The spinal cord is the part of the central nervous system that connects the lower part of the brain (the brain stem) to the peripheral nervous system. It consists of a bundle of nerve fibers (also called axons) that runs through the spine to the rest of the body. The last part of the spinal cord is called cauda equina (meaning "horse's tail") and although it is located in the lower part of the spine, it is not considered part of the central nervous system, but the peripheral nervous system.
The spinal cord consists of ascending and descending nerve bundles. Through ascending pathways, sensory information from the senses reaches the brain. These are also called afferent pathways. Through descending pathways, the brain controls the body. These are called the efferent pathways. At various points these pathways leave the spine. The nerves that go to the arms leave the spinal cord just below the neck and the nerves that control the legs leave the spinal cord at the level of the lower back. Therefore, if the spinal cord is severed, for example, by an accident, the brain can no longer receive information from the senses or control the muscles. The result is paralysis. The higher the location of the damage, the greater the paralysis.
By the way, the spinal cord is not only a connection between the peripheral nervous system and the central nervous system. It can also control movements itself. Consider the example of the headless chicken. Even after the head (and therefore the brain) is separated from the spinal cord, the chicken can continue to move. This is because certain reflexes and simple movement patterns can proceed without brain intervention.
The spinal cord consists of gray matter and white matter, surrounded by the spinal column. The gray matter is located in the centre of the spinal cord and is shaped like a butterfly. Surrounding it is the white matter that transmits information upward (afferent) and downward (efferent). The spinal cord has two horns on either side: anterior and posterior. Peripheral nerves enter the spinal cord on the posterior horns. There are sensory neurons that transmit information to the brain. The anterior horns contain the motor neurons. From there, the peripheral nerves leave for the muscles.
Author: Bart Aben (translated by Melanie Smekal)