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Revision of the Squid Giant Axon Experiment: The Altered -60mV and the Forgotten Physical Meaning

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2026-04-23

Revision of the Squid Giant Axon Experiment:The Altered -60mV and the Forgotten Physical Meaning

Sun Zuodong

The action potential measured by Hodgkin and Huxley on the squid giant axon is a landmark discovery in the history of neuroscience. The measured waveform clearly shows a resting potential of -60mV and a peak potential of +40mV, which represent unshakable scientific facts. In the subsequent development of the theory, however, two long-overlooked key issues persist on this important foundation: one is the continuous revision of original experimental data, and the other is that the physical meaning of the positive and negative signs of the potential has never been truly clarified.

The HH model and GHK equation, established based on the ionic theory, have always been unable to perfectly reproduce the smooth waveform from the original measurement. To achieve better self-consistency between theory and calculation, some studies and textbooks have revised the -60mV resting potential to other values such as -70mV or -75mV. In essence, this makes experimental data accommodate the theoretical model instead of making the theory fit the experiment. This tendency did not appear today; it existed from the early stage of research and has affected the direction of subsequent scientific exploration to a certain extent.

More crucially, traditional theories have never clearly answered: what is the physical nature of the positive and negative signs in the action potential? The answer comes from the structural characteristics of ions and the initial state of the cell. At the beginning of cell formation, chloride ions are enclosed inside the cell due to their large diameter and cannot move effectively across the membrane, forming a stable and fixed negative charge background. In the simplest unit measurement, about 260 chloride ions are locked inside the cell, while 300 sodium ions and 200 potassium ions can be dynamically exchanged outside the cell.

The famous German mathematician Emmy Noether put forward the well-known Noether’s Theorem in 1918, stating that every continuous symmetry corresponds to a conserved quantity, and every conserved quantity necessarily corresponds to a symmetry. The cell membrane maintains continuous symmetry in its overall structure during dynamic ion exchange, so there must be a corresponding conserved quantity, which is the Law of Cell Membrane Area Conservation. The occupiable sites on the membrane surface are a fixed conserved quantity. Ions cannot enter or leave without restriction and can only exchange under the constraint of the total conserved amount, which is the basic physical premise of the entire action potential process.

At the resting potential of -60mV, the inner surface of the cell membrane is theoretically almost entirely covered by potassium ions in a stable resting state. Between -60mV and -40mV, sodium ions enter the cell uniformly while potassium ions remain stationary. In this phase, only uniform influx and efflux of sodium ions occur with equal amounts entering and leaving, strictly following membrane area conservation without additional ion exchange, so the waveform shows a sharp peak instead of a smooth parabola. If cross-exchange of sodium influx and potassium efflux occurred at this stage, the waveform would inevitably take the shape of a parabola, which is obviously inconsistent with the measured sharp peak. From -40mV to +40mV is the rising phase of the action potential: massive influx of sodium ions fully stretches the elastic excretory opening (sphincter-like structure), sodium ions accelerate inward rapidly, and potassium ions accelerate outward synchronously, achieving rapid alternating occupation under the constraint of membrane area conservation. At the peak of +40mV, the inner surface of the cell membrane is theoretically almost entirely occupied by sodium ions. From +40mV back to -60mV is the falling phase of the action potential: sodium ions accelerate outward, potassium ions accelerate inward again, the direction of ion movement reverses, and the potential drops rapidly under membrane area conservation, eventually returning to the resting state.

The essence of the positive and negative signs is the charge state formed by the fixed negative charge background of chloride ions and the dynamic alternating occupation of the inner membrane by sodium and potassium ions. This explanation is not only logically concise and physically clear but also provides a verifiable new idea for further experimental design and examination of ion movement rules.

Traditional theories only fit the potential difference of about 100mV mathematically, but they neither clarify the origin of the positive and negative signs nor ignore the fixed background effect of chloride ions, nor introduce the core constraint of cell membrane area conservation. They fail to fully describe the real movement rules of sodium and potassium ions in each phase of the action potential, especially the key detail that a sharp peak instead of a parabola is formed by the uniform influx and efflux of sodium ions and stationary potassium ions between -60mV and -40mV.

Scientific progress never lies in clinging to existing models, but in continuously approaching the truth through the tension between experimental facts and theoretical interpretation. Re-examining the altered -60mV is not only correcting a set of data, but also regaining the scientific spirit of taking experiments as the foundation and physics as the root. Making theories obey facts and confronting contradictions directly is the most enduring enlightenment left to future generations by the squid giant axon experiment.

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