9.1 Neuron Structure, Neuroglia & Nerve Impulse Conduction

Neuron Structure, Neuroglia & Nerve Impulse Conduction

Core Concept: The nervous system is the body's primary high-speed communication and control network. It coordinates sensory detection, information integration, and motor response. This entire system operates through specialized cellular architecture: excitable neurons that transmit electrochemical impulses and supportive neuroglia that sustain, protect, and insulate them.

---

1. Nervous Tissue Histology: Neurons vs. Neuroglia

Nervous tissue consists of two distinct classes of cells:

1. Neurons (Nerve Cells): Highly specialized, excitable cells capable of generating, conducting, and transmitting rapid electrochemical signals termed action potentials. Neurons possess an exceptionally high metabolic rate, requiring continuous supplies of oxygen and glucose. They are characterized by extreme longevity (functioning over a human lifetime) and are predominantly amitotic in the mature adult (with very limited exceptions in the hippocampus and olfactory bulb).
2. Neuroglia (Glial Cells): Non-excitable accessory cells that outnumber neurons by approximately 5 to 10-fold. Glia do not initiate or propagate action potentials; instead, they provide mechanical support, form electrical insulation (myelin), regulate the chemical composition of interstitial fluid, synthesize neurotrophic factors, and defend against pathogens. Unlike mature neurons, neuroglia retain the capacity for cellular division (mitosis) throughout life—a biological characteristic explaining why virtually all primary adult central nervous system neoplasms (gliomas, astrocytomas) arise from glial cells rather than neurons.

---

2. Neuroglia: Supportive Cell Lines of the CNS and PNS

Glial cells are segregated into four distinct types within the Central Nervous System (CNS) and two distinct types within the Peripheral Nervous System (PNS).

                    ┌──────────────────────────────────────────────┐
                    │          NEUROGLIAL CELLS (GLIA)             │
                    └──────────────────────┬───────────────────────┘
                                           │
            ┌──────────────────────────────┴──────────────────────────────┐
            ▼                                                             ▼
┌──────────────────────────────┐                            ┌──────────────────────────────┐
│ Central Nervous System (CNS) │                            │Peripheral Nervous System(PNS)│
├──────────────────────────────┤                            ├──────────────────────────────┤
│ • Astrocytes (BBB, K+ buffer)│                            │ • Schwann Cells (Myelination,│
│ • Oligodendrocytes (Myelin)  │                            │   Neurolemma & Regeneration) │
│ • Microglia (Phagocytosis)   │                            │ • Satellite Cells (Ganglionic│
│ • Ependymal Cells (CSF flow) │                            │   Support & Microenvironment)│
└──────────────────────────────┘                            └──────────────────────────────┘

---

3. Anatomy of a Neuron

A representative multipolar neuron consists of three functionally integrated regions: a receptive input zone, an integrative trigger zone, and a conducting/secretory zone.

The Soma (Cell Body or Perikaryon)

The soma is the biosynthetic and metabolic core of the neuron, ranging from 5 to 140 micrometers in diameter. It houses:

• Nucleus & Nucleolus: A spherical, pale, euchromatic central nucleus containing a prominent dark nucleolus responsible for intense ribosomal RNA (rRNA) transcription.
• Nissl Bodies (Chromatophilic Substance): Dense, prominent clusters of rough endoplasmic reticulum (RER) and free polyribosomes that stain intensely with basic dyes. Nissl bodies produce structural proteins, membrane proteins, and peptide neurotransmitters to sustain axonal transit.
• Cytoskeletal Architecture: Abundant neurofibrils (intermediate neurofilaments) and microtubules arranged in parallel bundles that maintain cellular shape, provide mechanical tensile strength, and serve as intracellular railway tracks for axonal transport.

Dendrites (The Receptive Zone)

Dendrites are short, tapering, highly branched cytoplasmic extensions that project outward from the soma. In motor neurons and interneurons, dendrites provide an enormous surface area studded with thousands of minute projections termed dendritic spines. Spines form postsynaptic junctions with incoming axon terminals. Dendrites receive chemical signals from adjacent neurons and transduce them into localized, non-propagating graded potentials (depolarizations or hyperpolarizations) conducted toward the soma.

