Neurophysiology: Introduction, the Neuron, and Glial Cells

Overview and Major Subdivisions of the Mammalian Nervous System

• Structural Divisions:

  • Central Nervous System (CNS): Composed of the brain and spinal cord, anatomically encased and protected by bone (cranium/skull and vertebral column).

  • Peripheral Nervous System (PNS): Composed of sensory receptors, peripheral nerves (cranial and spinal nerves), and ganglia (clusters of neuronal cell bodies situated outside the CNS); establishes bidirectional communication pathways connecting the CNS to peripheral organs, tissues, and limbs.

• Primary Cell Populations:

  • Neurons: The primary functional signaling units of the nervous system (derived from the Greek neuron, meaning nerve); specialized for reception, integration, transmission, and processing of electrochemical information.

  • Neuroglia (Glial Cells): Supporting non-neuronal cells (derived from the Greek glia, meaning glue); critical for homeostatic maintenance, structural scaffolding, metabolic support, and defense.

Comparative Neuroanatomy and Neuron Distribution

• Interspecies Neuronal Population Comparisons:

  • Canines vs. Felines: Dogs possess a higher total number of cortical neurons than cats.

  • Humans vs. Elephants: Humans possess a higher absolute number of cerebral cortical neurons compared to elephants, despite the significantly larger total brain mass of the elephant.

  • Porifera (Sponges): Sponges possess zero ($0$) neurons, representing the baseline multicellular organisms lacking an organized nervous system.

Functional Organization of the Nervous System

• Central Nervous System Functions:

  • Brain: Receives and integrates sensory information, initiates motor responses, consolidates and stores memories, and generates cognitive thought patterns and emotional responses.

  • Spinal Cord: Conducts bidirectional afferent and efferent signals between the peripheral tissues and the brain; independently mediates and coordinates local reflex arcs.

• Peripheral Nervous System Divisions:

  • Sensory (Afferent) Division: Transmits action potentials generated by peripheral sensory receptors toward the CNS.

  • Visceral Sensory System: Monitors and delivers sensory input from visceral organs, internal baroreceptors, and chemoreceptors.

  • Somatic Sensory System: Delivers sensory feedback concerning physical position (proprioception), touch, pressure, pain (noxious stimuli), and thermal fluctuations.

  • Motor (Efferent) Division: Conveys descending motor commands and regulatory impulses from the CNS to peripheral effector tissues.

  • Somatic Motor System: Mediates voluntary control by conducting motor output directly to striated skeletal muscle fibers.

  • Visceral Motor System (Autonomic Nervous System): Mediates involuntary, autonomic control over cardiac muscle, smooth muscle, glands, and adipose tissue.

    • Sympathetic Division: Regulates physiological adaptations associated with the "Fight or Flight" stress response.

    • Parasympathetic Division: Coordinates homeostatic maintenance, energy conservation, and digestive functions associated with "Rest or Digest".

• Directional Mnemonic:

  • $\text{SAME}$: Sensory = Afferent; Motor = Efferent.

Functional Classification of Neurons and Reflex Circuitry

• Sensory (Afferent) Neurons:

  • Transmit action potentials from peripheral sensory receptors toward the spinal cord and brain.

  • Anatomically organized with cell bodies situated in peripheral ganglia (such as the dorsal root ganglia).

  • Axonal projections enter the dorsal horn of the spinal cord via the dorsal root.

• Interneurons (Association Neurons):

  • Confined entirely within the central nervous system (brain and spinal gray matter).

  • Form local processing circuits interposed between sensory input and motor output channels; determine reflex thresholds and integrated responses.

• Motor (Efferent) Neurons:

  • Originate with cell bodies located in the ventral horn of the spinal cord gray matter.

  • Axons exit the spinal cord via the ventral root and travel via peripheral nerves to terminate on effectors (skeletal muscle motor endplates or autonomic ganglia/viscera).

Comprehensive Functional and Anatomic Classification of the Nervous System

• Sensory (Afferent) System Categorization:

  • Somatic System:

  • General Somatic Afferent: Modalities include temperature, touch, and noxious stimuli. Innervation: all spinal nerves, cranial nerve $\text{V}$ (trigeminal nerve).

