Temperature and excitable cells: Testable predictions from a thermodynamic perspective

Abstract

Temperature affects a host of biological processes, one of which is the conduction velocity of action potentials (AP). The velocity-temperature profile of APs has remained remarkably conserved across excitable animal and plant cells. Herein, we will not analyze this behavior in terms of temperature sensitivities of single molecules (e.g., ion channels), but rather we present a phenomenological thermodynamic interpretation. By assuming that APs are acoustic phenomena, one arrives at testable predictions about the temperature-dependence of the macroscopic material properties of the excitable cell membrane. These material properties set constraints on the excitability of a cell membrane and allow us to hypothesize about its typical relaxation timescales. The presented approach-by virtue of its thermodynamic nature-is by no means limited to temperature. It applies equally well to all thermodynamic variables (e.g., mechanical stretch, pH, ion concentrations, etc.) and to underline this argument we discuss some implications and predictions for sensory physiology.

Keywords: acoustic pulse; action potential; conduction velocity; propagation; relaxation; sensory physiology; temperature; thermodynamics; thermosensing.

Figure 1. Temperature dependence of action potential propagation velocity in mollusc, mammal and amphibian. Velocity-temperature curve for squid giant axon (open squares; data taken from Figure 1 in and normalized to c ≈ 10.5 ms^–1 at 10 °C), sciatic nerve of cat and frog (stars and open triangles respectively; data taken from Figure 1 in and normalized to c ≈20 ms^–1 at 20 °C in the case of cats and c ≈22 ms^–1 at 15 °C in the case of frogs). Qualitatively identical results have been observed for excitable plant cells.^{16,17, unpublished data}

Figure 2. Speed of sound in water and bubble-free ice. A minimum of the sound velocity is observed at the freezing-melting point of water. Data was taken from Figure 3 and Figure 4 in and was normalized to sound velocity at ~17 °C (≈1472 ms^–1).

Figure 3. Temperature dependence of blood vessel pulsations and firing rate of thermosensory warm fibers. Heart rate of blackworms (shaded area; normalized to 4.4 ± 0.8 beats min^–1 at ~9 °C; see for details) compared with static discharge of warm fibers in the nasal region of cats (closed symbols; data taken from Figure 12 in ; normalized to 3s⁻¹ at ~36 °C).