Action Potential Physiology, Neuronal Morphology, and Conduction Biophysics

Somatosensory Pathway and Neural Electrical Communication

• Fundamental Basis of Neural Signaling: Electrical communication within the nervous system relies on specialized, transient alterations in membrane potential. The principal, regenerative electrical signal propagated over long anatomical distances is the action potential.

• Functional Architecture of the Three-Neuron Somatosensory Pathway:

  • Cutaneous Sensory Receptors (Cute): Located within the skin and peripheral tissues. These specialized structures function as transducers, converting mechanical, thermal, tactile, or noxious chemical stimuli into graded electrical receptor potentials.
  • First-Order Neuron and Primary Afferent Fiber (Neurone del ganglio delle radici dorsali e Fibra di primo ordine):
    • The pseudounipolar cell body (soma) is located outside the central nervous system within the dorsal root ganglion.
    • The peripheral process extends to the sensory receptor in the skin, while the central axon enters the dorsal horn of the spinal cord via the dorsal root and ascends ipsilaterally toward the brainstem.
  • Relay Nucleus and Second-Order Fiber (Nucleo di relais e Fibra di secondo ordine):
    • Primary afferent fibers terminate and form synaptic connections within specific relay nuclei situated in the medulla oblongata of the brainstem.
    • Axons emerging from the cell bodies of second-order neurons decussate (cross the midline of the neuroaxis) and ascend through the brainstem via sensory tracts toward the diencephalon.
  • Thalamus, Third-Order Fiber, and Cerebral Cortex (Talamo, Fibra di terzo ordine e Corteccia cerebrale):
    • The thalamus serves as the obligatory subcortical relay, gating, and processing center for sensory information.
    • Second-order fibers synapse onto third-order neurons in specific thalamic nuclei.
    • Third-order fibers project directly from the thalamus through the internal capsule to the primary somatosensory cortex located in the postcentral gyrus, where conscious perception, sensory discrimination, and spatial localization of the stimulus occur.

Functional Morphology of the Neuron

• The Neuron as the Cellular Excitable Unit: Neurons represent the primary excitable cells of the nervous system, structurally and functionally specialized for the generation, integration, and directional transmission of action potentials.

• Anatomical and Subcellular Compartments:

  • Dendrites (Dendriti): Highly branched arborizations extending from the cell body that provide an expanded surface area. They contain high densities of ligand-gated ion channels and dendritic spines (dendritic spine) to receive chemical and electrical synaptic inputs from upstream neurons.
  • Cell Body / Soma (Corpo cellulare):
    • The metabolic, trophic, and biosynthetic center of the neuron.
    • Contains the nucleus, a prominent nucleolus, extensive rough endoplasmic reticulum and free polyribosomes termed Nissl bodies (corpi di Nissl) for active protein synthesis, Golgi apparatus for vesicular sorting, and numerous mitochondria to supply adenosine triphosphate ($ATP$).
    • Integrates incoming post-synaptic graded potentials.
  • Axon Hillock (Cono d'emergenza dell'assone):
    • The specialized conical junction between the neuronal soma and the initial segment of the axon.
    • Functions as the physiological trigger zone due to a critically high density of voltage-gated sodium channels.
    • Integrates incoming spatial and temporal post-synaptic potentials; when the net depolarization reaches the threshold potential, an action potential is triggered.
  • Axon (Assone): A single, long cylindrical cytoplasmic projection that propagates the action potential away from the soma in an efferent, unidirectional manner toward target cells.
  • Myelin Sheath and Schwann Cells (Guaina mielinica e Cellule di Schwann):
    • A multi-layered lipid and protein coating produced by Schwann cells in the peripheral nervous system (PNS).
    • Acts as an electrical insulator around the axonal internodes to reduce membrane capacitance and increase transverse membrane resistance.
  • Nodes of Ranvier (Nodi di Ranvier): Uninsulated gaps occurring at regular intervals between adjacent myelin sheaths. These segments feature high concentrations of voltage-gated sodium channels, enabling saltatory propagation.
  • Collateral Axon (Assone collaterale): Side branches emerging from the primary axon shaft that permit divergence of the action potential to multiple target structures.
  • Terminal Boutons / Presynaptic Terminals (Bottoni terminali): Distal swellings of the axonal telodendria containing synaptic vesicles loaded with chemical neurotransmitters, mitochondria, and voltage-gated calcium channels, mediating synaptic transmission across the synaptic cleft.

