18.8
The Resting Membrane Potential
Overview
The relative difference in electrical charge, or voltage, between the inside and the outside of a cell membrane, is called the membrane poten…
The resting membrane potential is the difference in electrical potential between the inside and outside of the cell membrane of neurons at rest when they are not being stimulated. Typically, the value is around negative 70 millivolts, meaning that it is more negative inside.
Cell membranes are selectively permeable because most ions and molecules cannot passively diffuse across them. They often require transmembrane proteins, such as ion channels, to allow them to pass through. When a neuron is at rest, potassium channels are the main type of ion channel that are open.
Another transmembrane protein, the sodium potassium pump, uses energy to continuously move sodium out of the cell, and potassium in. This action creates a concentration gradient, with a higher concentration of potassium inside than outside.
The force of diffusion then causes potassium ions to move down their concentration gradient, through the open potassium channels, and out of the cell. The movement of these positive ions out combined with negatively charged proteins inside the cell creates a negative charge inside the membrane, a negative potential when a neuron is at rest.
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Q1: What is the resting membrane potential and why is it negative?
The resting membrane potential is the electrical potential difference between the inside and outside of a neuron at rest, typically around negative 70 millivolts. The inside is more negative because positive potassium ions move out of the cell through open potassium channels, while negatively charged proteins remain trapped inside, creating a polarized state essential for neural signaling.
Q2: How does the sodium-potassium pump establish ion concentration gradients?
The sodium-potassium pump is a transmembrane protein that continuously pumps three sodium ions out of the cell for every two potassium ions pumped in. This active transport process uses energy to create concentration gradients, establishing higher sodium concentrations outside the neuron and higher potassium concentrations inside, which drives the resting membrane potential.
Q3: Why is the membrane selectively permeable to potassium at rest?
At rest, potassium channels are the main type of ion channel open in the neuron membrane. Most ions cannot passively diffuse through the lipid bilayer because charged ions cannot cross its hydrophobic interior. Potassium channels allow potassium ions to move down their concentration gradient, making the resting membrane potential primarily determined by potassium movement.
Q4: What role do negatively charged proteins play in the resting membrane potential?
Negatively charged proteins are trapped inside the neuron and cannot cross the cell membrane. Combined with the outward movement of positive potassium ions, these internal negative charges create the relative negativity inside the membrane. This accumulation of negative charge inside contributes directly to the negative resting potential of approximately negative 70 millivolts.
Q5: How does tetrodotoxin affect the resting membrane potential?
Tetrodotoxin is a neurotoxin that selectively blocks voltage-gated sodium channels, preventing sodium ions from entering the cell. However, it does not disrupt the resting membrane potential because the resting state depends primarily on potassium permeability and the sodium-potassium pump, not sodium channel activity. It disrupts action potentials instead.
Q6: What happens when potassium ions reach electrochemical equilibrium?
As potassium ions diffuse out of the cell, positive charges accumulate outside the membrane, creating electrostatic repulsion that opposes further outward movement. Eventually, outward diffusion of potassium is balanced by this electrostatic repulsion, reaching electrochemical equilibrium. This balance produces the stable negative resting potential observed in neurons.
Q7: How is the resting membrane potential measured in neurons?
The resting membrane potential is measured by inserting a microelectrode into a neuron and comparing the electrical charge to a reference electrode in the extracellular fluid. This technique reveals the voltage difference between the inside and outside of the cell membrane, typically showing a negative value around negative 70 millivolts in resting neurons.