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MCAT

Unit 3: Lesson 1

Chemical and physical sciences practice passage questions

Neuronal membranes: Nature's capacitors

Problem

Neurons are specialized to facilitate communication from one part of the body to another, and these signals of communication are achieved through electrical potentials. The cell membrane of a neuron creates an ionic potential by separating various ions across its cell membrane. Any construct that can separate an electric charge can be considered a capacitor, even the lipid bilayer membrane of a neuron! Capacitance (C, measured in Farads) is defined formulaically as the amount of stored charge divided by voltage, as seen in the formula C= Q/V.
Various ion pumps and channels are utilized by neurons to maintain a transmembrane ionic gradient, which produces an electric potential energy (Voltage). The pumps and channels themselves act an electrical conductor, much like a wire in a circuit. Therefore, we can describe the electrical properties of a neuron membrane in the physical terms of capacitors and resistors, and accordingly calculate the voltage and resistance of this membrane.
An action potential is the means by which a neuron can send a signal elsewhere throughout the body. By allowing the transmembrane voltage to increase through ion channels, an electric current can form, which then propagates down the axon of a neuron. As shown in figure 1, a neuron will normally have a resting potential around $-70$ mV, because the inside of a cell is more negative than the outside. When the transmembrane voltage becomes more negative than the resting potential, it is said to hyperpolarize, while when it becomes more positive than the resting potential, it is said to depolarize. Over time, the ionic potential will naturally depolarize. Therefore the negative electric potential must be maintained by Na+/K+ pumps, which exchanges $3 $ Na+  ions out of the cell for every $2$ K+ ions in.  

 To initiate an action potential a stimulus is required to reach the threshold level of a voltage-gated ion channel. This stimulus opens the channel, allowing more Na+  to enter the cell to propagate the action potential. A careful interplay of Na+ entering and K+ leaving the cell creates depolarization and repolarization, respectively. 
Figure 1. Graph of voltage changes during an action potential
Adapted from wikipedia user Chris 73, CC-BY-SA 3.0
Ouabain is a toxin which blocks the action of the Na+/K+ pump. Given enough time, what effect will ouabain have on a neuron’s membrane potential?
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