Spinal root ganglia cells. Coloured transmission electron micrograph (TEM) of a section through healthy human neurones (nerve cells) in a spinal root ganglion. A ganglion is a grouping of nerve cells outside the brain and spinal cord. Each neurone consists of a cell body, in which a nucleus is found, along with various nerve fibres (not seen) down which impulses (messages) travel. The neurone is insulated by a myelin sheath which improves the efficiency of nerve transmission. Axon nerve fibres are the output processes, while branching dendrites are the neurone's sensory receptors. Magnification: x2500 when printed at 10 centimetres wide.