Chapter 12: Nervous System Overview, Neurophysiology, and Neural Integration

Comprehensive vocabulary flashcards covering the anatomical and functional divisions of the nervous system, neurocytology, axonal transport, glial cell classes, myelination, electrophysiology, synaptic mechanisms, neural circuits, and clinical pathologies.

Nerve

A bundle of nerve fibers (axons) wrapped in fibrous connective tissue.

Ganglion

A knot-like swelling in a nerve where neuron cell bodies of the peripheral nervous system are concentrated.

Central Nervous System (CNS)

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

Peripheral Nervous System (PNS)

The anatomical subdivision of the nervous system composed of all nerves and ganglia outside the central nervous system.

Sensory (Afferent) Division

The functional division of the peripheral nervous system that carries signals from sense organs (receptors) to the central nervous system.

Somatic Sensory Division

The subdivision of the sensory division that carries signals from receptors located in the skin, muscles, bones, and joints.

Visceral Sensory Division

The subdivision of the sensory division that carries sensory signals from the viscera, including the heart, lungs, stomach, and urinary bladder.

Motor (Efferent) Division

The functional division of the peripheral nervous system that carries outgoing signals from the central nervous system to effectors such as glands and muscles.

Somatic Motor Division

The motor subdivision that carries signals to skeletal muscles, producing voluntary muscle contractions as well as automatic somatic reflexes.

Visceral Motor Division (Autonomic Nervous System, ANS)

The motor subdivision that carries involuntary signals to glands, cardiac muscle, and smooth muscle to execute visceral reflexes.

Sympathetic Division

The division of the autonomic nervous system that stimulates and prepares the body for action.

Parasympathetic Division

The division of the autonomic nervous system that exerts a calming effect on bodily functions.

Enteric Plexus

A specialized neural network embedded within the digestive tract wall that coordinates local digestive communication and activity.

Excitability

A universal property of neurons describing their capacity to respond to environmental stimuli.

Conductivity

A universal property of neurons describing the production and propagation of electrical signals to other cells.

Secretion

A universal property of neurons wherein an electrical signal reaching an axon terminal triggers the release of a neurotransmitter to stimulate the adjacent cell.

Interneurons

Neurons confined entirely to the central nervous system that receive signals from other neurons, process information, and determine appropriate responses.

Neurosoma (Cell Body / Soma / Perikaryon)

The control center of a neuron containing the nucleus and major organelles, which lacks centrioles and cannot undergo mitosis after adolescence.

Neurofibrils

Bundles of actin filaments in the neuronal cytoskeleton that compartmentalize the rough endoplasmic reticulum into dark-staining chromatophilic substance.

Lipofuscin

A golden-brown pigment found in neuronal inclusions resulting from the lysosomal degradation of worn-out organelles.

Dendrites

Extensive, tree-branching extensions of a neuron that serve as the primary reception sites for incoming signals from other neurons.

Axon (Nerve Fiber)

A long, cylindrical neuronal extension specialized for the rapid conduction of nerve signals away from the soma.

Axon Hillock

A distinct mound on one side of the neurosoma from which the axon originates.

Axon Terminal (Terminal Bouton)

A bulbous swelling at the terminus of an axon branch that forms a synapse with a target cell.

Multipolar Neuron

A structural class of neuron with one axon and multiple dendrites; the most common structural type in the body and central nervous system.

Bipolar Neuron

A structural class of neuron featuring one axon and one dendrite, found in olfactory cells, sensory neurons of the ear, and certain retinal circuits.

Unipolar Neuron

A structural class of neuron featuring a single process exiting the cell body that divides into a peripheral process and a central process.

Anaxonic Neuron

A structural class of neuron containing multiple dendrites but lacking an axon, found in the brain, retina, and adrenal gland.

Anterograde Transport

Axonal transport carrying materials (such as enzymes and ion channels) away from the cell body toward the axon terminal, powered by the motor protein kinesin.

Retrograde Transport

Axonal transport returning materials from the axon terminal to the cell body for disposal or recycling, powered by the motor protein dynein.

Neuroglia (Glial Cells)

Non-neuronal supportive cells in the nervous system numbering approximately $86\text{ billion}$ in the adult brain, roughly equal to the number of neurons.

Oligodendrocytes

Glial cells of the central nervous system that extend multiple processes to form myelin sheaths around neighboring axons.

