How Neurons Communicate Electrically and Chemically

1. The 86-Billion-Node Electrical Computer

Inside your cranium lies an organ that consumes barely twenty watts of electrical power—roughly the energy required to illuminate a faint refrigerator lightbulb. Yet within this 1.4-kilogram mass of lipid and protein, 86 billion neurons are interconnected by more than 100 trillion synaptic junctions.

A single cortical neuron receives inputs from up to 10,000 other neurons simultaneously, computing and firing up to several hundred times per second.

How does biological tissue—composed of salty water, lipids, and delicate proteins—transmit information with the speed and precision necessary to track a flying baseball, recall a childhood scent, or compose a symphony?

                  THE HYBRID SIGNALING ARCHITECTURE
  
       INTRANEURONAL (Within Cell)              INTERNEURONAL (Between Cells)
  
   ┌──────────────────────────────────┐    ┌──────────────────────────────────┐
   │ ELECTRICAL: Action Potential     │    │ CHEMICAL: Neurotransmitters      │
   │ Fast (up to 120 m/s)             │    │ Microscopic diffusion (20 nm)    │
   │ Binary (All-or-Nothing pulse)    │    │ Analog & Modulatory (Graded)     │
   │ Non-decaying over 1+ meters      │    │ Excitatory or Inhibitory logic   │
   └──────────────────────────────────┘    └──────────────────────────────────┘

The nervous system achieves this through a hybrid electrical-chemical architecture:

• Electrical Transmission: Within the boundary of a single cell, information is propagated as a rapid, regenerative electrical wave—the action potential—traveling down a microscopic cable (the axon) over distances up to a meter.
• Chemical Transmission: When the electrical signal reaches the end of the wire (the synapse), it cannot cross the physical air or water gap. Instead, it triggers the release of chemical packets (neurotransmitters) that diffuse across a twenty-nanometer cleft to activate or inhibit the next cell in the circuit.

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2. The Battery of Life: The -70 mV Resting Potential

Before a neuron can fire an electrical pulse, it must first build up an electrical charge. A neuron is literally a microscopic biological battery.

Every living neuron maintains a steady voltage difference across its cell membrane: the interior of the cell is negatively charged relative to the fluid outside. In a resting human neuron, this potential difference is approximately -70 millivolts (-0.07 volts).

How does a cell establish this electrical voltage across a membrane barely five nanometers thick?

                  THE CHEMICAL CONCENTRATION ASYMMETRY
  
   Ion Species        Extracellular Fluid (mM)    Intracellular Cytosol (mM)
  ─────────────────────────────────────────────────────────────────────────────
   Potassium ($K^+$)   4 – 5 mM                   140 – 150 mM  (30x higher inside!)
   Sodium ($Na^+$)     140 – 145 mM               10 – 15 mM    (10x higher outside!)
   Chloride ($Cl^-$)   105 – 110 mM               4 – 10 mM     (10x higher outside!)
   Organic Anions ($A^-$) Very Low                 ~100 mM (Trapped proteins/ATP)

This voltage is created by two physical components working in concert:

1. The $Na^+/K^+$ ATPase Molecular Engine

Embedded in the cell membrane are millions of copies of an active transport protein: the Sodium-Potassium Pump ($Na^+/K^+$ ATPase).

Using the chemical energy stored in one molecule of ATP, the pump undergoes a cyclic shape change that:

• Captures three sodium ions ($3\ Na^+$) from inside the cell and ejects them into the extracellular fluid.
• Captures two potassium ions ($2\ K^+$) from outside and pumps them into the cytosol.

This pump is electrogenic: because it moves three positive charges out for every two positive charges brought in, it produces a net loss of positive charge inside the cell.

More importantly, it builds up massive chemical concentration gradients: crowding potassium inside the cell while packing sodium outside. This single molecular pump is so critical to nervous system survival that it consumes between thirty and forty percent of all metabolic ATP in the entire human brain.

2. Potassium Leak Channels and the Nernst Potential

The lipid bilayer is impermeable to ions. However, the resting membrane contains open, non-gated potassium leak channels.

Because potassium is 30 times more concentrated inside the cell, potassium ions begin to diffuse outward down their chemical concentration gradient through these leak channels. But as positive $K^+$ ions leave, they leave behind large, negatively charged intracellular proteins ($A^-$) that are too big to cross the membrane.