Electrophysiological Phases of the Action Potential

• General Time Course and Trajectory: The action potential is a rapid, transient reversal of the transmembrane electrical potential. When measured over time in milliseconds ($ms$ or $msec$), membrane potential ranges from a negative baseline of $-90\,mV$ (or $-70\,mV$ depending on neuronal cell type) to a positive peak of approximately $+20\,mV$ to $+30\,mV$.

• Resting Membrane Potential and Local Potential (Potenziale Locale):

  • Under baseline conditions, the intracellular environment is electrically negative relative to the extracellular fluid, resting between $-70\,mV$ and $-90\,mV$.
  • Subthreshold excitatory inputs trigger small, graded, localized shifts in membrane potential termed local potentials (potenziali locali).
  • These local potentials propagate passively with decrement; if the local depolarization reaches the critical threshold potential (soglia), a self-regenerating action potential is initiated.

• Depolarization Phase (Fase di Depolarizzazione):

  • Characterized by an explosive upward inflection in membrane potential, rising from the threshold level through $0\,mV$ up to a peak positive value (overshoot) of approximately $+20\,mV$ to $+30\,mV$.
  • Molecular Mechanism:
    • Depolarization to threshold induces a rapid conformational change in voltage-gated sodium channels, causing their activation gates to open ($Na^+$ channels open).
    • Driven by both a concentration gradient (higher extracellular $Na^+$) and an electrical gradient (negative internal charge), sodium ions flood into the cell ($Na^+$ ions diffuse into cell).
    • This rapid inward sodium current ($I_{Na}$) generates a positive feedback loop (Hodgkin cycle): inward positive charge further depolarizes the membrane, driving the recruitment and opening of additional neighboring voltage-gated $Na^+$ channels.

• Repolarization Phase (Fase di Ripolarizzazione):
  • Marked by a steep downward shift of membrane potential, returning from the peak positive value back toward negative baseline levels.
  • Molecular Mechanism:
    • The voltage-gated sodium channels undergo rapid, time-dependent inactivation via their cytoplasmic inactivation gate ($Na^+$ channels close), halting further $Na^+$ influx.
    • Simultaneously, voltage-gated potassium channels open with a delayed kinetic profile.
    • Potassium ions move out of the intracellular space down their electrochemical gradient into the extracellular fluid ($K^+$ channels open until end of the after potential).
    • This outward potassium current ($I_K$) removes positive charges from the axoplasm, re-establishing internal electronegativity.

• Afterpotential / Post-Hyperpolarization (Iperpolarizzazione Postuma / After Potential):
  • The terminal phase where the membrane potential becomes transiently more negative than the initial resting potential (undershoot).
  • Molecular Mechanism:
    • Voltage-gated potassium channels exhibit delayed closing kinetics, remaining open briefly even after the membrane potential has repolarized to the resting baseline.
    • The persistent outward flux of $K^+$ drives the membrane potential closer to the potassium equilibrium potential ($E_K \approx -90\,mV$).
    • Restoration of resting ionic concentrations and baseline voltage is subsequently achieved through the activity of constitutive background potassium leak channels and the electrogenic sodium-potassium adenosine triphosphatase ($Na^+/K^+$ ATPase pump), which pumps $3\,Na^+$ out in exchange for $2\,K^+$ imported per molecule of $ATP$ hydrolyzed.

Pharmacology and Toxicology of Voltage-Gated Sodium Channels

• The Voltage-Gated Sodium Channel as a Molecular Target: Voltage-gated $Na^+$ channels are membrane-spanning heterooligomeric proteins containing pore-forming domains, voltage-sensing segments, and distinct extracellular and intracellular regulatory gates. Multiple biological neurotoxins and synthetic drugs target specific sites on this protein complex.