  • Special Somatic Afferent: Modalities include vision (cranial nerve $\text{II}$, optic nerve) and audition (cranial nerve $\text{VIII}$, vestibulocochlear nerve).

  • Visceral System:

  • General Visceral Afferent: Modalities include organ luminal contents, luminal distention, and chemical concentrations. Innervation: spinal nerve splanchnic branches, cranial nerves $\text{VII}$ (facial), $\text{IX}$ (glossopharyngeal), and $\text{X}$ (vagus).

  • Special Visceral Afferent: Modalities include gustation/taste (cranial nerves $\text{VII}$, $\text{IX}$, and $\text{X}$) and olfaction (cranial nerve $\text{I}$, olfactory nerve).

  • Proprioception:

  • General Proprioception: Mechanoreception of muscle length and joint angles. Innervation: all spinal nerves, cranial nerve $\text{V}$.

  • Special Proprioception: Vestibular spatial orientation and equilibrium. Innervation: cranial nerve $\text{VIII}$.

• Motor (Efferent) System Categorization:

  • Somatic System:

  • General Somatic Efferent: Motor control of striated skeletal muscles. Innervation: all spinal nerves, cranial nerves $\text{III}$ (oculomotor), $\text{IV}$ (trochlear), $\text{V}$ (trigeminal), $\text{VI}$ (abducens), $\text{VII}$ (facial), $\text{IX}$ (glossopharyngeal), $\text{X}$ (vagus), $\text{XI}$ (accessory), and $\text{XII}$ (hypoglossal).

  • Visceral System:

  • General Visceral Efferent: Autonomic innervation of smooth muscle, cardiac muscle, and glandular parenchymas.

    • Sympathetic Division: All spinal nerves and splanchnic nerves.

    • Parasympathetic Division: Sacral spinal nerves and cranial nerves $\text{III}$, $\text{VII}$, $\text{IX}$, and $\text{X}$.

Structural Anatomy and Components of the Neuron

• Seven Core Structural Components:

  1. Dendrites: Extensive branched arborizations acting as the receptive field of the cell membrane to integrate incoming electrochemical inputs.

  2. Cell Body (Soma or Perikaryon): Metabolic and transcriptional core containing the nucleus, nucleolus, rough endoplasmic reticulum (Nissl substance), smooth endoplasmic reticulum, Golgi complexes, ribosomes, lysosomes, neurofilaments, and mitochondria.

  3. Axon Hillock (Trigger Zone): The specialized, funnel-shaped junction between the soma and axon possessing high concentrations of voltage-gated ion channels; serves as the initiation site for action potentials.

  4. Axon: A prolonged cylindrical cytoplasmic process specialized for rapid action potential propagation away from the soma; varies in length from a few micrometers ($\mu\text{m}$) to greater than $10\,\text{m}$ depending on organismal size.

  5. Myelin Sheath: Concentric lipid-rich membrane wrappings serving as an electrical and ionic insulator along the internodal axon.

  6. Nodes of Ranvier: Unmyelinated periodic gaps exposed between adjacent myelin internodes where voltage-gated channels are densely clustered.

  7. Presynaptic Terminals (Terminal Boutons): Distal arborizations at the terminal ends of the axon containing synaptic vesicles loaded with chemical neurotransmitters for junctional exocytosis.

• Replication and Longevity:

  • Differentiated neurons are terminally post-mitotic and do not undergo cell division upon reaching full maturity.

  • Irreversible neuronal necrotic or apoptotic death permanently alters the cytoarchitecture and functional capacity of the affected neural tissue.

Dendritic Plasticity and Synaptic Function

• Information Integration:

  • Dendrites integrate graded potentials (inhibitory and excitatory postsynaptic potentials) to dictate whether the depolarization threshold is met at the axon hillock.

• Dendritic Spines:

  • Small membranous micro-protrusions projecting from the dendritic shaft that serve as individual postsynaptic contact sites for presynaptic axonal boutons.

  • Enable a single dendritic tree to receive concurrent inputs from hundreds to thousands of individual axon terminals.

• Structural Neuroplasticity:

  • The spatial density and branching arborization of dendritic spines are dynamically regulated based on ambient neurochemical inputs.

  • Enriched, stimulatory cognitive environments, structured learning paradigms, and subsequent physiological sleep cycles stimulate spine morphogenesis and synaptogenesis.

