The Nervous System
Nerve cells that have a sheath of Schwann cells around their axons, for insulation and faster conduction, are known as myelinated nerves.
The Propagation of a Nerve Impulse
Like the other cells of the body neurons contain charged ions including potassium, sodium and chlorine. Potassium and sodium are positively charged ions whereas chlorine is a negatively charged ion. Unlike most other cells, neurons are able to depolarize, creating a nerve impulse, by rapidly changing the concentration of ions inside the cell relative to the outside of the cell.
When at rest a neuron is polarizedthat is, it has a negative charge on the inside of the cell relative to the outside of the cell. This is because it has a higher concentration of negatively charged ions on the inside. When stimulated, the nerve cell membrane becomes permeable to sodium ions, which rush in temporarily causing a positive charge to build up on the inside relative to the outside. For a short time afterward, referred to as the "refractory period," the nerve cell is unable to "fire" again. A mechanism in its cell membrane has to "pump" the sodium ions back out again, restoring the negative charge and the "action potential" to the cell. Once the action potential is restored the neuron is ready to transmit the next nerve impulse. This all takes place in a very short period of time, measured in microseconds, as a single nerve cell can fire hundreds of times during a single second.
The characteristic of nerve cells which sets them apart from the other cells of the body, and which allows them to transmit signals, is their ability to ________.
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