Recall that sodium potassium pumps brings two K + ions into the cell while removing three Na + ions per ATP consumed.
The actions of the sodium potassium pump help to maintain the resting potential, once established. Because more cations are leaving the cell than are entering, this causes the interior of the cell to be negatively charged relative to the outside of the cell. Therefore, potassium diffuses out of the cell at a much faster rate than sodium leaks in. However, the neurons have far more potassium leakage channels than sodium leakage channels. The cell possesses potassium and sodium leakage channels that allow the two cations to diffuse down their concentration gradient. In neurons, potassium ions are maintained at high concentrations within the cell while sodium ions are maintained at high concentrations outside of the cell. The negative charge within the cell is created by the cell membrane being more permeable to potassium ion movement than sodium ion movement. The negative resting membrane potential is created and maintained by increasing the concentration of cations outside the cell (in the extracellular fluid) relative to inside the cell (in the cytoplasm). When the membrane is at rest, K + ions accumulate inside the cell due to a net movement with the concentration gradient. The difference in the number of positively charged potassium ions (K +) inside and outside the cell dominates the resting membrane potential ( Figure 16.10). 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 16.1. If the membrane were equally permeable to all ions, each type of ion would flow across the membrane and the system would reach equilibrium. This voltage is called the resting membrane potential it is caused by differences in the concentrations of ions inside and outside the cell. After activation, they become inactivated for a brief period and will no longer open in response to a signal.Ī 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). Voltage-gated ion channels open in response to changes in membrane voltage. The difference in total charge between the inside and outside of the cell is called the membrane potential. Voltage-gated ion channels regulate the relative concentrations of different ions inside and outside the cell. Ion channels that change their structure in response to voltage changes are called voltage-gated ion channels. These ion channels are sensitive to the environment and can change their shape accordingly. Some ion channels need to be activated in order to open and allow ions to pass into or out of the cell. Ion channels have different configurations: open, closed, and inactive, as illustrated in Figure 16.9. To enter or exit the neuron, ions must pass through special proteins called ion channels that span the membrane. The lipid bilayer membrane that surrounds a neuron is impermeable to charged molecules or ions.
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