  • Sensory deprivation or prolonged stress leads to dendritic retraction and spine pruning.

Axonal Biology and Axoplasmic Transport

• Axoplasmic Composition:

  • The axon contains the vast majority of the total neuronal cytoplasmic volume.

  • Contains neurofibrils, neurotubules (microtubules), vesicular carriers, mitochondria, metabolic enzymes, and lysosomes.

  • Because ribosomes and rough endoplasmic reticulum are absent from axons, axonal proteins must be transcribed and translated in the soma and actively delivered via the cytoskeleton.

• Axoplasmic Transport Mechanisms:

  • An active, ATP-dependent intracellular trafficking process that occurs along longitudinal microtubule tracks; entirely distinct from electrochemical action potential propagation.

  • Anterograde Transport:

  • Direction: Movement from the soma outward toward the axon terminals.

  • Cargo: Synaptic components, neurotransmitter storage vesicles, membrane precursors, and mitochondria required at the terminal junction.

  • Retrograde Transport:

  • Direction: Movement from the terminal boutons back toward the soma.

  • Cargo: Endocytosed materials, target-derived neurotrophic signals, recycled membrane vesicles, and spent organelles targeted for lysosomal degradation.

Structural Classification of Neurons

• Multipolar Neurons:

  • Characterized by a single axon and multiple distinct dendritic trees emanating from the soma.

  • Constitutes the most abundant morphological category in mammals.

  • Exemplified by somatic motor neurons and central projection neurons.

• Bipolar Neurons:

  • Characterized by two distinct processes arising from opposite poles of the soma: one dendritic extension and one axon.

  • Typically found in specialized sensory circuits (e.g., retina, vestibular apparatus) and select interneurons.

• Pseudo-Unipolar Neurons:

  • Characterized by a single short continuous stem originating from the cell body that bifurcates into two distinct long axonal branches: a peripheral process extending to sensory receptors and a central process entering the CNS.

  • Typical architecture of general somatic and visceral afferent neurons residing in cranial and dorsal root ganglia.

• Unipolar Neurons:

  • Possess only one neurite emerging from the soma; typical of invertebrate (e.g., insect) nervous systems.

Neuroglia: Classification and Functional Profiles

• General Glial Characteristics:

  • Smaller individual soma diameter than typical projection neurons.

  • Lack axons, dendrites, and excitable membranes capable of firing all-or-none action potentials.

  • Populate all interstitial spaces in nervous tissue not directly occupied by neurons or blood vessels.

  • Substantially outnumber neurons within the central nervous system parenchyma.

  • Maintain persistent proliferative capacity (can divide post-developmentally).

• Central Nervous System Glial Subtypes:

  • Microglial Cells:

  • Constitute $10\text{--}20\%$ of the total CNS neuroglial population.

  • Function as resident myeloid-derived immunocompetent cells and macrophages.

  • Phagocytose cellular debris, myelin breakdown products, and apoptotic cells.

  • Secrete reactive nitrogen species (including nitric oxide, $NO$) to halt viral and microbial pathogen proliferation.

  • Mediate synaptic pruning during neurodevelopment to eradicate redundant connections.

  • Astrocytes:

  • Represent approximately $50\%$ of all glial cells in the CNS; exhibit a star-like morphology with elaborate radiating processes.

  • Form the outer and inner glial limiting membranes (glia limitans) of the CNS.

  • Establish extensive perivascular end-feet over capillary endothelium, regulating blood flow, metabolic substrate delivery (glucose transport), and maintaining the integrity of the blood-brain barrier (BBB).

  • Regulate extracellular fluid ion homeostasis by buffering microenvironmental concentrations of $K^+$, $Na^+$, $Ca^{2+}$, $Cl^-$, and $HCO_3^-$ via active ion channels.

  • Clear and metabolize neuroactive transmitters around synaptic clefts (notably glutamate reuptake and enzymatic processing).

  • Release neurotrophic growth factors that support neuronal survival, dendritic arborization, and axonal guidance.

  • Direct reactive astrogliosis and structural scar formation following physical trauma or ischemic lesions.

  • Oligodendrocytes:

  • Provide myelin sheaths for multiple adjacent axonal segments in the CNS.

