7.1 Neurons, Neuroglia & Action Potential Physiology

7.1 Neurons, Neuroglia & Action Potential Physiology

The nervous system represents the master controlling and communicating system of the human body. Every sensation, voluntary movement, autonomic regulation, thought, and reflexive response depends upon the rapid generation, integration, and transmission of electrical and chemical signals. To excel on nursing entrance examinations and build clinical diagnostic intuition, students must command the structural divisions of the nervous system, the cellular anatomy of neurons and supporting neuroglia, and the rigorous electrochemical mechanisms governing resting membrane potentials and action potential propagation.

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Structural & Functional Organization of the Nervous System

The nervous system is organized anatomically into two primary structural divisions, which continuously coordinate to monitor external and internal environments, integrate sensory information, and initiate appropriate motor responses:

1. Central Nervous System (CNS): Composed exclusively of the brain (housed within the cranium) and the spinal cord (enclosed within the vertebral canal). The CNS functions as the supreme integration and command center of the body. It processes diverse sensory inputs, synthesizes complex decisions, stores memories, generates emotions, and dispatches coordinated motor directives.
2. Peripheral Nervous System (PNS): Encompasses all neural structures residing outside the brain and spinal cord. Its primary anatomical components include 12 pairs of cranial nerves (which emerge directly from the brain and brainstem to supply the head, neck, and visceral organs) and 31 pairs of spinal nerves (which emerge segmental from the spinal cord to supply the neck, trunk, and extremities). The PNS functions as an extensive communication network linking all peripheral body tissues to the central processing centers of the CNS.

+-------------------------------------------------------------------------+
|                   ORGANIZATION OF THE NERVOUS SYSTEM                    |
|                                                                         |
|                     +---------------------------+                       |
|                     |   CENTRAL NERVOUS SYSTEM  |                       |
|                     |   (Brain & Spinal Cord)   |                       |
|                     +-------------+-------------+                       |
|                                   |                                     |
|                     +-------------+-------------+                       |
|                     | PERIPHERAL NERVOUS SYSTEM |                       |
|                     | (Cranial & Spinal Nerves) |                       |
|                     +------+-------------+------+                       |
|                            |             |                              |
|         +------------------+             +------------------+           |
|         |                                                   |           |
|  +------+------+                                     +------+------+    |
|  |   SENSORY   |                                     |    MOTOR    |    |
|  | (AFFERENT)  |                                     |  (EFFERENT) |    |
|  |  DIVISION   |                                     |  DIVISION   |    |
|  +-------------+                                     +------+------+    |
|  - Somatic Sensory (skin, joints, muscle)                   |           |
|  - Visceral Sensory (internal organs)                       |           |
|                                                             |           |
|                     +---------------------------------------+           |
|                     |                                                   |
|              +------+------+                                     +------+------+|
|              |   SOMATIC   |                                     |  AUTONOMIC  ||
|              |   NERVOUS   |                                     |   NERVOUS   ||
|              |   SYSTEM    |                                     |   SYSTEM    ||
|              +-------------+                                     +------+------+|
|              - Voluntary                                                |       |
|              - Skeletal muscle effectors                                |       |
|                                           +-----------------------------+       |
|                                           |                                     |
|                                    +------+------+                       +------+------+|
|                                    | SYMPATHETIC |                       |PARASYMPATHETIC|
|                                    |  DIVISION   |                       |  DIVISION   ||
|                                    +-------------+                       +-------------+|
|                                    - Fight-or-Flight                     - Rest-and-Digest
|                                    - Catabolic arousal                   - Anabolic maintenance
+-------------------------------------------------------------------------+

Functional Subdivisions of the Peripheral Nervous System

Functionally, peripheral nerve fibers are grouped according to the direction in which they conduct nerve impulses:

