Neurons are the basic cells of the nervous system. These ~nerve~ cells are responsible for receiving and transmitting ~nerve~ impulses and forming long |fibers| by linking together. They consist of a |cell body|, which contains a |nucleus|, with one or more |axons| and |dendrites| extending from the body. The |dendrites| are the multi-branched portions which receive impulses, while the |axons| are the elongated structures which carry the impulse away from the body of the cell. Clustered neurons are grey, though some ~nerve~ cells are covered with a white |myelin sheath|. This accounts for the grey and white matter in the |brain| and |spinal cord|. Neurons transmit their impulses across ~nerve~ connections (synapses) using a neurotransmitter chemical, such as acetylcholine. They may be unipolar, bipolar, or multipolar, depending upon which directions the impulses are carried (characteristics based upon the structure of the neuron). Billions of neurons are located in the body's nervous system and they are so efficient that a ~nerve~ impulse (of pain, for example) can be transmitted from the hand to the |brain| and back again to allow a reflex movement in a fraction of a second.
