Nerve cells are a type of cell that is capable of transmitting impulses over a distance. Nerve cells are longer than any other cell in the body, often several centimeters long. They carry all sensory information (touch, sight, smell, taste and hearing) to the brain through nerve fibres, and also carry impulses from the brain to the muscles to control voluntary and involuntary movement. Although their ends are spread out all over sensory surfaces (the skin, retina, nasal cavities, tongue and inner ear respectively), they eventually become bundled together as they travel through the body towards the brain. The spinal cord contains the majority of nerve fibres in the body, but they can be found throughout the body.
Nerves work by pumping sodium ions along their length once stimulated at one end. The end of each nerve cell is sensitive to a different sensation, such as pain, light touch, heavy pressure, red light, low light, acidic substances, etc. When the nerve is stimulated, sodium ions are pumped out of the nerve cell along its length, releasing chemicals at the other end that set off a similar reaction in the next nerve until the impulse reaches the brain and is interpreted. Similarly, instructions from neurons in the brain are sent down nerves until they reach a muscle.
Because nerve impulses are relatively slow, the setting off of certain sensations will automatically trigger a muscle before the sensation reaches the brain. This is referred to as a reflex. For example, a pain sensation in the hand will cause the bicep to contract, drawing the hand away.
Because nerve fibres travelling through the limbs have to pass areas without a great deal of muscle, they are often very close to the surface, particularly in the elbow. Striking a nerve bundle in this area will often set off the sensation of intense pain along the length of the limb - i.e. hitting the "funny bone". There are similar pinch points in the knee and shoulder.
Because nerve fibres are bundled together, they are particularly vulnerable to injury. Severing of nerve bundles is a serious event and almost always results in permanent paralysis below the break.
In addition, there are several diseases that attack only nerve cells, usually causing progressive numbness and paralysis.