• Sensory (Afferent) Division: Carries electrical nerve impulses toward the CNS from sensory receptors located throughout the body:
  • Somatic Sensory Fibers: Convey sensory impulses from the skin, skeletal muscles, bones, and synovial joints (e.g., touch, pain, temperature, pressure, and proprioception).
  • Visceral Sensory Fibers: Transmit impulses from internal visceral organs within the thoracic and abdominopelvic cavities (e.g., stomach distension, bladder fullness, and chemical changes in blood).
• Motor (Efferent) Division: Transmits electrical impulses away from the CNS to peripheral effector organs (muscles and glands) to elicit a physiological response. The motor division is further bifurcated into:
  • Somatic Nervous System (SNS): Conducts motor impulses from the CNS directly to skeletal muscles. Because it operates under conscious control, it is classically designated the voluntary nervous system (though it also executes involuntary somatic reflexes like the patellar stretch reflex).
  • Autonomic Nervous System (ANS): Regulates the involuntary visceral activities of cardiac muscle, smooth muscle, and endocrine/exocrine glands. Often termed the involuntary or visceral nervous system, it is subdivided into two physiologically opposing branches: the sympathetic division (mobilizes bodily systems during activity, stress, and emergency; "fight-or-flight") and the parasympathetic division (conserves energy, promotes digestion, and directs restorative cellular processes; "rest-and-digest").

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Cellular Anatomy of the Neuron

Nervous tissue consists of two distinct cell populations: excitable conducting cells called neurons (nerve cells) and non-excitable supporting cells called neuroglia (glial cells). Neurons are highly specialized, amitotic cells possessing extreme longevity, exceptionally high metabolic rates, and an absolute dependence on continuous supplies of glucose and oxygen.

+-------------------------------------------------------------------------+
|                      ANATOMY OF A MULTIPOLAR NEURON                     |
|                                                                         |
|          Dendrites                                                      |
|           \  |  /                                                       |
|            \ | /           Axon Hillock                                 |
|          +-------+               \                                      |
|          | Soma  |-------------------------------------===---===--->    |
|          | (Cell |       Initial   |      Axon         |     |          |
|          | Body) |       Segment   |   (Axolemma)      |  Telodendria   |
|          +-------+                 v                   v        |       |
|            / | \             Schwann Cell           Node of  Synaptic   |
|           /  |  \            (Myelin Sheath)        Ranvier  Terminals  |
+-------------------------------------------------------------------------+

1. Cell Body (Soma or Perikaryon)

The cell body represents the major biosynthetic and metabolic hub of the neuron. It contains a prominent spherical nucleus with an active nucleolus, surrounded by typical cytoplasmic organelles, with several notable specializations:

• Nissl Bodies (Chromatophilic Substance): Dense arrays of rough endoplasmic reticulum and free ribosomes that stain intensely with basic histological dyes. They synthesize the immense volume of structural proteins and enzymes required for neurotransmitter production and axoplasmic maintenance.
• Neurofibrils: Bundles of intermediate neurofilaments that form an extensive intracellular scaffolding, maintaining cellular shape and serving as structural tracks for intracellular transport.
• Lipofuscin: Golden-brown pigment granules that accumulate as harmless metabolic byproducts within aging neuronal lysosomes.
• Anatomical terminology distinction: A cluster of neuronal cell bodies within the CNS is termed a nucleus (plural: nuclei); an identical cluster of cell bodies residing within the PNS is termed a ganglion (plural: ganglia).

2. Dendrites

Dendrites are short, tapering, highly branched cytoplasmic processes that extend directly from the soma. In motor neurons, dendrites provide an expansive surface area studded with hundreds to thousands of microscopic dendritic spines, which form points of close contact with presynaptic terminals. Dendrites act as the primary receptive or input regions of the neuron. They do not generate action potentials; instead, incoming electrical stimuli generate short-distance, graded changes in membrane potential (graded potentials) that conduct passively toward the cell body.

3. Axon

Each neuron possesses exactly one axon, a slender cylindrical conducting process that arises from a specialized cone-shaped elevation of the cell body termed the axon hillock. The narrow transition zone between the hillock and the fully formed axon is the initial segment, which possesses the lowest electrical threshold in the cell and serves as the trigger zone where action potentials are generated.

