How does the nerve impulse travel? Mechanical, structural and thermal dimensions of nerve conduction

The propagating nerve impulse is universally recorded as an electrical event, and the Hodgkin-Huxley cable framework describes this event with remarkable quantitative fidelity. However, the same propagating impulse is accompanied by mechanical and thermal changes and travels through an axon that is never a bare membrane but a structured, ensheathed conduit. This review asks a deliberately physical question: how does the impulse travel along the fibre? We survey the correlates of conduction in one physical dimension at a time, namely electrical, mechanical, thermal, and structural, together with the constraint imposed by the spatial extent of the active region, and we set the principal proposed frameworks beside the classical one, where each is relevant to a given phenomenon. The aim is to organise the available data, to ask which physical substrate would integrate them, and to commit, in a calibrated form, to the reading that this survey undertakes to develop.