The brain is an organ, like the heart, liver and kidneys, and like them it is composed of cells, the neurons. There are some one hundred billion neurons, together with as many supporting cells of one kind or another. A typical neuron (Figure 9) consists of a cell body, from which emerges a major process or fibre, the axon, and a number of smaller branches, the dendrites. Both the axon and dendrites may branch repeatedly, but axons are distinguished from dendrites on the basis that the dendrites and cell body receive incoming signals, while the axon transmits them onward to affect the activity of other nerve cells or muscles. The incoming signals received by the dendrites and cell body are collated and integrated by the neuron and if the outcome leads to the generation of an outgoing signal then that electrical change is passed to the axon which transports the signal (a change in the electrical charge) down its length to the axon terminals, which contact other cells. The axon may be very long, as in the case of a motor neuron concerned with limb movement, or short, as in cells in the cerebral cortex. Connections between neurons are almost unbelievably numerous, with one cell making connections with hundreds or thousands of other cells and receiving an input from a similar number.
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