• Axoplasm and Axolemma: The cytoplasm of the axon is termed the axoplasm, and its surrounding plasma membrane is designated the axolemma.
• Axon Length and Collaterals: Axons vary in length from a fraction of a millimeter to over a meter (such as motor axons extending from lumbar spinal segments to the intrinsic muscles of the foot). Axons may give off right-angle branches called axon collaterals.
• Telodendria and Synaptic Terminals: At its distal terminus, an axon branches profusely into thousands of delicate terminal arborizations called telodendria. Each branch ends in a bulbous expansion known as a synaptic knob, synaptic terminal, or bouton. These terminals contain abundant mitochondria and membrane-bound synaptic vesicles packed with chemical neurotransmitters.
• Axonal Transport: Because axons lack rough ER and Golgi complexes, they depend entirely on the cell body for protein synthesis. Materials travel along neurotubule tracks via two mechanisms: anterograde transport (movement from soma toward axon terminal, mediated by the motor protein kinesin, delivering enzymes, organelles, and membrane components) and retrograde transport (movement from axon terminal back to soma, mediated by dynein, returning degraded vesicular membranes, nerve growth factor, and inadvertently transporting neurotropic pathogens such as rabies, polio, herpes simplex, and tetanus toxin).

4. Myelin Sheath & Nodes of Ranvier

Many large-diameter axons are enveloped by a segmented, whitish, lipid-rich covering termed the myelin sheath. Myelin consists of roughly 80% lipid and 20% protein, presenting a glistening white appearance that gives white matter its characteristic coloration (in contrast to unmyelinated somas, dendrites, and neuroglia that form gray matter).

• Function: The primary role of myelin is to provide electrical insulation around the axolemma, preventing ionic leakage across the membrane and dramatically increasing the velocity of electrical impulse conduction.
• Nodes of Ranvier (Neurofibril Nodes): The myelin sheath is not continuous along the axon length. Adjacent myelinating glial cells are separated by regular gaps measuring approximately 1 micrometer, termed the nodes of Ranvier. At these uninsulated nodes, the axolemma is exposed directly to extracellular fluid and contains an exceptionally high concentration of voltage-gated sodium channels, making rapid saltatory conduction possible.

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Structural & Functional Classifications of Neurons

Neurons are classified both structurally (by the number of cytoplasmic processes extending from the soma) and functionally (by the direction in which nerve impulses travel relative to the CNS).

+-------------------------------------------------------------------------+
|                  STRUCTURAL CLASSIFICATIONS OF NEURONS                  |
|                                                                         |
|   MULTIPOLAR NEURON          BIPOLAR NEURON         UNIPOLAR (PSEUDO)   |
|      (Most Common)              (Special)               (Sensory)       |
|                                                                         |
|         Dendrites                 Dendrite               Receptive      |
|          \  |  /                     |                    Endings       |
|          +-----+                  +-----+                    |          |
|          |Soma |                  |Soma |                Peripheral     |
|          +--+--+                  +--+--+                  Process      |
|             |                        |                       |          |
|            Axon                     Axon                 +---+---+      |
|             |                        |                   | Soma  |      |
|             v                        v                   +---+---+      |
|         Terminals                Terminals                   |          |
|                                                          Central        |
|                                                          Process        |
|                                                              v          |
|                                                          Terminals      |
+-------------------------------------------------------------------------+

Structural Classification

1. Multipolar Neurons: Feature three or more distinct processes extending from the soma—specifically, multiple branching dendrites and a single conducting axon. Multipolar neurons represent the most common structural type in the human body, accounting for over 99% of all neurons. They encompass all motor (efferent) neurons and virtually all central interneurons.
2. Bipolar Neurons: Possess exactly two processes extending from opposite poles of an oval soma: one fused dendrite and one axon. Bipolar neurons are rare specialized sensory cells found only in primary sensory receptor organs: the retina of the eye, the olfactory epithelium of the nasal cavity, and the inner ear cochlea and vestibular apparatus.
3. Unipolar (Pseudounipolar) Neurons: Originate embryonically as bipolar cells whose two processes fuse during development into a single, short stem that bifurcates like an inverted "T" into a peripheral process (associated with sensory receptors) and a central process (entering the CNS). Unipolar neurons constitute virtually all primary sensory (afferent) neurons whose somas reside within sensory cranial nerve ganglia and dorsal root ganglia (DRG) of spinal nerves.

