Transmission electron micrograph of tissue from the medial prefrontal cortex in a mouse brain. This region of the brain is important for social behaviour, emotions and psychiatric disorders such as depression. In the centre of the image are two nerve fibres (axons; colored green and purple) each surrounded by myelin (thick black line). Axons are long projections which extend out from nerve cells (neurones) and carry electrical impulses or signals to other cells. Myelin forms a protective sheath or insulating layer around some nerve fibres. It allows electrical impulses to travel quickly along the nerve cells. If it is damaged these impulses slow down and can cause diseases such as multiple sclerosis. Width of image is approximately 4 micrometres. Supplied by Mikaela Laine, University of Helsinki, courtesy of Wellcome Images.