                  THE EQUILIBRIUM BALANCE FOR POTASSIUM
  
       Chemical Gradient: Wants to push K+ OUT of cell (High conc ──► Low conc)
                              ▲
                              │ Exactly balanced at -90 mV!
                              ▼
       Electrical Gradient: Wants to pull K+ IN to cell (Attracted to negative A-)

As more positive potassium leaves, the inside of the cell becomes increasingly negative. This growing interior negative charge exerts an electrical pull that attracts positive potassium ions back inside.

Eventually, a point of dynamic equilibrium is reached where the outward chemical push is exactly balanced by the inward electrical pull. This equilibrium potential is described by the Nernst Equation: $$E_{\text{ion}} = \frac{RT}{zF} \ln \left( \frac{[\text{ion}]{\text{outside}}}{[\text{ion}]{\text{inside}}} \right)$$

For potassium at human body temperature ($37^\circ\text{C}$), the Nernst equilibrium potential is approximately -90 millivolts.

Because the resting membrane has a tiny residual leak for sodium (which pulls voltage slightly upward toward $+60\text{ mV}$), the overall resting membrane potential settles at -70 millivolts.

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3. The Action Potential: The All-or-Nothing Spark

When a neuron is stimulated by sensory inputs or upstream synaptic connections, positive charges flow into the dendrites, making the interior slightly less negative—a process called depolarization.

If depolarization remains faint (shifting from -70 mV to -65 mV), nothing happens; the leak channels simply restore the baseline. But if the depolarization reaches a critical threshold of -55 millivolts, the neuron fires an action potential.

                    THE ANATOMY OF AN ACTION POTENTIAL
  
     Membrane
     Potential (mV)
          ▲
     +40 ─┼                    Peak (+30 to +40 mV)
          │                   / \
      0 ─┼                  /   \  Repolarization (K+ exits)
          │   Depolarization/     \
    -55 ─┼─── Threshold ───/       \
          │               /         \
    -70 ─┼── Resting ───/           ╰──── Undershoot / Hyperpolarization (-80 mV)
          └────────────────────────────────────────────────────────► Time (ms)
               0             1             2             3             4

Between 1939 and 1952, British biophysicists Alan Hodgkin and Andrew Huxley dissected the giant axon of the North Atlantic squid (Loligo pealeii)—a nerve fiber nearly one millimeter in diameter, large enough to insert internal glass capillary electrodes. Their work established the exact mathematical biophysics of the three action potential phases:

Phase 1: Rapid Depolarization (0 to 1 ms)

At -55 mV, voltage-sensitive sensors inside voltage-gated sodium channels ($Nav$) twist, opening a central aqueous pore.

• Sodium ions ($Na^+$) rush into the cell, driven simultaneously by two irresistible forces: their chemical concentration gradient (ten-fold higher outside) and the electrical attraction of the negative interior.
• This influx of positive charge depolarizes the membrane further, which opens even more neighboring $Nav$ channels—a runaway positive-feedback loop known as the Hodgkin cycle.
• The membrane potential reverses polarity entirely, rocketing from -55 mV to +30 or +40 millivolts in less than half a millisecond.

Phase 2: Inactivation and Repolarization (1 to 2 ms)

Why does the action potential not stay at +40 mV forever? Two safety mechanisms terminate the spike:

1. The Inactivation Gate ("Ball-and-Chain"): At peak positive voltage, a tethered intracellular peptide loop on the sodium channel swings into the inner mouth of the pore, physically plugging it. The channel enters an inactivated state: sodium influx ceases instantly.
2. Voltage-Gated Potassium Channels ($Kv$): Sluggish to respond, voltage-gated potassium channels fully open just as sodium channels inactivate. Positive potassium ions, now repelled by the interior positive charge and pushed by their concentration gradient, rush outward into the extracellular space, rapidly stripping positive charge from the cell and driving membrane voltage back down toward negative territory.

Phase 3: Hyperpolarization and Refractory Periods (2 to 4 ms)

Because $Kv$ channels close slowly, potassium efflux briefly overshoots the resting potential, driving the membrane down to -80 millivolts (hyperpolarization) before resetting to -70 mV.