The Axon Hillock & Initial Segment (The Trigger Zone)

The axon emerges from the cell body at a funnel-shaped, specialized elevation called the axon hillock. The hillock transitions into the unmyelinated initial segment of the axon. Together, this region constitutes the trigger zone. The trigger zone contains the cell's lowest electrical threshold because it possesses the highest physiological density of voltage-gated Na+ channels. Here, all incoming graded potentials conductively converge; if the net membrane potential reaches threshold (approximately -55 mV), an all-or-none action potential is initiated.

The Axon (The Conducting Zone)

The axon (nerve fiber) is a solitary, cylindrical, elongated cytoplasmic process that propagates action potentials away from the soma toward target effectors (muscles, glands, or other neurons). Axons can vary in length from less than a millimeter in the brain to over a meter (e.g., sciatic nerve motor fibers running from the lumbosacral spinal cord to the foot). The axon's cytoplasm is designated axoplasm, enclosed by the axolemma (plasma membrane). Because the axon contains mitochondria and cytoskeletal elements but completely lacks Nissl bodies and Golgi cisternae, it is entirely dependent upon the soma for protein maintenance.

• Axonal Transport:
  • Anterograde Transport: Movement of materials from the soma forward to the axon terminal, propelled along microtubules by the motor protein kinesin (transports mitochondria, synaptic vesicles, structural proteins, and neurotransmitter-synthesizing enzymes).
  • Retrograde Transport: Movement of materials from the axon terminal backward to the soma, propelled by the motor protein dynein (transports recycled membrane vesicles, nerve growth factor, and pathological neurotropic agents such as rabies virus, tetanus toxin, polio virus, and herpes simplex).

Myelin Sheath & Nodes of Ranvier

Many axons are enveloped by a multi-layered lipid-and-protein wrap termed the myelin sheath, which provides electrical insulation to prevent ion dissipation across the axolemma:

• In the PNS, myelin is produced by Schwann cells. Each Schwann cell repeatedly spirals its plasma membrane around an axon segment (internode), squeezing its cytoplasm and nucleus to the outermost perimeter to form the neurolemma (sheath of Schwann). The neurolemma is critical for peripheral nerve regeneration.
• In the CNS, myelin is formed by oligodendrocytes, whose cytoplasmic processes myelinate multiple adjacent axons without forming a protective neurolemma.
• Nodes of Ranvier (Neurofibril Nodes): Unmyelinated gaps of exposed axolemma (approximately 1 micrometer in length) occurring at regular 1-millimeter intervals between adjacent myelin segments. Nodes of Ranvier contain an exceptionally high concentration of voltage-gated Na+ and K+ channels.

Axon Terminals (The Secretory Zone)

Distally, the axon branches profusely into hundreds to thousands of fine terminal filaments termed telodendria. Each telodendrion terminates in a bulbous swelling designated the axon terminal (synaptic knob or end bulb). Terminals are densely packed with mitochondria and membrane-bound synaptic vesicles storing neurotransmitter molecules ready for calcium-dependent exocytosis into the synaptic cleft.

---

4. Structural and Functional Classification of Neurons

Neurons are categorized based on their morphological branching patterns and their directional role within physiological circuits.

       MULTIPOLAR                   BIPOLAR               UNIPOLAR (PSEUDOUNIPOLAR)
       
        ┌───┐ Dendrites              ┌───┐ Dendrite                 Peripheral
      ┌─┤   ├─┐                      │   │                           Process
    ┌─┤ └───┘ ├─┐                    │   │                       ═══════════════════
    │ └─┬───┬─┘ │                    ▼   ▼                                │
    │   │   │   │                  ┌───────┐                              │
    └───┼───┼───┘                  │ Soma  │                              ▼
        ▼   ▼                      └───────┘                           ┌───────┐
      ┌───────┐                        │                               │ Soma  │ (In DRG)
      │ Soma  │                        │ Axon                          └───────┘
      └───────┘                        ▼                                  │
          │                        ┌───────┐                              │ Central
          │ Axon                   │Terminal                          ═══════════════════
          ▼                        └───────┘                             Process
      ┌───────┐                                                         (To CNS)
      │Terminal
      └───────┘
(Motor / Interneurons)         (Special Senses)                    (Primary Sensory)

