The insides of cells are electrically negative in comparison to the outsides. The difference in electric potential, or voltage, across the plasma membrane of a cell is called its membrane potential. In an unstimulated neuron, this voltage difference is called a resting potential.
Membrane potentials can be measured with electrodes. An electrode can be made from a glass pipette pulled to a very sharp tip and filled with a solution containing ions that conduct electric charges. Using such electrodes, we can record very tiny local electrical events. If a pair of electrodes is placed one on each side of the plasma membrane of a resting axon, they measure a voltage difference of about 60 millivolts (mV) (Figure 44.4).
The resting potential provides a means for neurons to respond to a stimulus. A neuron is sensitive to any chemical or physical factor that causes a change in the resting potential across a portion of its plasma membrane. The most extreme change in membrane potential is an action potential, which is a sudden and rapid reversal in the voltage across a portion of the plasma membrane. For 1 or 2 milliseconds, a bioelectric current crosses the membrane and the inside of the cell becomes more positive than the outside. Nerve impulses are action potentials that move along axons.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.