Spinal nerves are the vessels that carry information from the brain out to the body and vice verse. As part of the peripheral nervous system, they are found not in the spinal cord, which like the brain is a part of the central nervous system, but exiting the spinal cord and traveling to the various parts of the body that they innervate. Spinal nerves are paired, meaning that for any given nerve there is one supplying the right side of the body and one supplying the left side. Thirty-one pairs in total exit the spine, with approximately one pair leaving each vertebra: eight pairs come from the cervical region, 12 pairs exit the thoracic region, five pairs leave the lumbar region, five pairs come from the sacral region, and one pair exits the coccyx, or tailbone.
The role of spinal nerves is to carry signals to and from the brain in response to stimuli occurring both inside and outside of the body. Motor signals link the brain to the muscles and either instruct the body to move voluntarily or cause the body to react involuntarily. Sensory signals connect the brain to receptors in the skin and tell the body if a stimulus is, for example, hot or cold or painful.
They also send environmental and proprioceptive cues that tell the brain to adjust movement, such as whether a surface one is walking on is hard or soft or slippery. Autonomic signals communicate with the brain regarding internal bodily functions, such as whether one’s kidneys are functioning properly. Spinal nerves transmit all three of these signals between the brain and the body.
Like arteries and veins in the circulatory system, spinal nerves work in two directions to carry signals toward and away from the brain. The dorsal root of a spinal nerve, which exits the spinal cord on its posterior side, carries what are known as afferent sensory signals, or those carrying information from sensory receptors to the central nervous system. On the anterior side of the spinal cord at the same vertebral segment is the ventral root of a given spinal nerve, which carries the efferent motor signals from the central nervous system to the body. In other words, the ventral root tells the body how to react in response to the information brought in by the dorsal root, such as snatching one’s hand away from a pan that, as the sensory receptors have told the brain, is hot.