal nerve roots.

Ventral Horn

The anterior region of spinal gray matter containing motor neuronal cell bodies whose axons depart through the ventral nerve roots.

Somatic Nervous System

The efferent division in which single myelinated motor axons project uninterrupted from the $\text{CNS}$ to innervate skeletal muscle fibers using acetylcholine.

Autonomic Nervous System (ANS)

The efferent division utilizing a two-neuron relay chain (preganglionic and postganglionic fibers) to innervate smooth muscle, cardiac muscle, and glandular tissues.

Sympathetic Division

The thoracolumbar autonomic branch mediating 'fight or flight' responses, signaling via adrenergic receptors at target tissues and innervating the adrenal medulla.

Parasympathetic Division

The craniosacral autonomic branch mediating 'rest and digest' responses, using acetylcholine acting on muscarinic receptors at target organs.

Meninges

The three connective tissue membranes—dura mater, arachnoid mater, and pia mater—that envelop and protect the brain and spinal cord.

Blood-Brain Barrier

A protective permeability barrier formed by tightly joined continuous brain capillary endothelial cells and astrocyte foot processes that restricts entry of substances into brain tissue.

Neuron

The fundamental cellular unit of the nervous system specialized to transmit electrical signals and release chemical messengers to communicate with target cells.

Neuronal Signal Directionality

The sequential transmission of electrical impulses receiving at dendrites, processing at the soma, triggering at the initial segment, and conducting along the axon toward target terminals.

Myelin Sheath Formation

The concentric wrapping of $20\text{ to }200\text{ layers}$ of specialized glial membrane around an axon to provide electrical insulation and accelerate conduction velocity.

Axonal Transport

The active, motor-protein-driven movement of vesicles, organelles, and proteins along microtubules between the cell body and the synaptic terminals.

Axon vs. Nerve

A single microscopic cellular process extending from an individual soma versus a macroscopic cord-like organ composed of bundled nerve fibers enveloped in connective tissue.

Synaptic Transmission Relationship

The functional arrangement where an upstream presynaptic cell releases chemical messengers across a fluid-filled cleft to bind receptors on a downstream postsynaptic cell.

Dual Synaptic Status of Interneurons

The organizational feature of central integrating cells in which each cell simultaneously serves as a postsynaptic target receiving input and as a presynaptic source delivering output.

Supportive Functions of Glial Cells

Providing metabolic nourishment, ionic buffering, myelin insulation, structural support, and immune surveillance for excitable neural tissue without firing action potentials.

Fundamental Law of Electrical Charges

The electrostatic principle stating that particles with identical polarities exert mutual repulsive forces, whereas particles with opposite polarities attract each other.

Resting Membrane Potential Determinants

The combined effect of unequal ion concentration gradients established by the $\text{Na}^+/\text{K}^+\text{-ATPase}$ pump and the significantly greater resting membrane permeability to $\text{K}^+$ than to $\text{Na}^+$.

Membrane Depolarization Mechanism

A reduction in the magnitude of charge separation across the plasma membrane that shifts the internal voltage toward zero, making it less negative.

Decremental Spread of Graded Potentials

The progressive dissipation of local depolarizing current through resting membrane leak channels that causes signal amplitude to decay with distance from the stimulus origin.

Graded Potentials vs. Action Potentials

Localized, variable-amplitude signals capable of summation that decay with distance, compared to all-or-none, propagated impulses maintaining uniform amplitude.

Role of Ligand- and Mechanically-Gated Channels

Membrane proteins responding to chemical transmitters or physical deformation that generate the initial graded potentials needed to drive the membrane to threshold.

Ion Permeability Sequence of an Action Potential

A rapid, dramatic rise in $\text{Na}^+$ conductance causing the depolarizing upstroke, followed by channel inactivation and a delayed increase in $\text{K}^+$ conductance causing repolarization.

All-or-None Principle

The physiological rule that an excitable cell responds to a threshold stimulus with a maximal, stereotyped impulse, while any subthreshold stimulus generates no impulse at all.

After-Hyperpolarization

The transient dip in membrane potential below normal resting levels that occurs because voltage-gated $\text{K}^+$ channels remain open for a brief period following repolarization.

Presynaptic $\text{Ca}^{2+}$ Influx

The entry of divalent cations through voltage-gated channels stimulated by terminal depolarization that activates synaptotagmin to induce neurotransmitter vesicle exocytosis.

Excitatory Synapse

A communication junction where neurotransmitter binding generates a localized net inward positive current, driving the postsynaptic membrane closer to threshold.

Inhibitory Synapse

A communication junction where neurotransmitter binding produces a net outward positive or inward negative current, stabilizing or hyperpolarizing the postsynaptic membrane away from threshold.

Chemical Synapse Sequence of Events

Action potential arrival depolarizes the terminal, opening voltage-gated $\text{Ca}^{2+}$ channels, triggering SNARE-mediated vesicle fusion, followed by transmitter diffusion across the cleft to postsynaptic receptors.

Three Early Brain Divisions

The initial embryonic regions of the central nervous system comprising the forebrain (prosencephalon), midbrain (mesencephalon), and hindbrain (rhombencephalon).

Gray Matter vs. White Matter

Centrally located tissue packed with unmyelinated somas, dendrites, and synapses versus tracts dominated by lipid-rich, myelinated axons specialized for conduction.

Enteric Division

The autonomous network of sensory and motor neurons embedded within the gastrointestinal tract wall that coordinates digestive reflexes independently of the central nervous system.

Cranial Nerves vs. Spinal Nerves

$12\text{ pairs}$ of peripheral nerves innervating head and neck structures directly from the brain versus $31\text{ pairs}$ of peripheral nerves distributing segmentally from the spinal cord.