Ependymal Cells

Central nervous system glial cells that line the internal fluid-filled cavities of the brain and function to produce and circulate cerebrospinal fluid (CSF).

Microglia

Phagocytic glial cells of the central nervous system that engulf cellular debris and defend against foreign pathogens.

Astrocytes

The most abundant glial cells in the central nervous system; their perivascular feet induce the blood-brain barrier, supply lactate, regulate tissue fluid, and form scar tissue.

Astrocytosis (Sclerosis)

The proliferation of astrocytes to form hardened scar tissue in damaged areas of the central nervous system.

Schwann Cells (Neurolemmocytes)

Glial cells that envelop peripheral nervous system axons, form the peripheral myelin sheath, and aid in the regeneration of damaged nerve fibers.

Satellite Cells

Glial cells that surround neuron cell bodies within peripheral ganglia to provide electrical insulation and regulate the chemical microenvironment.

Gliomas

Rapidly growing, highly malignant brain tumors originating from mitotically active glial cells.

Myelin Sheath

A multi-layered insulating spiral of plasma membrane around an axon composed of approximately $20\%$ protein and $80\%$ lipid.

Neurilemma (Neurolemma)

The thick, outermost coil of a Schwann cell containing its nucleus and the bulk of its cytoplasm.

Endoneurium

A thin sleeve of fibrous connective tissue surrounding a peripheral nerve fiber external to the neurilemma and basal lamina.

Myelin Sheath Gap (Node of Ranvier)

An unmyelinated gap along an axon between adjacent myelin sheath segments where ion channels are concentrated.

Multiple Sclerosis (MS)

A central nervous system disease in which oligodendrocytes and myelin sheaths deteriorate and are replaced by hardened scar tissue, disrupting nerve signal conduction.

Tay–Sachs Disease

A hereditary disorder characterized by the abnormal accumulation of a glycolipid (ganglioside) in the myelin sheath due to a missing lysosomal enzyme, causing blindness and dementia.

Electrical Potential

A difference in the concentration of charged particles between one point and another, representing potential energy measured in volts.

Current

The physical flow of charged particles from one point to another.

Resting Membrane Potential (RMP)

The charge difference across the plasma membrane of an unstimulated, resting neuron, typically measured at $-70\,\text{mV}$.

Depolarization

A reduction in membrane polarity that shifts the intracellular membrane voltage toward a less negative value (such as when $\text{Na}^+$ flows into the cell).

Local Potential

A short-range, graded, decremental, and reversible voltage fluctuation across the plasma membrane produced by a localized stimulus.

Threshold

The critical minimum membrane voltage (typically around $-55\,\text{mV}$) required to open voltage-gated ion channels and fire an action potential.

Action Potential

A rapid, dramatic up-and-down voltage fluctuation produced by the sequential opening and closing of voltage-gated ion channels along an excitable membrane.

Repolarization

The phase of an action potential during which the membrane potential shifts back toward negative resting levels due to the outflow of $\text{K}^+$.

Hyperpolarization

A temporary state during which the membrane potential becomes $1\text{ to }2\,\text{mV}$ more negative than the resting membrane potential due to lingering $\text{K}^+$ outflow.

All-or-None Law

The principle stating that if a neuron reaches threshold, it fires at full maximum voltage; if threshold is not reached, it does not fire an action potential at all.

Absolute Refractory Period

The period following an action potential during which no stimulus of any strength can initiate a new action potential because voltage-gated $\text{Na}^+$ channels are inactivated.

Relative Refractory Period

The interval during hyperpolarization when a new action potential can be triggered, but only by an unusually strong stimulus sufficient to overcome the negative voltage overshoot.

Continuous Conduction

The mode of signal conduction in unmyelinated axons where action potentials and sodium influx occur sequentially along the entire length of the fiber like falling dominoes.

Saltatory Conduction

The rapid mode of signal conduction in myelinated nerve fibers where action potentials jump from one node of Ranvier to the next.

Synapse

The specialized junction where an axon terminal meets and communicates with another cell, such as a neuron, muscle cell, or gland cell.

Electrical Synapse

A junction where gap junctions directly connect adjacent cells, allowing ions to spread immediately without chemical diffusion delay but lacking the ability to integrate signals.

Synaptic Cleft

A narrow extracellular gap (typically approximately $20\,\mu\text{m}$ wide) separating the presynaptic and postsynaptic membranes at a chemical synapse.

