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Resting Membrane Potential

Resting Membrane Potential

For the nervous system to function, neurons must be able to send and receive signals. These signals are possible because each neuron has a charged cellular membrane (a voltage difference between the inside and the outside), and the charge of this membrane can change in response to neurotransmitter molecules released from other neurons and environmental stimuli. To understand how neurons communicate, one must first understand the basis of the baseline or “resting” membrane charge.

Neuronal Charged Membranes

The lipid bilayer membrane that surrounds a neuron is impermeable to charged molecules or ions. To enter or exit the neuron, ions must pass through special proteins called ion channels that span the membrane. Ion channels have different configurations: open, closed, and inactive, as illustrated in Figure 1. Some ion channels need to be activated in order to open and allow ions to pass into or out of the cell. These ion channels are sensitive to the environment and can change their shape accordingly. Ion channels that change their structure in response to voltage changes are called voltage-gated ion channels. Voltage-gated ion channels regulate the relative concentrations of different ions inside and outside the cell. The difference in total charge between the inside and outside of the cell is called the membrane potential.
The first image shows a voltage-gated sodium channel that is closed at the resting potential. In response to a nerve impulse the channel opens, allowing sodium to enter the cell. After the impulse the channel enters an inactive state. The channel closes by a different mechanism and, for a brief period does not reopen in response to a new nerve impulse.
Figure 1. Voltage-gated ion channels open in response to changes in membrane voltage. After activation, they become inactivated for a brief period and will no longer open in response to a signal.

Resting Membrane Potential

A neuron at rest is negatively charged: the inside of a cell is approximately 70 millivolts more negative than the outside (−70 mV, note that this number varies by neuron type and by species). This voltage is called the resting membrane potential; it is caused by differences in the concentrations of ions inside and outside the cell. If the membrane were equally permeable to all ions, each type of ion would flow across the membrane and the system would reach equilibrium. Because ions cannot simply cross the membrane at will, there are different concentrations of several ions inside and outside the cell, as shown in Table 1.
Table 1. Ion Concentration Inside and Outside Neurons
IonExtracellular concentration (mM)Intracellular concentration (mM)Ratio outside/inside
Na+1451212
K+41550.026
Cl−120430
Organic anions (A−)—100
The resting membrane potential is a result of different concentrations inside and outside the cell. The difference in the number of positively charged potassium ions (K+) inside and outside the cell dominates the resting membrane potential (Figure 2).
The resting membrane potential of minus seventy volts is maintained by a sodium/potassium transporter that transports sodium ions out of the cell and potassium ions in. Voltage gated sodium and potassium channels are closed. In response to a nerve impulse, some sodium channels open, allowing sodium ions to enter the cell. The membrane starts to depolarize; in other words, the charge across the membrane lessens. If the membrane potential increases to the threshold of excitation, all the sodium channels open. At the peak action potential, potassium channels open and potassium ions leave the cell. The membrane eventually becomes hyperpolarized.

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