Functional Classification

• Sensory (Afferent) Neurons: Conduct nerve impulses originating from sensory receptors in cutaneous, muscular, or visceral tissues toward the brain and spinal cord. Most are unipolar, with their cell bodies residing in dorsal root ganglia outside the CNS.
• Motor (Efferent) Neurons: Carry action potentials away from the CNS to effector organs (skeletal, cardiac, and smooth muscles, and glands). Structurally, all somatic motor neurons are multipolar, with their somas located in the ventral gray horns of the spinal cord or motor cranial nerve nuclei.
• Interneurons (Association Neurons): Confined entirely within the CNS. Interneurons lie functionally between sensory and motor pathways, integrating sensory inputs, coordinating complex responses, and driving higher cognitive functions. More than 99% of the roughly 86 billion neurons in the human body are interneurons, and virtually all exhibit multipolar morphology.

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Neuroglia: Supporting Cells of the Nervous System

Neuroglia ("nerve glue"), or glial cells, are roughly as numerous as neurons in the human brain (older textbooks cited ratios as high as 10 to 1). Unlike neurons, neuroglia retain their capacity for mitotic cell division throughout adult life; consequently, almost all primary intracranial adult brain neoplasms (gliomas) arise from glial cells rather than non-dividing neurons. Six distinct types of neuroglia exist—four residing exclusively in the CNS and two in the PNS.

Neuroglia of the Central Nervous System (CNS)

1. Astrocytes: The most abundant and versatile glial cells in the CNS, exhibiting a delicate, star-shaped morphology with radiating cytoplasmic processes. Astrocytes anchor neurons directly to neighboring blood capillaries through expanded perivascular feet. These feet stimulate cerebral capillary endothelial cells to produce uninterrupted tight junctions, creating the structural foundation of the blood-brain barrier (BBB). Astrocytes also regulate the chemical composition of extracellular brain fluid by buffering excess extracellular potassium ions ($K^+$) and absorbing, recycling, and metabolizing released neurotransmitters (notably glutamate and GABA). Furthermore, they supply neurons with metabolic substrates (such as lactate derived from blood glucose) and form glial scar tissue (gliosis) following CNS trauma.
2. Oligodendrocytes: Possess fewer cytoplasmic processes than astrocytes. Oligodendrocytes line up along thick CNS nerve fibers and extend flattened paddle-like cytoplasmic extensions that wrap tightly around multiple axon segments simultaneously. These concentric layers of plasma membrane form the myelin sheaths of the CNS. A single oligodendrocyte can myelinate segments of up to 30 to 50 separate CNS axons. Unlike peripheral Schwann cells, oligodendrocytes lack a peripheral neurilemma and secrete neurite growth-inhibitory proteins that actively arrest axonal regeneration following spinal cord injury.
3. Microglia: Small, ovoid cells featuring thorny, branching cytoplasmic processes. Derived embryonically from mesodermal monocyte lineages, microglia function as the resident macrophages and specialized immune defense of the CNS. Because circulating white blood cells are barred from entering healthy brain parenchyma by the blood-brain barrier, microglia continuously monitor neural tissue health. Upon detecting cellular damage, necrotic debris, or invading microorganisms, microglia transform into large, active phagocytes that engulf and digest apoptotic cells and foreign pathogens.
4. Ependymal Cells: Range in morphology from cuboidal to columnar epithelium and possess prominent surface cilia. Ependymal cells form a continuous, permeable epithelial lining covering the fluid-filled ventricles of the brain and the central canal of the spinal cord. Working in direct conjunction with specialized capillary networks, ependymal cells form the choroid plexuses, which continuously synthesize and filter cerebrospinal fluid (CSF). The coordinated beating of ependymal cilia generates hydrodynamic currents that circulate CSF through the ventricular system and subarachnoid space.

Neuroglia of the Peripheral Nervous System (PNS)

1. Schwann Cells (Neurolemmocytes): The primary glial cells of the PNS. Schwann cells wrap around large peripheral axons to form the PNS myelin sheath. Unlike CNS oligodendrocytes, which myelinate multiple axons, each individual Schwann cell wraps around only a single 1 mm segment of a single peripheral axon. During the winding process, the Schwann cell's cytoplasm and nucleus are squeezed outward into the outermost layer, forming a nucleated cytoplasmic bulge called the neurilemma (sheath of Schwann). The neurilemma is essential for peripheral nerve repair: following axon transection (Wallerian degeneration), Schwann cells proliferate, secrete neurotrophic growth factors, and construct a hollow regeneration tube that guides the regenerating axonal sprout back to its original target effector.
2. Satellite Cells: Small, flattened cells that cluster around and envelop individual neuronal cell bodies located within sensory and autonomic ganglia of the PNS. Satellite cells provide vital structural support, regulate metabolic exchange between the neuronal soma and interstitial fluid, and maintain an optimal ionic microenvironment analogous to the homeostatic role of CNS astrocytes.

