Explaining the action potential with channels, ions, and voltages Explaining the action potential with channels, ions, and voltages. In (A), the voltage that would be recorded between the inside and outside of a single neuron during an action potential is illustrated in black. The green curve indicates how easily Na+ can flow through the cell membrane (i.e., Na+ conductance) during the action potential, and the blue curve shows K+ conductance. Threshold is the voltage level required for an action potential. Once the membrane potential exceeds the threshold, and action potential will definitely occur. RMP stands for resting membrane potential. EK is the voltage the membrane would reach if potassium channels were open and stayed open (called the reversal potential), and ENA is the reversal potential for sodium. ARP stands for absolute refractory period, the time when a new action potential cannot begin. RRP stands for relative refractory period, the time when it is relatively difficult to make a new action potential. In (B), the three possible states for voltage-gated sodium channels are illustrated. Sodium can flow only during the open, active state. (Adapted with permission from Kandel et al. Principles of Neural Science, New York, NY: McGraw-Hill; 2013. Figure 7-13 on page 163.) Source: Neuronal Structure and Function, Neurologic Rehabilitation: Neuroscience and Neuroplasticity in Physical Therapy Practice Citation: Nichols-Larsen DS, Kegelmeyer DA, Buford JA, Kloos AD, Heathcock JC, Basso D. Neurologic Rehabilitation: Neuroscience and Neuroplasticity in Physical Therapy Practice; 2016 Available at: http://accessphysiotherapy.mhmedical.com/DownloadImage.aspx?image=/data/books/1760/larsen_ch02_fig-02-02.png&sec=120047424&BookID=1760&ChapterSecID=120047406&imagename= Accessed: March 05, 2018 Copyright © 2018 McGraw-Hill Education. All rights reserved