Question

2. Neuron during an action potential: a. What triggers the first action potential (which ion)? b. What is the typical threshold potential of a neuron? c. The calculated equilibrium potential of Na+ (EN) is approx. +60mV (calculated). Explain how this is related to the rising phase of an action potential (depolarization). d. The calculated equilibrium potential of K+ (EK) is approx. -90mV. Explain how this is related to the falling phase (repolarization). e. Direction of Nation movement (influx/efflux). f. Direction of K+ ion movement (influx/efflux). g. The role of voltage-gated Nat and K+ channels, and when they are open/inactive/closed. h. The role of Na+/K+ pumps in reestablishing and maintaining the electrochemical gradients after an action potential.

Answer 1

a) sodium ion triggers the first action potential

b) A threshold potential is the critical level to which a membrane potential must be depolarised to initiate an action potential. Most often , the thresothr potential is a membrane potential value between -50 to -55mV

C) Relatively small initial changes in membrane potential (from -70 mV to - 50 mV ) leads to the rapid but transient opening of Na⁺ channels.This allow Na⁺ to flow into the cell, driven by both it's concentration gradient and the membrane potential. The sudden entry of Na⁺ leads to change in membrane potential , which increases to nearly +40 mV, approaching the Na⁺ equilibrium potential of approx. +60 mV . At this time , the Na⁺ channels are inactivated .

D) As the sodium channels are inactivated and voltage gated K⁺ channels open , substantially increasing the permeability of the membrane to K⁺ . K⁺ then flow rapidly out of the cell , driven by both the membrane potential and the K⁺ concentration gradient, leading to a rapid decrease in membrane potential to about -95 mV (the K⁺ equilibrium potential). This change in membrane potential inactivates the voltage - gated K⁺ ​​​​​​channels and the membrane potential returns to its resting potential of -70 mV.