About 200 years ago, scientists removed muscle from dead animals and passed electric current through it. The scientists discovered that the current caused the muscle to contract just as it did in life. Since then, scientists have learned a great deal about neuron structure, shown in Figure 49-1 and Figure 49-2, and how neurons affect other parts of the body, such as muscles. Scientists have also learned that neuron function is dependent on electrical activity.
All cells, including neurons, have an electrical charge inside the cell that is different from the electrical charge outside the cell. A difference in the electrical charge across the cell membrane is called a membrane potential. Membrane potentials are produced by the movement of ions across the cell membrane. The movement of ions depends on the ability of the ions to diffuse through the cell membrane, the concentrations of ions inside and outside the cell, and the electrical charge of the ions. As with batteries, membrane potential is expressed as voltage.
Ions diffuse across a neuron's cell membrane by passing through proteins that act as ion channels. Each type of channel allows only specific ions to pass. Certain channels are voltage gated—that is, whether these channels are open or closed depends on the membrane potential. Even a small change in membrane potential can affect the permeability of the cell membrane to certain ions. As ions move into or out of the neuron, they, in turn, affect the membrane potential.
Neurons consist of dendrites, which bring signals toward the cell body, and axons, which carry signals away from the cell body. At the tip of the axon, the axon terminal makes contact with a target cell, such as a muscle cell, gland cell, or other neuron, at a synapse.
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