TEM of myelinated axon. Magnification 15,000x. Myelin is a white fatty material composed chiefly of alternating layers of lipids and lipoproteins that encloses the axons of myelinated nerve fibers. An axon is a portion of a nerve cell (neuron) that carries nerve impulses away from the cell body. A neuron typically has one axon that connects it with other neurons or with muscle or gland cells. Some axons may be quite long, reaching, for example, from the spinal cord down to a toe. Most axons of vertebrates are enclosed in a myelin sheath, which increases the speed of impulse transmission; some large axons may transmit impulses at speeds up to 300 feet per second.