Like all organ systems, the nervous system can do its specialized functions because the cells that make up the nervous system are specialized. The cells in the nervous system are specialized both in how they work individually and how they are connected to each other. The nervous system contains two kinds of cells:
- neurons are the cell type (primarily) responsible for communication and integration in the nervous system.
- glia, which protect the neurons, but also modify their action.
Neurons (nerve cells) have three parts that carry out the functions of communication and integration: dendrites, axons, and axon terminals. They have a fourth part the cell body or soma, which carries out the basic life processes of neurons. The figure at the right shows a "typical" neuron.
Neurons have a single axon is the output of the neuron. Axons are long (up to several feet long), but thin - - sort of like a wire. They are designed both in shape and function to carry information reliably and quickly over long distances (communication). Axons usually branch to connect to go to different neurons. Axon terminals at the end of axons make the actual connection to other neurons.
Axons carry information from the senses to the CNS (Central Nervous System, brain and spinal cord), from one part of the CNS to another, or from the CNS to muscles and glands, which generate the behaviors you do.
Neurons usually have several dendrites (from the Greek dendron, for tree branches) are the input to a neuron. Dendrites are designed both in shape and function to combine information the information they get (integration). Most neurons have several dendrites, each of which may branch up to six times to collect signals from the axon terminals from other neurons that cover it. They are covered with synapses (connections) from many other neurons and combine the signals they get from these synapses.