The College of New Jersey
April 30, 2013
By this time in our scientific understanding, the process of a nerve signal transmission is a well understood topic in the subject of biology. The most well-known facet of the process, known as the Hodgkin-Huxley Model, was proposed by Lloyd Hodgkin and Andrew Huxley in 1952. This model is electrically based. Hodgkin and Huxley suggested that a differential concentration of ions, namely Sodium, Potassium, and Chlorine, both inside and outside the cell membrane creates a resting membrane potential of about -60 mV. Through a series of voltage-gated trans-membrane proteins opening and closing in response to corresponding voltage fluctuations across the membrane, what is known as an action potential is propagated down the axon of a nerve cell. These voltage-gated proteins are ion specific, allowing only the select ion of their type through, changing the electrical potential across the cellular membrane. In this manner, the membrane potential rises sharply followed by a falling below the -60 mV resting potential before reestablishing the resting potential. Subsequently, this action potential triggers another action potential and so on as the signal travels down the axon (Hodgkin et al, 1952). As an effect of what Hodgkin and Huxley have shown, signal propagation in a nerve cell is also a mechanical and thermally, or temperature, associated process. As a nerve signal propagates, and an action potential fires, the cell membrane of an axon also undergoes a mechanical deformation due to exposure to the electric field of an action potential as a consequence of the piezoelectric effect (Tasaki, 1999). In addition, during the course of the action potential, it has been shown that an adiabatic, that is a thermally conserved and reversible, release and reabsorption of heat occurs in which the membrane undergoes a phase change. Note here that phase change does not refer to a change in the classical definition of phase (solid, liquid, or gas) but simply that the state of the membrane is altered. Changes in phase, as well as altering of any number of intensive thermodynamic variables (pressure, temperature, etc.,) affects the permeability of a cell membrane and thus how the electrical signal is transferred down an axon. However, it should be noted that there are some conflicts in studying these topics in thermodynamics as related to a biological standpoint (Heimburg, 2009). The following will attempt to explain the mechanical and thermodynamic aspects of a nerve signal in conjugation with the electrical. First, however, the problems of reconciling thermal physics with biology will be tackled.
Problems using Thermodynamic Principles on a Biological System
For the purposes of this research, thermodynamics is used, in part, to elaborate on the multifaceted character of a nerve signal propagating down an axon. Namely, the membrane can be alternatively permeable depending upon environmental changes, or better put as changes in intensive thermodynamic variables such as temperature, pressure, and volume. However historically, thermodynamics deals only in macroscopic views, looking at ensembles. That is, the whole is viewed rather than the individual sum of its parts. In biology, however, when studying nerve signals one often cites specific ions or proteins. This cannot fit into a thermodynamic model, which will not work on a microscale. This can, however, be reconciled when, instead of viewing the movement of proteins or ions, the macroscopic views such as wave propagation through the membrane or a phase change of the membrane are made (Heimburg, 2009). Another important aspect when attempting to apply thermodynamics to signal transmission is that experimental results show that heat is conserved (Ritchie et al, 1985). When the heat is integrated over the course of an action