• Natural Neurotoxins and Their Sources:

  • Tetrodotoxin (TTX): Extracted from the tissues of the pufferfish (pesce palla), produced by symbiotic bacterial flora including Pseudoalteromonas tetraodonis.
  • Batrachotoxin (BTX): Derived from the skin secretions of neotropical poison dart frogs (rana dorata/frettolosa, family Dendrobatidae).
  • $\alpha$-Toxins and $\beta$-Toxins: Peptidic venoms isolated from scorpion stings.

• Molecular Binding Sites and Mechanisms of Action:

  • Tetrodotoxin (TTX): Binds selectively to receptor site 1 located in the outer vestibule of the channel pore on the extracellular side (versante esterno). TTX acts as a physical plug, occluding the ion conduction pathway, selectively eliminating voltage-dependent $Na^+$ currents and arresting action potential generation.
  • Batrachotoxin (BTX): Binds to receptor site 2 situated within the intramembrane domain of the channel protein. It locks the channel in a permanently open state by shifting the activation voltage to more hyperpolarized potentials and eliminating normal inactivation, causing sustained depolarization, cellular calcium overload, and electrical inexcitability.
  • Scorpion $\alpha$-Toxins and $\beta$-Toxins: Interact with external voltage-sensing loops (neurotoxin receptor site 3 and site 4). They slow or prevent normal inactivation kinetics or alter the voltage threshold of channel activation, resulting in prolonged action potentials and uncontrolled repetitive neuronal firing.
  • Pronase and N-bromoacetamide: Experimental chemical and enzymatic agents applied to the intracellular cytoplasmic face (versante interno/citoplasmatico). They selectively cleave or chemically modify the peptide tether of the inactivation gate (inactivation ball/hinged lid), abolishing channel inactivation while preserving normal channel opening.
  • Local Anesthetics (e.g., Lidocaine):
    • Amphiphilic compounds that cross the lipid bilayer in an uncharged form and bind to a specific receptor site located in the inner pore cavity from the cytoplasmic side.
    • By physically blocking the pore and stabilizing the inactivated conformation of the channel, they inhibit $Na^+$ permeability in a use-dependent and voltage-dependent manner.
    • Prevents action potential initiation and transmission along nociceptive afferent nerve fibers, producing reversible regional analgesia and anesthesia.

The All-or-None Law

• General Definition and Biophysical Concept: The generation of an action potential in an excitable cell follows the all-or-none law (la legge del "tutto o nulla"). An action potential is not a graded phenomenon proportional to stimulus strength; an applied depolarizing stimulus either fails to reach the critical threshold value, yielding zero action potential ("nulla"), or reaches/surpasses the threshold, triggering an action potential of stereotypic, maximal, and invariant amplitude ("tutto").

• Quantitative Threshold Parameters:

  • Resting Membrane Potential: $-70\,mV$, establishing baseline electrical polarization.
  • Threshold Potential (Potenziale Soglia): Approximately $-55\,mV$, representing the critical membrane voltage at which inward sodium currents equal and then exceed outward potassium currents.
  • Peak Amplitude: Approximately $+30\,mV$, representing the maximum overshoot reversal of polarity.

• Experimental Stimulus Intensities and Membrane Responses:

  • Subthreshold Stimuli (Stimoli Sottosoglia):
    • Represented by low-amplitude stimulus pulses.
    • Induce minor local depolarizing shifts in membrane potential that remain below the $-55\,mV$ threshold.
    • Because the number of activated voltage-gated $Na^+$ channels is insufficient to generate self-sustaining regenerative current, the potential decays exponentially via passive electrotonic spread, returning directly to the $-70\,mV$ resting baseline without firing an action potential.
  • Threshold Stimulus (Stimolo Soglia):
    • An intermediate-intensity stimulus that depolarizes the membrane potential to precisely $-55\,mV$.
    • At $-55\,mV$, the Hodgkin positive feedback cycle engages, activating sufficient voltage-gated $Na^+$ channels to overcome outward leak currents.
    • Produces a full-scale action potential that peaks at $+30\,mV$ before proceeding through complete repolarization.
  • Suprathreshold Stimulus (Stimolo Soprasoglia):
    • A high-intensity stimulus well above threshold strength.
    • Evokes an action potential that peaks at precisely $+30\,mV$, displaying an amplitude, configuration, and trajectory identical to that generated by the threshold stimulus.
    • Because the peak voltage is governed by the equilibrium potential of sodium ($E_{Na}$) and channel gating properties rather than stimulus magnitude, suprathreshold stimuli cannot alter action potential height; stimulus strength is encoded biologically by action potential firing frequency rather than amplitude.