This cycle enforces two mandatory functional constraints:

• Absolute Refractory Period: While sodium channels are inactivated by their plug, no stimulus, no matter how intense, can fire another action potential. This guarantees that every nerve spike is an isolated, discrete digital event.
• Unidirectional Propagation: Because the patch of membrane immediately behind an advancing action potential is locked in its refractory state, the electrical wave can only travel forward down the axon toward the terminals, never backward.

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4. The Myelin Revolution: Saltatory Conduction

In an unmyelinated axon, an action potential moves via continuous conduction: the local sodium influx depolarizes the adjacent micrometer of membrane, which opens its sodium channels, which depolarizes the next micrometer, like a burning fuse.

This process is slow: unmyelinated fibers conduct at velocities between 0.5 and 2.0 meters per second (one to four miles per hour). If you were relying on unmyelinated nerves to pull your foot away from a hot stove, the sensory signal would take nearly a full second just to reach your spinal cord.

Vertebrates solved this speed barrier through an ingenious evolutionary innovation: the myelin sheath.

                       SALTATORY CONDUCTION ALONG AN AXON
  
    Schwann Cell (Myelin)       Node of Ranvier       Schwann Cell (Myelin)
  ┌───────────────────────┐         ┌───┐         ┌───────────────────────┐
  │ Multilayered Fatty    │         │Nav│         │ Multilayered Fatty    │
  │ Lipid Insulation      │         │   │         │ Lipid Insulation      │
  │ (Rm High, Cm Low)     │         │Nav│         │ (Rm High, Cm Low)     │
  └───────────────────────┘         └───┘         └───────────────────────┘
  ════════════════════════════════════╪════════════════════════════════════
             Axon Cytosol             │ Action Potential regenerated!
  ════════════════════════════════════╪════════════════════════════════════
              ▲                       │                       ▲
              │                       ▼                       │
              └────────────── Fast Electrotonic Flow ─────────┘
                               (Near speed of light)

The Biophysics of Myelin

Specialized glial cells—Schwann cells in the peripheral nervous system and oligodendrocytes in the central nervous system—wrap their membranes tightly around the axon dozens of times, creating a thick, white, fatty insulating jacket.

In electrical circuit terms, myelin does two critical things:

1. Increases Transmembrane Resistance ($R_m$): It seals the membrane so ionic charges cannot leak out across the lipid bilayer.
2. Decreases Membrane Capacitance ($C_m$): By physically separating the intracellular fluid from the extracellular fluid by hundreds of nanometers of lipid, it reduces the electrostatic attraction between opposite charges across the membrane, meaning fewer ions are needed to change the voltage.

Nodes of Ranvier and the Saltatory Leap

Myelin does not cover the axon continuously. Every one to two millimeters, the sheath is interrupted by a microscopic uninsulated gap roughly one micrometer wide: a Node of Ranvier.

These nodes are packed with an astonishing density of voltage-gated sodium channels—up to 10,000 channels per square micrometer, compared to barely 100 in unmyelinated axons.

The result is saltatory conduction (from the Latin saltare, "to leap"):

1. When an action potential fires at Node 1, a massive flood of sodium enters.
2. Because the myelinated internode is tightly insulated, the positive electrical charge does not leak out. Instead, it flashes passively through the intracellular fluid at near the speed of light.
3. When this electrical pulse hits Node 2, it still carries sufficient voltage to immediately trip the high-density sodium channels, regenerating the full +30 mV action potential instantly.

Saltatory conduction accelerates nerve transmission speeds from 2 m/s to over 120 meters per second (over 270 miles per hour), while reducing metabolic energy consumption by ninety-nine percent because sodium-potassium pumps only need to clear ions at the tiny nodes rather than across the entire axon length.

In demyelinating diseases such as Multiple Sclerosis (MS), the body’s immune system attacks and strips myelin away. Without insulation, the electrical current leaks out through the exposed axon membrane before reaching the next Node of Ranvier. The signal decays below threshold, causing conduction block, muscular weakness, blindness, and paralysis.

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5. The Chemical Junction: Synaptic Vesicle Exocytosis

When the electrical wave reaches the swollen terminal tip of the axon (the presynaptic bouton), it hits a physical dead end: the synaptic cleft, an extracellular fluid gap twenty nanometers wide separating the neuron from its target cell.

To cross this chasm, the electrical pulse must be translated into a chemical messenger.

