Neuronal Membranes Contain a Resting Electrical Membrane Potential
Neurons, as with other cells of the body, have an electrical potential, or voltage, that can be measured across their cell membrane (resting membrane potential). However, the electrical membrane potential in neurons and muscle cells is unique in that its magnitude and sign can be changed as the result of synaptic signaling from neighboring cells, or it can change within a sensory organ receptor as a response to transduction of some environmental energy.
When the change in membrane potential of a neuron or muscle cell reaches a threshold value, a further and dramatic change in the membrane potential, called an action potential, occurs; this action potential spreads along the entire length of the neuronal axon (see later discussion).The origins of the resting electrical membrane potential are complicated, particularly in a quantitative way. In qualitative terms, however, the resting membrane potential is the result of the differential separation of charged ions, especially sodium (Na’) and potassium (IC), across the membrane and the resting membrane’s differential permeability to these ions as they attempt to move back down their concentration and electrical gradients (see Chapter I). Even though the net concentration of positive and negative charges is similar in the intracellular and extracellular fluid, an excess of positive charges accumulates immediately outside the cell membrane, and an excess of negative charges accumulates immediately inside the cell membrane (Figure 4-2). This makes the inside of the cell negatively charged with regard to the outside of the
FIGURE 4-2 Concentrations of positively and negatively charged ions are similar in both the intracellular space and the extracellular space. However, more positively charged ions accumulate immediately outside the cell membrane (blue), and more negatively charged ions accumulate immediately inside the cell membrane (lighter blue).
cell. The magnitude of the resulting electrical difference (or voltage) across the membrane varies from cell to cell, ranging from about 40 to 90 millivolts (mV), and is usually about 70 mV in mammalian neurons. Because the extracellular fluid is arbitrarily considered to be 0 mV, the resting membrane potential is -70 mV, more negative on the inside than on the outside.