Beyond Hodgkin-Huxley—The Ionic-Mechano-Hydraulic (IMH) Model of Nerve Conduction

The axonal membrane is not the seat of nerve conduction: it is the boundary between two osmotic reservoirs whose asymmetry is the thermodynamic engine of the action potential. Voltage-gated ion channels are not the generators of the nerve signal -- they are its osmotic amplifiers, and their spatial distribution along the axon is a geometric necessity, not an arbitrary anatomical feature. The Ionic-Mechano-Hydraulic (IMH) model formalises this principle: intracellular K$^{+}$ adsorbed on the cytoplasmic polyelectrolyte gel triggers an ionic phase transition; extracellular Na$^{+}$ amplifies the resulting hydraulic wave via Nav channels; Kv channels close the osmotic cycle and enforce the refractory period. Conduction velocity is predicted from myelin elastic modulus, not sodium channel density. The model resolves a 75-year-old anomaly that Huxley and St\"{a}mpfli themselves described as impossible in a purely electrical system: positive current enters a node before the membrane potential at the preceding node has reached its maximum. Nine falsifiable predictions are presented -- among them, a graded reduction in conduction distance under partial tetrodotoxin block, a bell-shaped relationship between node length and conduction velocity, and an upper diameter limit for unmyelinated fibres derived from first physical principles. The Hodgkin-Huxley model is not discarded: it is explained.