Neuron. Coloured scanning electron micrograph (SEM) of a freeze-fractured neuron (nerve cell, red). The fracture plane cuts through the nucleus (dark grey) of the cell, and also the nucleolus (round, green, lower centre). The nucleolus is the site of ribosome assembly within the nucleus. The nucleus, a cell's control centre, contains the DNA (deoxyribonucleic acid) genetic material. Nerve cells have branching processes (two seen at upper left and upper right) called axons. These connect nerve cells together, allowing the transmission of electrical impulses around the nervous system. This neuron is from the spinal cord. Magnification unknown.