The internal structures shown here include the cell nucleus, the mitochondria, and the endoplasmic reticulum. Neurons are responsible for passing information around the central nervous system and to the rest of the body. A nerve cell comprises a central nerve cell body surrounded by numerous extensions. Most of these are dendrites, which collect information from other nerve cells or from sensory cells. Each neuron also has an extension called an axon (lower left) through which information passes outwards to other nerve cells and muscle fibers. The axons are myelinated (covered in a myelin sheath, seen at lower left) to improve electrical conduction of nerve signals.