Cholinergic Synapse

A chemical synapse that utilizes acetylcholine (ACh) as its neurotransmitter to activate postsynaptic ligand-gated ion channels.

GABA-ergic Synapse

An inhibitory synapse that utilizes $\gamma\text{-aminobutyric acid}$ (GABA), which binds to postsynaptic chloride channels to cause $\text{Cl}^-$ influx and hyperpolarization.

Adrenergic Synapse

A synapse that employs norepinephrine (NE) to bind G-protein coupled receptors, generating intracellular second messengers like cAMP with the advantage of enzyme amplification.

Acetylcholinesterase (AChE)

An enzyme located in the synaptic cleft that terminates synaptic signaling by degrading acetylcholine into choline and acetate.

Monoamine Oxidase (MAO)

An intracellular enzyme that breaks down reabsorbed amino acid and monoamine neurotransmitters within the presynaptic axon terminal.

Neuromodulators

Chemicals secreted by neurons that produce long-term modulatory adjustments in neurotransmitter release, synthesis, or receptor sensitivity across groups of neurons.

Neural Integration

The cellular capacity of neurons to process, store, and recall multiple inputs to execute physiological decisions.

Excitatory Postsynaptic Potential (EPSP)

A partial depolarization of the postsynaptic membrane (typically caused by $\text{Na}^+$ entering the cell) that moves the membrane potential closer to threshold.

Inhibitory Postsynaptic Potential (IPSP)

A hyperpolarization of the postsynaptic membrane that drives the membrane voltage farther below resting potential, reducing the likelihood of firing an action potential.

Temporal Summation

The additive process occurring when a single synapse generates EPSPs in rapid succession before previous potentials can fade, allowing threshold to be reached.

Spatial Summation

The additive process wherein simultaneous EPSPs originating from multiple distinct synapses converge and summate to threshold at the axon hillock.

Presynaptic Facilitation

A mechanism in which one presynaptic neuron enhances signal transmission by another (such as releasing serotonin to keep voltage-gated $\text{Ca}^{2+}$ channels open longer).

Presynaptic Inhibition

A mechanism in which an inhibitory neuron suppresses neurotransmitter release from another axon terminal (such as releasing GABA to block $\text{Ca}^{2+}$ channel opening).

Labeled Line Code

A sensory coding mechanism in which qualitative stimulus information is identified based strictly on which specific nerve fiber transmits the signal to the central nervous system.

Neural Pool

A functional ensemble consisting of thousands of interneurons dedicated to regulating a specific bodily task, such as breathing rhythm or walking movements.

Discharge Zone

The central zone within an input neuron's territory in a neural pool where its dense synapses can independently induce postsynaptic cells to fire.

Facilitated Zone

The peripheral zone of a neural pool where an input neuron forms fewer synapses and can stimulate target cells only with collaborative input from other fibers.

Diverging Circuit

A neural circuit arrangement in which one input nerve fiber branches to synapse with and stimulate multiple downstream postsynaptic neurons.

Converging Circuit

A neural circuit arrangement in which input from many different nerve fibers converges onto a single output neuron or neural pool.

Reverberating Circuit

A neural circuit in which neurons stimulate one another in a linear chain, but later neurons send collateral branches back to restimulate early cells, creating a self-sustaining loop.

Parallel After-Discharge Circuit

A circuit in which an input neuron branches into multiple parallel chains containing differing synapse counts, converging onto a common output with variable delays that sustain prolonged firing.

Memory Trace (Engram)

The physical pathway of altered or newly formed synapses through the brain that constitutes the anatomical basis of memory.

Synaptic Plasticity

The biological capacity of synapses to remodel, strengthen, weaken, or form anew in response to use and activity.

Posttetanic Potentiation

A form of short-term potentiation where calcium lingering inside the axon terminal following rapid stimulation causes an exceptionally large burst of neurotransmitter upon subsequent activation.

Long-Term Depression (LTD)

A synaptic forgetting mechanism triggered by low-frequency stimulation that activates protein phosphatases to disassemble actin microfilaments and tear down dendritic spines.

Alzheimer Disease (AD)

A degenerative neurological disease characterized pathologically by cortical gyri atrophy, neurofibrillary tangles, senile plaques of $\beta\text{-amyloid}$, and deficiencies in acetylcholine.

Parkinson Disease (PD)

A progressive motor disorder caused by the loss of dopamine-releasing neurons in the substantia nigra/basal nuclei, resulting in tremors, rigidity, slurred speech, and slow gait.