Introduction
Two unique categories of cells are found in the nervous system; nerve cells known as neurons and glial cells or glia. Neurons are excitable; they have the ability to produce and transmit electrical signals, known as nerve impulses or potentials. The majority of neurons have long extensions called axons, which enable them to execute nerve impulses over long distances. Glial cells do not conduct nerve impulses; they support neurons physically, immunologically and metabolically. A nerve (as opposed to a neuron), is a bundle of axons which originate from many different neurons.

Nerve System Networks
Neurons are arranged into networks in the nervous system, allowing information to be processed. These networks are comprised of three functional categories of cells, involved with input, integration and output. In the first category, afferent neurons transport sensory information into the nervous system, which information comes from specialised sensory neurons which convert or transducer various types of sensory information into action potentials. Next, efferent neurons carry commands to physiological and behavioural effectors, such as muscles and glands. The third category of cells, known as interneurons, integrate and store information and assist communication between sensors and effectors.

Complexity of Networks
Simple animals such as sea anemones can process information with simple networks of neurons that do little more than provide direct lines of communication from sensory cells to effectors. A sea anemone's nerve net is most developed in the area around its tentacles and the oral opening, where it serves to detect food or danger and causes tentacles to expand or contract.

Animals which are more complex and which need move around the environment to search for food and mates need to process and integrate larger amounts of information. Their increased need for information processing is met by elevated numbers of neurons organised into clusters, called ganglia. Ganglia serving different functions may be dispersed around the body, such as in the earthworm or squid. In animals that are bilaterally symmetrical, ganglia frequently occur in pairs, one on each side of the body. Furthermore, as animals increase in complexity, typically one pair of ganglia is larger than the other, and is therefore given the designation of brain.

In vertebrates, most cells of the nervous system are found in the brain and spinal cord, the sites of most processing information, and retrieval. Therefore, the brain and spinal cord are known collectively as the central nervous system (CNS). Information is transmitted from sensory cells to the CNS and from the CNS to effectors via neurons that extend to, or are sited outside of the brain and spinal cord. These neurons and their supporting cells are called the peripheral nervous system (PNS). Vertebrates differ a great deal in their behavioural complexity and in their physiological specialisations, and their nervous systems echo this diversity.

The Human Nervous System
The human nervous system contains approximately 1011 neurons. Information is passed from one neuron to another where they meet in close proximity at synapses. The cell that sends the message is the presynaptic neuron, and the cell receiving the message is the postsynaptic neuron. A neuron in the brain can receive information from more than a thousand synapses. Thus, the human brain may contain up to 1014 synapses, which can strengthen with use and weaken with disuse. The human brain, therefore, has the incredible ability to process information, to learn, to carry out complex and difficult tasks, to remember and have emotions. This vast number of neurons and synapses are separated into thousands of individual but interacting networks which function alongside each other.

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