Nerve cells in the brain. Artwork showing the different types of nerve cells in the grey matter of the brain. Neurons (yellow, for example at lower right) relay nerve signals around the brain. They are supported by astrocytes (red, centre left), which link to blood vessels (across top) and provide the neurons with nutrients such as glucose. They also provide structural support. The axons (output processes) of the neurons are protected in sheaths of the fatty substance myelin, which is provided by extensions of oligodendrocyte cells (dark red, centre). Microglia (small green cells) act as immune system cells, cleaning up debris and protecting the other cells.