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Summary of Neuroglial Cell Types & Functions

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Electrophysiology of the Resting Membrane Potential

All living cells maintain an electrical voltage difference across their plasma membrane, termed the membrane potential. In resting, non-stimulated excitable cells (neurons and muscle fibers), this electrical charge separation is designated the resting membrane potential (RMP). In a typical resting human multipolar neuron, the RMP measures approximately -70 millivolts (mV). By convention, the minus sign denotes that the inner cytoplasmic face of the plasma membrane is electrically negative relative to the extracellular fluid bathing its outer surface.

+-------------------------------------------------------------------------+
|               IONIC BASIS OF THE RESTING MEMBRANE POTENTIAL             |
|                                                                         |
|   EXTRACELLULAR FLUID (Positive Net Charge Relative to Inside):         |
|   - High [Na+] (~142 mEq/L)                                             |
|   - High [Cl-] (~103 mEq/L)                                             |
|   -------------------- A X O L E M M A --------------------             |
|     [Na+/K+ Pump: 3 Na+ pumped OUT <--- 2 K+ pumped IN (Uses 1 ATP)]    |
|     [K+ Leak Channels: K+ leaks OUT freely down concentration gradient] |
|     [Na+ Leak Channels: Very few; minimal Na+ leaks inward]             |
|   ---------------------------------------------------------             |
|   INTRACELLULAR AXOPLASM (Negative Net Charge, -70 mV):                 |
|   - High [K+] (~140 mEq/L)                                              |
|   - Impermeant negatively charged proteins & organic phosphates (A-)    |
+-------------------------------------------------------------------------+

Factors Establishing the Resting Membrane Potential

The negative resting potential of -70 mV is established and sustained by three interacting physical and biochemical conditions:

1. Differences in Ionic Composition Across the Axolemma:
   • Extracellular Fluid (ECF): Possesses a high concentration of sodium ions ($Na^+$, ~142 mEq/L) and chloride ions ($Cl^-$, ~103 mEq/L), with a very low concentration of potassium ions ($K^+$, ~4 mEq/L).
   • Intracellular Fluid (ICF/Axoplasm): Possesses a high concentration of potassium ions ($K^+$, ~140-150 mEq/L) and a low concentration of sodium ions ($Na^+$, ~10-14 mEq/L). Crucially, the intracellular compartment is packed with large, non-diffusible, negatively charged organic molecules—including cytoplasmic proteins, amino acids, ATP, and organic phosphates ($A^-$)—that are physically trapped inside because they cannot cross the hydrophobic lipid bilayer.
2. Differential Membrane Permeability via Leakage Channels:
   • The resting axolemma possesses non-gated leakage channels that remain open continuously. Because the membrane contains roughly 50 to 75 times more $K^+$ leakage channels than $Na^+$ leakage channels, resting membrane permeability to potassium is vastly higher than its permeability to sodium.
   • Driven by its steep chemical concentration gradient, $K^+$ diffuses steadily out of the cell through these open leakage channels. As positively charged potassium ions leave the axoplasm, they leave behind the trapped, impermeable organic anions ($A^-$). This loss of positive charge makes the internal face of the membrane progressively negative.
   • If potassium diffusion were unopposed, it would reach an electrochemical equilibrium at -90 mV (the potassium equilibrium potential, $E_K$). However, because a small trickle of $Na^+$ leaks inward down its steep chemical and electrical gradients, it introduces a slight positive charge that pulls the resting potential up to its actual baseline of -70 mV.
3. Maintenance by the $Na^+/K^+$ ATPase Pump:
   • Because potassium leaks outward and sodium leaks inward continuously, these ion gradients would eventually run down, causing the membrane potential to dissipate to zero.
   • To prevent run-down, the axolemma utilizes the sodium-potassium pump ($Na^+/K^+$ ATPase). This electrogenic primary active transport pump consumes cellular energy by hydrolyzing ATP to pump 3 $Na^+$ ions outward while pumping 2 $K^+$ ions inward against their respective concentration gradients.
   • By extruding three positive charges for every two positive charges it imports, the pump contributes directly (roughly -3 to -5 mV) to the electronegativity of the interior while permanently sustaining the steep ionic gradients required for electrical excitability.

