THE NERVE IMPULSE
The events of an electrical nerve impulse are the same as those of the electrical impulse generated in muscle fibers. Stated simply, a neuron not carrying an impulse is in a state of polar-ization, with Na+ ions more abundant outside the cell, and K+ ions and negative ions more abundant inside the cell. The neuron has a positive charge on the outside of the cell membrane and a relative nega-tive charge inside. A stimulus (such as a neurotrans- mitter) makes the membrane very permeable to Na+ ions, which rush into the cell. This brings about depolarization, a reversal of charges on the mem-brane. The outside now has a negative charge, and the inside has a positive charge.
As soon as depolarization takes place, the neuron membrane becomes very permeable to K+ ions, which rush out of the cell. This restores the positive charge outside and the negative charge inside, and is called repolarization. (The term action potential refers todepolarization followed by repolarization.) Then the sodium and potassium pumps return Na+ ions outside and K+ ions inside, and the neuron is ready to respond to another stimulus and transmit another impulse. An action potential in response to a stimulus takes place very rapidly and is measured in milliseconds. An indi-vidual neuron is capable of transmitting hundreds of action potentials (impulses) each second. A summary of the events of nerve impulse transmission is given in Table 8–2.
Transmission of electrical impulses is very rapid. The presence of an insulating myelin sheath increases the velocity of impulses, since only the nodes of Ranvier depolarize. This is called saltatory conduc-tion. Many of our neurons are capable of transmitting impulses at a speed of many meters per second. Imagine a person 6 feet (about 2 meters) tall who stubs his toe; sensory impulses travel from the toe to the brain in less than a second (crossing a few synapses along the way). You can see how the nervous system can communicate so rapidly with all parts of the body, and why it is such an important regulatory system.
At synapses, nerve impulse transmission changes from electrical to chemical and depends on the release of neurotransmitters. Although diffusion across synapses is slow, the synapses are so small that this does not significantly affect the velocity of impulses in a living person.
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