Here we see the many layers of nerve cells in the retina of a ground squirrel. The top layer (green) is made up of cells called photoreceptors. These cells convert light into electrical signals that travel to the brain. The two best-known types of photoreceptors are rod and cone cells. Rods help us see under low-light conditions, and cones allow us to see vibrant colors in daylight.
Axones nerviosos - A cross section of a bundle of nerve fibers. Axons (orange) are wrapped in myelin (purple); an extension of the membrane of glial cells. Myelin has high lipid content, and acts to insulate the axons, causing them to transmit nerve impulses much faster.