The human nervous system is a wonderfully subtle mechanism for acquiring and processing information about our environment, and for planning and implementing adaptive response. I am amazed by the simplicity of its underlying construction. There are very few components, which achieve complex and subtle regulatory and intellectual function through adaptation of their interconnection.
The neuron is the cell that differentiates the function of the nervous system from that of the other organ systems. It has a characteristic structure: a large body, from which a long, thick axon extends to stimulate either another neuron, or a tissue. In sensory cells, the cell body is embedded in a tissue, and produces signals in response to chemical or mechanical stimuli. These signals travel along the axon to their destination.
In other neurons, a branched dendritic tree extends opposite from the emergent axon. The dendrites of transmitting neurons are generally tightly coupled to the originating sensory axons, or the control centers of the brain. In the information processing centers of the nervous system, the dendrites can branch profusely, and interconnect with a large number of axons. It is through this complexity of interconnection that intelligence arises.