Action Potential Propagation and Refractory Dynamics in Unmyelinated Axons

• Requirement for Continuous Conduction: In an unmyelinated axon (assone amielinico), action potential propagation requires a continuous, uninterrupted distribution of voltage-gated sodium and potassium channels along the entire length of the axolemma.

• Sequential Stages of Continuous Propagation:

  • (a) Resting State (Resting):
    • The entire unmyelinated axonal membrane maintains a resting charge distribution: excess positive charges distributed along the outer leaflet in the extracellular fluid and excess negative charges along the inner leaflet in the intracellular fluid.
    • Action potentials initiate at the axon hillock.
  • (b) Initiation (Initiation):
    • At the initial active site (Site A), the membrane depolarizes past threshold, opening voltage-gated $Na^+$ channels and reversing membrane polarity (intracellular side becomes electropositive relative to the extracellular side).
    • This establishes longitudinal electrical potential differences between Site A and the adjacent resting region (Site B).
    • Local circular electric currents flow: intracellular positive charges flow toward the negative interior of Site B, while extracellular positive charges flow from Site B toward the negative exterior of Site A.
  • (c) Propagation (Propagation):
    • The local current loop depolarizes the adjacent resting patch (Site B) to threshold, triggering opening of its local voltage-gated $Na^+$ channels and initiating a newly generated action potential at Site B.
    • Concurrently, Site A enters the refractory state (stato refrattario), during which its voltage-gated $Na^+$ channels are closed and inactivated.
    • Local currents emerging from Site B begin to flow downstream toward the next resting segment (Site C).
  • (d) Continuation of Propagation (Propagation Continues):
    • Site C is depolarized to threshold, generating an active action potential.
    • Site B shifts into the refractory state, while Site A repolarizes and returns to the resting state (region of repolarization / resting state).
    • This cycle repeats consecutively point-by-point along the entire axonal length.

• Mechanism of Unidirectional (Orthodromic) Propagation:

  • Local circuit currents spread bidirectionally within the axoplasm, moving forward toward resting tissue and backward toward recently depolarized tissue.
  • Backward propagation is physically prevented because the immediately upstream axonal membrane segment is locked in a refractory state (absolute refractory period due to sodium channel inactivation, followed by relative refractory period due to elevated potassium conductance).
  • This ensures that the action potential propagates strictly in an orthodromic direction—away from the soma and axon hillock toward the presynaptic terminal boutons.

Cellular Mechanics of Axonal Myelination

• Myelination in the Peripheral Nervous System (PNS):

  • Conducted exclusively by Schwann cells.
  • A single Schwann cell associates with a single internodal segment of a single axon.
  • The Schwann cell repeatedly wraps its plasma membrane concentrically around the axonal cylinder.
  • The cellular cytoplasm and nucleus are squeezed into the outermost wrapping layer (neurilemma), while the tightly packed, multilayered lipid membranes form the insulating myelin sheath.
  • Small, periodic exposed gaps of bare axolemma between adjacent Schwann cells constitute the nodes of Ranvier.

• Myelination in the Central Nervous System (CNS):

  • Conducted exclusively by oligodendrocytes.
  • Unlike Schwann cells, a single oligodendrocyte projects multiple cytoplasmic processes that wrap and myelinate internodes on up to dozens of different neighboring axons simultaneously.

