Vertebrates have neurons with one or more fibers. In neurons with two or more fibers, the fibers are usually of two different types. Dendrites are the fibers that receive excitation from other cells and transmit the stimulation to the cell body. A single neuron often has many dendrites which are often multi-branched themselves. Dendrites are relatively short in length in comparison with a neuron's axon. The axon leads impulses away from the cell body and toward other neural cells. Axons are ordinarily longer than dendrites although there is usually only one axon per neuron. An axon is connected to its cell body by a pyramid-shaped structure called an axon hillock. An axon's free end is multi-branched; each branch possesses a terminal
swelling called a synaptic bouton. In the central nervous system, the length of an axon is often enveloped by specialized neuroglia or glial cells that wrap around the axon, from a point just after the axon arises from the cell body to the point at which the axon branches, forming a heavily-lipid myelin sheath. Outside the central nervous system, Schwann cells form the myelin sheaths around axons. The sheaths are interrupted at regular intervals by gaps, called the nodes of Ranvier, where one sheath cell stops and another one begins. The sheaths function to speed up the conduction of impulses along axons. Bundles of nerve cells bound together by connective tissue constitute a nerve. Packed together for structural convenience, each nerve cell is insulated from all others by its sheath and conducts its impulses separately from the impulses in other cells.