The pipeline diagram below illustrates the six sequential steps of neural signaling, from resting charge to postsynaptic chemical activation:

The Molecular Machine of Exocytosis: SNARE Proteins

Resting inside the presynaptic terminal are hundreds of tiny spherical lipid bubbles (synaptic vesicles), each loaded with roughly 5,000 molecules of neurotransmitter (such as acetylcholine, glutamate, or GABA).

The fusion of these vesicles with the plasma membrane is governed by a remarkable molecular machine called the SNARE complex, whose structural biology was elucidated by Nobel laureate Thomas Südhof:

• Synaptobrevin (VAMP): A protein anchored in the membrane of the synaptic vesicle (v-SNARE).
• Syntaxin-1 and SNAP-25: Target proteins anchored in the presynaptic plasma membrane (t-SNAREs).

                  THE SNARE MEMBRANE FUSION COMPLEX
  
         Synaptic Vesicle Membrane
        ┌─────────────────────────┐
        │       (v-SNARE)         │
        │      Synaptobrevin      │
        └────────────┬────────────┘
                     │
         ════════════╧════════════  ◄── 4-Helix SNARE Bundle "Zippers" Together!
                     │                  Pulls vesicle into plasma membrane.
        ┌────────────┴────────────┐
        │   Syntaxin + SNAP-25    │
        │       (t-SNAREs)        │
        └─────────────────────────┘
         Presynaptic Plasma Membrane
  
         [Ca2+ Influx] ──► Binds to Synaptotagmin-1 ──► Snaps Fusion Pore Open!

1. Vesicle Docking & Priming: The alpha-helical coils of synaptobrevin, syntaxin, and SNAP-25 interlock, winding together like strands of a rope into a tight four-helix bundle. This mechanical winding exerts immense physical force, pulling the vesicle membrane to within a single nanometer of the cell membrane.
2. The Calcium Trigger: The primed vesicle sits cocked, blocked from fusing by an inhibitory protein called complexin. When the action potential invades the terminal, voltage-gated P/Q-type and N-type calcium channels open. Extracellular $Ca^{2+}$ floods into the microdomain around the vesicle.
3. Synaptotagmin-1 Action: Calcium ions bind to synaptotagmin-1, the calcium-sensing protein on the vesicle. Synaptotagmin undergoes an immediate conformational shift that kicks off complexin and plunges its hydrophobic loops into the plasma membrane.
4. Instantaneous Fusion: The membranes merge, opening a fusion pore in less than 200 microseconds (0.2 milliseconds). The neurotransmitter molecules blast out into the synaptic cleft.
5. Diffusion Across the Cleft: Because the synaptic cleft is only 20 nanometers wide, diffusion is virtually instantaneous: according to Fick's laws of diffusion, a small molecule like glutamate crosses the cleft in ten to twenty microseconds.

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6. Postsynaptic Integration: EPSPs, IPSPs, and Computation

Once neurotransmitters cross the cleft, they bind to specialized receptor proteins embedded in the postsynaptic membrane of the receiving neuron.

These receptors operate through two distinct mechanisms:

                  POSTSYNAPTIC RECEPTOR ARCHITECTURES
  
   Feature              Ionotropic Receptors           Metabotropic (GPCR) Receptors
  ──────────────────────────────────────────────────────────────────────────────────
   Structure            Ligand-gated ion channel       7-transmembrane G-protein receptor
   Speed                Ultra-fast (0.5 – 5 ms)        Slow (100 ms to minutes)
   Operating Mechanism  Direct channel pore opens     Activates second messenger cascades
   Primary Examples     AMPA, NMDA, $GABA_A$, Nicotinic $GABA_B$, Dopamine, Serotonin (5-HT)
   Computational Role   Fast binary excitation/inhibit Modulatory; plastic learning

Excitatory vs. Inhibitory Signals

Receptor binding alters the electrical potential of the postsynaptic cell:

• Excitatory Postsynaptic Potential (EPSP): When glutamate (the brain's primary excitatory neurotransmitter) binds to postsynaptic AMPA receptors, a central pore opens that allows sodium ($Na^+$) to rush in. This depolarizes the receiving cell, nudging its voltage closer to the -55 mV firing threshold.
• Inhibitory Postsynaptic Potential (IPSP): When GABA (gamma-aminobutyric acid, the primary inhibitory neurotransmitter) binds to $GABA_A$ receptors, it opens a channel selective for chloride ($Cl^-$). Because chloride is negatively charged and concentrated outside, it rushes into the cell, driving membrane voltage down to -75 mV (hyperpolarization). This holds the neuron below threshold, preventing it from firing.

