DOUGLAS & MARTIN / SCIENCE PHOTO LIBRARY DOUGLAS & MARTIN / SCIENCE PHOTO LIBRARY
Light micrograph of a pyramidal neuron (nerve cell) from mammalian visual cortex. At centre is a cell body (20 microns wide); branched dendrites with spines receive input stimuli from neighbour- ing neurons; a single axon (fine central fibre, partly disappearing & leading downwards) transmits the cell's response. On stimulation, a sudden local change in nerve cell-membrane permeability occurs. Sodium and potassium ions diffuse through, causing an electrical potential that travels wave- like along the nerve. Impulses are thus created. Nerve cells are able to adapt to incoming stimuli and may even alter their response.
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