  • Emit multiple tongue-like membrane processes, each wrapping around a separate axon of diameter $> 1\,\mu\text{m}$.

  • The high lipid content of compacted oligodendrocyte membranes forms the anatomical substrate for CNS white matter (contrasted with gray matter, which is dominated by neuronal somata and unmyelinated neuropil).

  • Ependymal Cells:

  • Simple ciliated columnar to cuboidal epithelial-like lining covering the ventricular walls of the brain and the central canal of the spinal cord.

  • Participate directly in the secretion, circulation, and filtration of cerebrospinal fluid (CSF).

• Peripheral Nervous System Glial Subtypes:

  • Schwann Cells (Neurolemmocytes):

  • Form the myelin sheath encircling peripheral axons larger than $1\,\mu\text{m}$ in diameter.

  • Unlike oligodendrocytes, a single Schwann cell myelinates only a single internodal segment ($25\text{--}1000\,\mu\text{m}$ in length) of a single axon.

  • Direct functional recovery and structural regrowth following peripheral nerve injury.

Myelin Sheath Architecture and Saltatory Conduction

• Membrane Architecture:

  • A specialized extension of the glial cell plasma membrane (Schwann cell in PNS; oligodendrocyte in CNS) wrapped repetitively in concentric spirals around an axon.

  • During myelination, cytoplasm is largely extruded from between the wrapping layers, leaving compacted multi-layered lipid bilayers.

  • Residual Schwann cell cytoplasm forms an internal and external ring surrounding the dense internodal sheath.

• Biophysical Mechanism of Conduction:

  • Serves as an electrical and ionic insulator along internodes by decreasing membrane capacitance and increasing transmembrane electrical resistance.

  • Action potential generation is restricted to the uninsulated Nodes of Ranvier, where voltage-gated $Na^+$ channels are clustered at high density.

  • Influx of $Na^+$ creates local intracellular current loops that traverse electrotonically through the axoplasm to the subsequent node with negligible leakage.

  • Produces Saltatory Conduction (derived from the Latin saltare, meaning to jump), wherein electrical depolarization jumps node-to-node.

  • Saltatory propagation increases conduction velocities by orders of magnitude compared to unmyelinated fibers of equivalent axon diameter.

Wallerian Degeneration and Peripheral Nerve Regeneration

• Pathophysiology of Wallerian Degeneration:

  • Refers to the standardized cascade of anterograde structural breakdown that occurs in the axon segment isolated distally from the soma following transection or severe crush injury.

  • Trophic and physical degeneration initiates at the lesion site and proceeds distally within $24\text{--}36\,\text{hours}$ post-injury.

  • The distal axon disintegrates into granular fragments, followed by collapse and fragmentation of the associated myelin sheaths.

  • Circulating macrophages invade the lesion site alongside resident reactive Schwann cells to phagocytose axonal fragments and lipid-dense myelin debris.

  • Complete clearing and degradation of the distal tract requires approximately $7\text{--}21\,\text{days}$.

• Regenerative Environment and Limitations:

  • Functional regeneration occurs in the PNS, facilitated by Schwann cells, but fails in the mammalian CNS due to inhibitory factors and astroglial scarring.

• Schwann Cell-Guided Regrowth Cascade:

  • Proliferating Schwann cells align longitudinally within the basal lamina to form cylindrical cellular guidance tracts known as the Bands of Büngner.

  • Schwann cells secrete neurotrophic growth factors that establish a molecular chemotactic gradient.

  • Between $7\text{--}21\,\text{days}$ post-trauma, the sealed proximal axon stump extends terminal regenerative exploratory sprouts (growth cones).

  • Growth cones navigate toward the trophic signals and enter the lumen of the Bands of Büngner.

  • Trapped within the Schwann cell tube, the axon elongates distally at a continuous velocity of approximately $1\,\text{mm/day}$.

  • The sprout traverses the length of the nerve conduit until it reaches and reinnervates its original peripheral effector tissue (e.g., neuromuscular junction).

• Factors Leading to Regenerative Failure:

  • Physical gaps between proximal and distal stumps that are excessively wide.

  • Interposition of dense collagenous fibrotic scar tissue or local hematoma/hemorrhage blocking sprout access to the Bands of Büngner.

  • Aberrant sprout migration resulting in neuroma formation rather than functional terminal reinnervation.