Summation at the Axon Hillock: The Neural Logic Gate

A single EPSP changes voltage by barely 0.5 to 1.0 millivolt—nowhere near enough to cross the fifteen-millivolt gap from -70 mV to the -55 mV threshold.

To make a decision, the receiving neuron performs continuous spatial and temporal summation:

                  DENDRITIC INTEGRATION AT THE AXON HILLOCK
  
       Dendrite 1: EPSP (+1.0 mV) ───┐
       Dendrite 2: EPSP (+1.5 mV) ───┼──► [Cell Body] ──► [AXON HILLOCK]
       Dendrite 3: IPSP (-2.0 mV) ───┤                    Does net voltage
       Dendrite 4: EPSP (+0.8 mV) ───┘                    cross -55 mV?
                                                               │
                                         ┌─────────────────────┴─────────────────────┐
                                         ▼ YES                                       ▼ NO
                                [FIRE ACTION POTENTIAL!]                     [SILENCE / NO FIRING]

• Temporal Summation: If a single presynaptic axon fires repeatedly in rapid succession (100 Hz), the second and third EPSPs arrive before the first has decayed, stacking on top of one another to push voltage over threshold.
• Spatial Summation: If five hundred excitatory synapses across the dendritic arbor fire at the exact same millisecond, their electrical currents wash through the cell body toward the axon hillock (the axon initial segment).

The axon hillock is the neuron's central decision processor. It possesses the lowest threshold and highest concentration of voltage-gated sodium channels in the entire cell.

If the algebraic sum of all incoming excitatory and inhibitory currents reaches -55 millivolts at the axon hillock, a new action potential is ignited, racing down the axon to begin the cycle anew.

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7. Comparative Matrix: Electrical vs. Chemical Synapses

While the vast majority of synapses in the human central nervous system are chemical, human neural tissue also retains primitive, direct electrical connections:

Property	Electrical Synapse (Gap Junction)	Chemical Synapse (Vesicular)
Intercellular Distance	3.5 nanometers (Direct cytoplasmic bridge)	20 – 40 nanometers (Fluid synaptic cleft)
Physical Substrate	Hexameric connexin hemichannels	Synaptic vesicles, SNARE complex, receptors
Transmission Delay	Virtually instantaneous (< 0.05 ms)	0.3 – 1.0 millisecond (Synaptic delay)
Directionality	Bidirectional (Current flows either way)	Strictly Unidirectional (Presynaptic ──► Postsynaptic)
Modulation / Plasticity	Low; inflexible, hardwired synchronization	Extremely High; long-term potentiation, learning
Primary Anatomical Sites	Vestibular nucleus, cardiac muscle, retinal cells	Cerebral cortex, hippocampus, cerebellum, NMJ

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8. Summary: The Inscribed Operating System of the Mind

Every memory you cherish, every thought you generate, and every muscle contraction you execute is the product of this hybrid neurochemical cascade:

• Energetic Priming: The ATP-powered $Na^+/K^+$ pump and leak channels maintain a cocked electrical spring at -70 millivolts.
• Digital Precision: Voltage-gated sodium and potassium channels convert analog dendritic summation into an all-or-nothing, non-decaying electrical pulse.
• High-Speed Saltatory Jumping: Myelin sheaths and Nodes of Ranvier accelerate conduction to over one hundred meters per second, linking distant anatomical regions in milliseconds.
• Chemical Computation: SNARE proteins and neurotransmitter receptors turn binary electrical spikes into nuanced, analog synaptic dialogues that encode the entire cognitive architecture of human consciousness.

In our companion explainers across the Human Body & Physiology Series, we explore how this neural wiring orchestrates peripheral organ systems:

• How the Heart Pumps Blood traces how autonomic sympathetic and parasympathetic nerves modulate sinoatrial pacemaker frequencies.
• How the Lungs Exchange Oxygen and Carbon Dioxide examines how brainstem respiratory centers monitor blood $pH$ to drive diaphragm contraction.
• How the Kidneys Filter Blood and Maintain Fluid Balance details how neurohypophyseal hormones like vasopressin adjust renal water permeability.
• How Binary and Logic Gates Became Computation contrasts this wet biological neural computing with dry solid-state silicon transistors.