ectrical impulses through the axon until the impulse reaches the synapse of any neurons near it. Once the signal is sent, the ion balances out and becomes at rest. The electrical impulse that runs down the axon releases a chemical called acetylcholine, only one of many chemicals that transmits signals across the synapse. These substances are called neurotransmitters because they transmit data from one neuron to another. Once the chemical binds to the dendrites of another neuron, it is converted back to an electrical impulse, which is brought to the cell body. The impulse is then sent to another neuron, and the process repeats until the nerves are at rest. The effect of the signals depends on what the target is. If the target of the signal is a muscle cell, the effect might be a muscle contraction. The speed of the electrical impulse depends on the size of the nerve fiber. In small nerves, the rate it transmits impulses is from a half to two meters a second. The larger the diameter of the nerve fiber, the higher rate of conducting impulses. There is less electrical resistance in thick fibers. When nerve impulse jumps from one node (gaps in nerve fibers) to the next, it is called saltatory conduction. Saltatory conduction conducts faster because it contains an insulator that prevents leakage of currents. The rate of conduction is 2 to 120 meters a second. Not all nerves conduct impulse electrochemically. Some impulses jump from nerve to nerve, bypassing the synapse. Unlike other cells, once neurons are lost, they can’t be regenerated. Fortunately, there are about 10 billion neurons and they have other cells to aid them in carrying messages to other nerves. But if nerves are severed, the nerve fibers can regenerate if the two ends are reattached precisely. However, restored functions may produce different actions because the nerves might not be connected to the right channel. There are three main parts of the nervous system: the cent...
