The Nernst equation describes the voltage across a cell membrane at which there would be no net flow of a particular ion (assuming there are ion channels that passed that ion). This voltage is called any of the following names “reversal potential,” “Nernst potential,” or “equilibrium potential” of an ion.<\/p>\n
<\/p>\n
The Nernst potential (Eion) is the voltage needed to counterbalance the diffusion of a charged ion down its concentration gradient. The potential is therefore a function of the concentration difference ([ion outside] vs. [ion inside]) across the membrane as well as the charge on the ion (z):<\/p>\n
Eion = RT\/zF (ln([ion outside]\/[ion inside]))<\/em><\/p>\n R is the gas constant (8.314472 J K-1 mol-1) T is temperature in Kelvin (body temperature is 273 + 37 = 310K) z is the charge on the ion (eg., +1 for Na+, +2 for Ca2+, -1 for Cl-) F is Faraday’s constant (9.65\u00d710^4 C mol^-1) [ion outside] = concentration of the ion in the extracellular space [ion inside] = intracellular concentration of the ion<\/p>\n For simplicity’s sake, students often assume “standard conditions” (temperature of 27 deg C) and write the equation with log base 10, rather than a natural log (ln). This makes RT\/F close to 60mV, and the equation can be written:<\/p>\n Eion = (60\/z) ( log([ion out]) – log([ion in]) )<\/p>\n The dominant ions in neurons are Na+, K+, Cl-, Ca2+. It is important to know the equilibrium potentials for each of these ions, and the concentrations inside and outside of the cell.<\/p>\n Prof. Francesco Bezanilla of the University of Chicago has an excellent primer on neurophysiology, which includes an animated demonstration of the factors that determine the Nernst potential.<\/p>\n Medical computing has calculators and explanations for several neurophysiological concepts like the Action potential and Nernst potential.<\/p>\n","protected":false},"excerpt":{"rendered":" The Nernst equation describes the voltage across a cell membrane at which there would be no net flow of a particular ion (assuming there are ion channels that passed that ion). This voltage is called any of the following names “reversal potential,” “Nernst potential,” or “equilibrium potential” of an ion.<\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"om_disable_all_campaigns":false,"_monsterinsights_skip_tracking":false,"_monsterinsights_sitenote_active":false,"_monsterinsights_sitenote_note":"","_monsterinsights_sitenote_category":0,"footnotes":""},"categories":[21],"tags":[],"aioseo_notices":[],"yoast_head":"\nSimplifying Assumptions for Calculating the Nernst Potential<\/h3>\n
Typical Equilibrium Potentials for Common Ions<\/h3>\n
\n\n
\n Ion<\/td>\n [intracellular] (mM)<\/td>\n [extracellular] (mM)<\/td>\n Equilibrium potential (mV)<\/td>\n<\/tr>\n \n Na+<\/td>\n 12<\/td>\n 145<\/td>\n +64 mV<\/td>\n<\/tr>\n \n K+<\/td>\n 140<\/td>\n 5<\/td>\n -86 mV<\/td>\n<\/tr>\n \n Ca2+<\/td>\n 0.0001 mM<\/td>\n 2 mM<\/td>\n Equilibrium potential not defined<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n Reference Materials<\/h3>\n