Structural Classification (Number of Processes)

1. Multipolar Neurons: Possess one axon and two or more dendrites emerging from the cell body. Multipolar neurons represent the most common anatomical class (>99% of all neurons in the human body). Examples include somatic motor neurons of the spinal cord anterior horn and pyramidal cells of the cerebral cortex.
2. Bipolar Neurons: Possess exactly two distinct processes extending from opposite poles of the soma: one fused dendrite and one axon. These rare neurons are confined to specialized sensory pathways: the olfactory epithelium of the nasal cavity, the retina of the eye, and the spiral ganglion of the cochlea (inner ear).
3. Unipolar (Pseudounipolar) Neurons: Originate embryologically as bipolar cells whose two processes fuse into a single short stem emerging from the soma. This stem immediately bifurcates into a T-shape: a peripheral process (dendritic/receptive branch extending into peripheral tissues) and a central process (axonal branch entering the spinal cord or brainstem). These comprise all primary sensory (afferent) neurons whose somas reside within the dorsal root ganglia of spinal nerves and sensory ganglia of cranial nerves.
4. Anaxonic Neurons: Small neurons with multiple dendrites but no discernible axon; they generate local graded potentials without action potentials (e.g., amacrine and horizontal cells of the retina, interneurons of the brain).

Functional Classification (Direction of Impulse)

1. Sensory (Afferent) Neurons: Transmit sensory action potentials inward from peripheral sensory receptors toward the central nervous system. Structurally, virtually all primary sensory neurons are unipolar (somas in dorsal root ganglia), with bipolar neurons serving special senses.
2. Motor (Efferent) Neurons: Transmit motor action potentials outward from the CNS to peripheral target effectors (skeletal muscle, smooth muscle, cardiac muscle, or glands). Structurally, all somatic and autonomic motor neurons are multipolar.
3. Interneurons (Association Neurons): Reside entirely within the gray matter of the brain and spinal cord, positioned between sensory afferent and motor efferent pathways. They account for >99% of all neurons in the human body. Interneurons process, integrate, analyze, and store sensory data and coordinate motor outputs. Structurally, all interneurons are multipolar.

---

5. Neurophysiology: Membrane Potentials & Ion Dynamics

Neurons communicate across distances by manipulating the electrical voltage across their plasma membranes.

The Resting Membrane Potential (RMP)

In an unstimulated, resting neuron, a stable electrical potential difference exists across the axolemma, termed the Resting Membrane Potential (RMP), calibrated at approximately -70 mV (the interior of the cell is negative relative to the extracellular fluid). The RMP is established and maintained by three essential biophysical factors:

1. Unequal Electrolyte Distribution: Extracellular fluid (ECF) has a high concentration of sodium (Na+ ~145 mM) and chloride (Cl- ~105 mM). Intracellular fluid (cytosol) has a high concentration of potassium (K+ ~140–150 mM) and non-diffusible organic anions (A-: proteins, amino acids, phosphate, and sulfate groups).
2. Differential Membrane Permeability (Leak Channels): The resting axolemma contains far more non-gated K+ leak channels than Na+ leak channels (resting permeability to K+ is approximately 50 to 100 times greater than to Na+). Consequently, K+ diffuses rapidly down its steep chemical concentration gradient out of the cell. As positively charged K+ leaves, the impermeable organic anions (A-) are left behind trapped inside the cytoplasm, establishing a negative electrical charge along the inner surface of the membrane.
3. The Electrogenic Na+/K+ ATPase Pump: A transmembrane protein pump driven by ATP hydrolysis that actively transports 3 Na+ ions out of the cell for every 2 K+ ions imported. Because it pumps three positive charges out for every two brought in, it is electrogenic, generating a net outward current that contributes directly (-3 to -4 mV) to the negative baseline and continuously preserves the chemical concentration gradients of Na+ and K+.

---

6. The Action Potential: Phases, Channels & Propagation

An action potential (nerve impulse) is a rapid, transient, all-or-none reversal of the resting membrane potential from -70 mV to approximately +30 mV, followed by rapid restoration of the resting state.

