Neuronal fibers (axons and dendrites) joined together by connective tissue form compound structures called nerves. Within a nerve, each neuronal fiber conducts its impulses separately, insulated from all others by its glial cell sheath. Nerves, thus, consist of up to thousands of completely separate impulse pathways which are bound together for structural convenience. Some nerves contain only sensor neurons, some only effector neurons, and some both types at the same time. In the latter case, the nerves are known as mixed nerves. The 31 spinal nerve pairs are all made up of mixed nerves. These nerves branch repeatedly after leaving the spinal chord, reaching most parts of the body below the head. The 12 pairs of cranial nerves are made up of some purely sensory nerves, some purely effector nerves, and some mixed nerves. These pairs connect directly with the brain (or brain stem) from the body rather than entering the spinal chord first.
The cell bodies of neurons aggregate into structures called ganglia. The dorsal-root ganglia are located just outside of the spinal chord and contain the cell bodies of somatic sensory neurons. The anterior horn, located within the gray matter of the spinal chord, consist of the cell bodies of somatic motor neurons.