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Action Potential Generation & Phased Voltage-Gated Channel Kinetics

An action potential (AP), or nerve impulse, is a rapid, temporary, and large reversal of the membrane potential along the axolemma, transitioning from a resting state of -70 mV to a positive peak of approximately +30 mV, before restoring the resting state. Unlike graded potentials, action potentials do not decay with distance; they propagate along the entire length of an axon adhering strictly to the all-or-none law: if a stimulus depolarizes the axolemma to a critical voltage known as the threshold (typically -55 mV), voltage-gated channels open and an action potential of maximal, uniform amplitude is triggered. If threshold is not attained, no action potential occurs.

+-------------------------------------------------------------------------+
|                        ACTION POTENTIAL VOLTAGE PHASES                  |
|                                                                         |
|   Membrane                                                              |
|   Potential                                                             |
|   (mV)                                                                  |
|   +30 |                  /\ (Peak Depolarization: +30 mV)               |
|       |                 /  \                                            |
|       |                /    \  Repolarization                           |
|     0 |               /      \ (K+ efflux)                              |
|       |  Depolarization       \                                         |
|   -55 |     (Na+ influx)       \                                        |
|       |         /--- Threshold  \                                       |
|   -70 |--------/ (Resting: -70)  \           /-- Return to RMP          |
|       |                           \_________/                           |
|   -85 |                        Hyperpolarization                        |
|       +--------------------------------------------------------> Time   |
+-------------------------------------------------------------------------+

The Four Consecutive Phases of an Action Potential

1. Resting State (-70 mV):
   • All voltage-gated sodium ($Na^+$) and voltage-gated potassium ($K^+$) channels remain closed. Resting permeability is governed entirely by leakage channels and the $Na^+/K^+$ ATPase pump.
   • Each voltage-gated $Na^+$ channel possesses two distinct gates: a voltage-sensitive activation gate (closed at rest) and an inactivation gate (open at rest). Both gates must be open for sodium to enter.
   • Voltage-gated $K^+$ channels possess a single activation gate, which remains closed at rest.
2. Depolarization Phase (-55 mV to +30 mV):
   • Incoming graded potentials (dendritic EPSPs) depolarize the axolemma at the axon hillock toward threshold. When the membrane potential reaches -55 mV, voltage-gated $Na^+$ activation gates open rapidly.
   • Sodium ions rush into the cell driven by both a chemical concentration gradient and an electrical attraction to the negative interior (an immense electrochemical gradient).
   • As $Na^+$ floods the axoplasm, the interior becomes positive, which in turn opens additional voltage-gated $Na^+$ channels. This explosive positive feedback loop drives the membrane potential rapidly upward through 0 mV to a positive peak of +30 mV.
3. Repolarization Phase (+30 mV down toward -70 mV):
   • At the +30 mV peak, two critical channel events occur simultaneously:
     1. The voltage-gated $Na^+$ channel inactivation gates close abruptly, plugging the channel pores and halting further sodium influx. Sodium permeability plummets back to resting levels.
     2. The slow-acting voltage-gated $K^+$ channels finally open fully.
   • With $Na^+$ entry blocked and $K^+$ channels wide open, potassium ions rush out of the axoplasm down their electrochemical gradient. The rapid efflux of positive charges restores the electrical negativity of the cell interior, driving the membrane potential back downward toward the resting baseline.
4. Hyperpolarization Phase (Undershoot: -70 mV to ~-85 mV):
   • The voltage-gated $K^+$ channels exhibit sluggish closing kinetics, remaining open for a brief period after the membrane potential has returned to -70 mV.
   • This excessive, transient efflux of potassium drives the membrane potential below the normal resting baseline, dipping to approximately -75 to -85 mV.
   • As the voltage-gated $K^+$ channels finally snap shut, the membrane potential returns to -70 mV. Although the electrical potential is restored, subtle ionic shifts are corrected over time by the continuous activity of the $Na^+/K^+$ ATPase pump.