Membrane Potential (mV)
   +30 |                         ,-. (Peak Depolarization: Na+ channels inactivate)
       |                        /   \
       |                       /     \  Repolarization (K+ efflux)
     0 |----------------------/-------\----------------------------
       |                     /         \
   -55 |                    /           `---- (Threshold)
       |     Depolarization/             \
   -70 |------/ (Na+ influx)              `-------/--- (Resting Potential: -70 mV)
       |     /                             `-----'  (Hyperpolarization: ~ -80 mV)
       +-----+-------------------------------+---------------------> Time (ms)
             Stimulus    Absolute Refractory  Relative Refractory

Step-by-Step Phases of the Action Potential

1. Resting State (-70 mV): All voltage-gated Na+ and K+ channels are closed. Resting potential is maintained by leak channels and the Na+/K+ pump.
2. Stimulus & Threshold Depolarization (-55 mV): Excitatory synaptic inputs produce localized graded potentials that spread to the axon hillock. If the membrane depolarizes to the critical threshold voltage of -55 mV, voltage-gated Na+ channel activation gates snap open instantaneously.
3. Depolarization Phase (-55 mV to +30 mV): As voltage-gated Na+ channels open, Na+ ions rush into the axoplasm driven by both chemical concentration gradients (high outside to low inside) and electrical attraction (negative interior). This massive influx of positive charge causes explosive membrane depolarization, overshooting 0 mV to reach a peak of +30 mV.
4. Repolarization Phase (+30 mV to -70 mV): Two events occur at +30 mV:
   • Voltage-gated Na+ channel inactivation gates close automatically, immediately halting the influx of Na+.
   • Voltage-gated K+ channels open (their opening is delayed relative to Na+). Driven by both chemical repulsion from the positive interior and their high internal concentration, K+ ions rush out of the axoplasm into the ECF. This efflux of positive charge restores the internal negativity of the membrane.
5. Hyperpolarization (After-Hyperpolarization, ~ -80 mV): Because voltage-gated K+ channels are slow to close, K+ efflux continues briefly beyond the resting level, driving the membrane potential down to approximately -80 to -85 mV. Once the voltage-gated K+ gates close completely, non-gated leak channels and the active Na+/K+ pump restore the resting potential to -70 mV.

The All-or-None Law

The action potential obeys the All-or-None Law: if a depolarizing stimulus fails to reach threshold (-55 mV), no action potential is generated (subthreshold response). If a stimulus reaches or exceeds threshold, a maximal action potential of constant amplitude and duration (+30 mV) is triggered. Stronger stimuli do not create larger action potentials; instead, stimulus intensity is coded by the frequency of action potentials fired per second.

Refractory Periods

• Absolute Refractory Period: The interval from threshold until repolarization is one-third complete. During this window, voltage-gated Na+ channels are either completely open or inactivated (inactivation gates shut). A second action potential cannot be generated under any circumstances, regardless of stimulus strength. This guarantees that each impulse is a discrete event and enforces strictly unidirectional (forward) propagation.
• Relative Refractory Period: Follows the absolute refractory period, corresponding to the late repolarization and hyperpolarization phases. Voltage-gated Na+ channels have reverted to their resting closed state (activation gates shut, inactivation gates open), but voltage-gated K+ channels remain open. A second action potential can be fired, but only if an exceptionally strong, suprathreshold stimulus arrives to overcome the ongoing hyperpolarization.

Saltatory Conduction vs. Continuous Conduction

• Continuous Conduction: Occurs in unmyelinated axons. Voltage-gated Na+ and K+ channels are distributed along the entire length of the axolemma. Every contiguous millimeter of membrane must depolarize and repolarize, yielding slow conduction velocities (0.5 to 2.0 m/s).
• Saltatory Conduction: Occurs in myelinated axons. The lipid-rich myelin sheath acts as an electrical insulator, preventing ion flow across the internodal membrane. Depolarization and ion exchange occur exclusively at the unmyelinated nodes of Ranvier, where voltage-gated channels are concentrated. The local electrical current travels through the axoplasm from one node to the next, causing the action potential to effectively "leap" (Latin saltare, to leap) from node to node. Saltatory conduction increases transmission speeds up to 100 to 120 m/s while consuming significantly less metabolic energy (ATP) because far fewer ions cross the membrane.

