Nerves attached to the brain are called cranial nerves, while nerves attached to the spinal cord are called spinal or spinal nerves.
Mammals, birds and reptiles have twelve pairs of cranial nerves, which are responsible for the innervation of the sense organs, muscles and glands of the head, as well as some internal organs. Amphibians and fish have only ten pairs of cranial nerves.
The spinal nerves are arranged in parts along the medulla, one pair per vertebra. Each nerve in the pair connects laterally to the medulla through two “roots”, one located in a more dorsal position and another in a more ventral position.
The dorsal root of a spinal nerve is formed by sensory fibers and the ventral root by motor fibers. If the dorsal (sensitive) root of a spinal nerve is injured, the part innervated by it will lose its sensitivity without, however, muscle paralysis. If there is injury to the ventral root (motor), paralysis of the innervated muscles will occur, but without loss of pressure, temperature, pain, etc.
At the dorsal root of each spinal nerve there is a ganglion, the spinal ganglion, where the cell bodies of sensory neurons are located. The motor neuron cell bodies are located within the medulla, in the gray matter.
The spinal nerves branch off near the medulla and the different branches innervate the muscles, skin and viscera. Branches of different nerves can still join together, forming true nerve networks called nerve plexuses.