  Recent studies have revealed the complex structure of nerve signals in axons.
Besides the electrical signal, mechanical and thermal effects are also detected
in many experimental studies. In this paper, the mathematical models of heat
generation are analysed within the framework of a general model derived earlier
by the authors. The main mechanisms of the heat generation are seemingly the
Joule heating and endo- and exothermic reactions. The concept of internal
variables permits to model the heat relaxation typical to these reactions. The
general energy balance of the whole signal is analysed based on physical
mechanisms responsible for emerging the components of a signal. Some open
questions are listed for further studies.