---

7. Synaptic Transmission & Neurotransmitters

A synapse is the functional junction between a neuron and a target cell (another neuron, muscle fiber, or gland). Synapses can be electrical (direct ionic flow through gap junctions, such as in cardiac intercalated discs) or chemical (neurotransmitter release across a cleft).

Mechanism of Chemical Synaptic Transmission

1. Action Potential Arrival: The action potential depolarizes the presynaptic axon terminal.
2. Calcium Influx: Depolarization opens voltage-gated Ca2+ channels in the terminal axolemma; Ca2+ rushes into the terminal from the high extracellular pool.
3. Exocytosis of Neurotransmitter: Intracellular Ca2+ binds to regulatory synaptotagmin proteins, triggering the SNARE complex to dock and fuse synaptic vesicles with the presynaptic membrane, releasing neurotransmitter molecules into the synaptic cleft (a 20 to 30 nm fluid-filled gap).
4. Receptor Binding: Neurotransmitters diffuse across the cleft and bind to specific ligand-gated receptor proteins on the postsynaptic membrane.
5. Postsynaptic Potentials Generated:
   • Excitatory Postsynaptic Potential (EPSP): Neurotransmitter binding opens ligand-gated Na+ or Ca2+ channels, causing local depolarization that brings the postsynaptic membrane closer to threshold.
   • Inhibitory Postsynaptic Potential (IPSP): Neurotransmitter binding opens ligand-gated Cl- channels (Cl- influx) or K+ channels (K+ efflux), causing local hyperpolarization that drives the membrane further away from threshold.
6. Summation: A single EPSP is rarely sufficient to trigger an action potential. The postsynaptic neuron sums all incoming inputs:
   • Temporal Summation: Rapidly successive firing from a single presynaptic terminal adds together over time.
   • Spatial Summation: Simultaneous firing from multiple different presynaptic terminals at various locations on the soma and dendrites adds together.
7. Signal Termination: Neurotransmitter action is rapidly terminated via three mechanisms:
   • Enzymatic Degradation: Enzymes in the synaptic cleft break down the transmitter (e.g., acetylcholinesterase [AChE] hydrolyzes acetylcholine into choline and acetate).
   • Presynaptic Reuptake: Active transporter proteins pump the intact transmitter back into the presynaptic terminal or into adjacent astrocytes (e.g., serotonin, dopamine, norepinephrine).
   • Diffusion: Transmitter molecules drift away from the cleft into extracellular fluid.

Key Neurotransmitters in Human Physiology

---

8. Clinical Traps & Therapy Applications

Clinical Trap: Do not confuse the myelin-producing cells of the CNS with those of the PNS. Oligodendrocytes myelinate up to 50 axons simultaneously in the CNS and lack a neurolemma, which prevents CNS axonal regeneration. Schwann cells myelinate only a single internodal segment in the PNS and form an outer neurolemma, which is vital for guiding peripheral axonal regeneration after injury.

Demyelinating Pathologies

• Multiple Sclerosis (MS): An autoimmune disease in which cytotoxic T-cells and autoantibodies selectively attack and destroy oligodendrocytes and the myelin sheath in the CNS. Demyelination exposes axolemmal channels, causing ion leakage, conduction slowing, or complete conduction block. Symptoms include muscle weakness, spasticity, visual loss (optic neuritis), paresthesias, and ataxia.
• Guillain-Barré Syndrome: An acute post-infectious autoimmune demyelination targeting Schwann cells in the peripheral nervous system, causing ascending flaccid motor paralysis and areflexia.

Application to Manual and Body Therapies

• Gate Control Theory & Endorphin Release: Rhythmic touch, effleurage, and moderate pressure activate large-diameter, myelinated cutaneous A-beta mechanoreceptors. These fibers synapse on inhibitory interneurons in the dorsal horn of the spinal cord that release GABA and enkephalins, closing the "gate" to slow, unmyelinated C pain fibers and blunting nociceptive signaling to the brain.
• Autonomic Tone Modulation: Gentle, continuous touch downregulates central sympathetic outflow while elevating parasympathetic cholinergic activity, reducing systemic cortisol levels and muscle tension.