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Human Body

36 nErvEs your electrical intranet The major divisions of the nervous system are sensory, motor, and central the brain and spinal cord. Sensory dendrites bring data from the five peripheral senses into the central nervous system, where all the processing, integration and thought take place. Apart from a few ganglia, all nerve cell bodies, neurosomes, live in the brain and spinal cord, each supported by several glial cells, the space between the stars. Motor axons extend to fire muscles, and are voluntary, to skeletal muscles, or autonomic, subconsciously controlling smooth muscles in your tubes, glands, and organs. The voltage gradient across nerve cell membranes is similar to that at the centre of a thunderstorm see p.14, and it fluctuates both rhythmically and in response to incoming nerve impulses. When the sum of local voltages reaches a threshold at the axon hillock, the cell fires, and a wave of temporary voltage gradient reversal thrills along the axon, as successive voltagegated channels open to let sodium ions flood in for a millisecond. The impulse moves at walking pace in slow sensory nerves, and up to 250mph as it hops along sleeves of myelin around the fastest motor axons. Axons terminate at synapses, where a neurotransmitter is released, which travels to the next cell, usually to another nerves dendrite, or to a muscle, or rarely to another axon. Here it makes the muscle contract p.42, or excites or inhibits the next nerve cells frequency of firing. All nerve cells fire regularly, rate and rhythm defining their signal in the circuitry, and patterns of inhibition are what carve meaning in the white noise of the nervous system. 37
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