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Action Potential Phases & Ion Channel Kinetics Summary

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Refractory Periods & Impulse Conduction Velocity

Absolute vs. Relative Refractory Periods

During an ongoing action potential, a neuron cannot generate another action potential in response to ordinary stimuli. This period of electrical recovery is subdivided into two distinct refractory stages:

1. Absolute Refractory Period:
   • Spans from the initial opening of voltage-gated $Na^+$ activation gates at threshold (-55 mV) until the $Na^+$ inactivation gates reset to their resting state during late repolarization.
   • During this window, the sodium channels are either already completely open or locked in an inactivated conformation. Therefore, no stimulus, regardless of its magnitude or duration, can trigger a second action potential.
   • Clinical Importance: The absolute refractory period enforces two fundamental neural rules: it sets an absolute upper limit on the maximal firing frequency of a neuron, and it ensures strictly unidirectional (one-way) forward propagation of action potentials away from the axon hillock toward the terminals, preventing backward reverberation.
2. Relative Refractory Period:
   • Immediately follows the absolute period, coinciding with the hyperpolarization phase when $Na^+$ channels have reset to their resting state, but slow voltage-gated $K^+$ channels remain open.
   • Because the membrane is hyperpolarized (-80 mV) and $K^+$ continues to leave the cell, the axon hillock is further away from the -55 mV threshold. However, an exceptionally strong suprathreshold stimulus can force enough sodium channels open to overcome the potassium efflux and trigger a new action potential.

Conduction Velocity: Continuous vs. Saltatory Conduction

The speed at which an action potential propagates along an axon depends upon two physical factors: axon diameter (larger diameters offer lower internal resistance to electrical current flow, conducting faster) and the presence of a myelin sheath.

• Continuous Conduction: Occurs in unmyelinated axons (such as autonomic postganglionic fibers and C-type slow pain fibers). Because the entire axolemma lacks insulation, voltage-gated sodium and potassium channels are distributed densely along its entire length. Depolarization in one membrane segment slowly triggers adjacent channels, requiring every adjacent micrometer of membrane to undergo full depolarization and repolarization. This process is metabolically expensive and slow, conducting at speeds of only 0.5 to 2.0 meters per second (~1 to 4 mph).
• Saltatory Conduction: Occurs in myelinated axons (such as somatic motor fibers and A-type sensory fibers). The thick myelin sheath provides high electrical resistance and low capacitance, preventing ions from leaking through the internodal axolemma. Consequently, local electrical currents travel rapidly through the internal axoplasm without decaying until they encounter an uninsulated node of Ranvier.
  • At each node of Ranvier, voltage-gated $Na^+$ channels are packed at very high density, far higher than along the insulated internodes. The electrical current instantly depolarizes the node to threshold, regenerating the action potential. The nerve impulse appears to "leap" or dance (Latin saltare = to leap) from one node to the next.
  • Saltatory conduction achieves speeds of up to 100 to 120 meters per second (~220 to 270 mph) while conserving immense amounts of cellular ATP, as the $Na^+/K^+$ pump is only required to restore ion gradients at the nodes rather than across the entire axon.

Clinical Correlate: Demyelinating Pathologies

• Multiple Sclerosis (MS): A progressive autoimmune disease of the CNS characterized by immune-mediated destruction of oligodendrocytes and the myelin sheaths they produce. As myelin is replaced by hardened, non-conductive sclerotic plaques, the uninsulated internodal axolemma lacks sufficient voltage-gated sodium channels to sustain current flow. Electrical current leaks out, leading to conduction slowing, conduction block, and clinical symptoms including visual deficits (optic neuritis), muscle weakness, spasticity, paresthesias, and ataxia.
• Guillain-Barré Syndrome (GBS): An acute, post-infectious autoimmune polyneuropathy targeting the Schwann cells and myelin sheaths of peripheral motor and sensory nerves. Characterized by rapidly ascending symmetrical flaccid paralysis starting in the lower extremities, GBS represents a medical emergency when demyelination ascends to involve the phrenic nerves (C3-C5), threatening catastrophic respiratory failure.