Metabolic efficiency with fast spiking in the squid axon

TL;DR

This study investigates how ion overlap during squid axon action potentials affects energy efficiency, revealing that higher temperatures lead to more efficient sodium use and lower metabolic costs, with optimal sodium conductance identified.

Contribution

It introduces a novel approach to compute neuronal energy cost that accounts for ion overlap and temperature effects, enhancing understanding of metabolic efficiency in neurons.

Findings

Higher temperatures increase firing frequency and efficiency.

Increased sodium conductance can minimize energy cost.

Ion overlap impacts ATP consumption during action potentials.

Abstract

Fundamentally, action potentials in the squid axon are consequence of the entrance of sodium ions during the depolarization of the rising phase of the spike mediated by the outflow of potassium ions during the hyperpolarization of the falling phase. Perfect metabolic efficiency with a minimum charge needed for the change in voltage during the action potential would confine sodium entry to the rising phase and potassium efflux to the falling phase. However, because sodium channels remain open to a significant extent during the falling phase, a certain overlap of inward and outward currents is observed. In this work we investigate the impact of ion overlap on the number of the adenosine triphosphate (ATP) molecules and energy cost required per action potential as a function of the temperature in a Hodgkin-Huxley model. Based on a recent approach to computing the energy cost of neuronal AP generation not based on ion counting, we show that increased firing frequencies induced by higher temperatures imply more efficient use of sodium entry, and then a decrease in the metabolic energy cost required to restore the concentration gradients after an action potential. Also, we determine values of sodium conductance at which the hydrolysis efficiency presents a clear minimum.