Saltatory Conduction and Biophysics of Myelinated Axons

• Discrete Spatial Distribution of Voltage-Gated Channels: In myelinated nerve fibers, voltage-gated sodium channels are highly concentrated at the uninsulated nodes of Ranvier. Underneath the internodal myelin sheath, voltage-gated channels are sparse or absent.

• Electrical Insulation and Passive Cable Properties:

  • The compact multi-lamellar myelin sheath acts as an effective electrical insulator.
  • By increasing membrane thickness, myelin dramatically decreases transmembrane electrical capacitance and increases transverse electrical resistance across the internode.
  • Consequently, transmembrane ionic leakage is minimized across internodal segments.
  • Current entering an active node cannot leak outward across the internodal membrane and is forced to flow longitudinally through the low-resistance intracellular axoplasm.

• Mechanism of Saltatory Conduction:

  • Phase (a): An action potential is actively generated at an initial node of Ranvier, driven by $Na^+$ influx that reverses local membrane polarity.
  • Phase (b): Electropositive intracellular charge flows electrotonically and rapidly through the internodal axoplasm toward the next downstream node of Ranvier, while extracellular current completes the circuit.
  • Phase (c): The electrotonic current depolarizes the downstream node to threshold, activating its clustered voltage-gated $Na^+$ channels and generating a new action potential.
  • The upstream node enters the refractory state, preventing retrograde current spread.
  • Because electrical regeneration occurs exclusively at successive nodes of Ranvier rather than continuously along every micrometer of membrane, the action potential appears to jump discontinuous from one node to the next (conduzione saltatoria).

• Physiological and Energetic Advantages:
  • Increased Velocity: Rapid passive electrotonic transmission through internodes substantially outpaces continuous regenerations required in unmyelinated fibers.
  • Metabolic Conservation: Because transmembrane sodium and potassium fluxes are restricted strictly to the nodal surfaces (representing less than $1\%$ of the total axonal membrane area), the total number of ions crossing the membrane per impulse is reduced. This lowers the metabolic workload and $ATP$ consumption required by $Na^+/K^+$ ATPase pumps to restore electrochemical gradients.

Comparative Biophysics of Conduction Velocity

• Velocity as a Function of Axon Diameter: Action potential conduction velocity ($v$, expressed in $m/s$ or $m/sec$) depends on axonal diameter ($d$, expressed in micrometers, $\mu m$), internal axoplasmic resistance, and membrane insulation properties.

• Mathematical Relationships Governing Conduction:

  • Unmyelinated Nerve Fibers (Fibre Amieliniche):
    • Represented by a non-linear, concave-downward curve.
    • Conduction velocity is proportional to the square root of axonal diameter:             $v \propto \sqrt{d}$
    • Because velocity scales only with the square root of the caliber, achieving high conduction speeds in unmyelinated systems requires large increases in fiber diameter (as observed in invertebrate giant axons).
  • Myelinated Nerve Fibers (Fibre Mieliniche):
    • Represented by a linear relationship.
    • Conduction velocity is directly proportional to axonal diameter:             $v \propto d$
    • Velocity increases linearly with diameter, enabling rapid signal transmission in compact nerve trunks.

• Critical Diameter Threshold and Biophysical Crossover ($1\,\mu m$):

  • Analysis of the Dashed Line Segment ($d < 1\,\mu m$):
    • The dashed portion of the linear curve for myelinated fibers delineates a theoretical diameter range below approximately $1\,\mu m$.
    • If myelinated fibers existed below $1\,\mu m$, their conduction velocity would be lower than that of unmyelinated fibers of equivalent outer diameter.
  • Underlying Biophysical Basis:
    • In axons with an external diameter smaller than $1\,\mu m$, wrapping the fiber in a myelin sheath of sufficient thickness severely constricts the internal diameter of the cytoplasmic core.
    • This marked reduction in internal cross-sectional area increases longitudinal axoplasmic electrical resistance, attenuating electrotonic current spread between nodes.
    • Consequently, for diameters below $1\,\mu m$, continuous unmyelinated conduction provides superior velocity and efficiency.
    • For axonal calibers exceeding $1\,\mu m$, myelination provides higher conduction velocities without requiring large increases in total axonal cross-sectional area.