Neurons (nerve cells) in the brain, spinal cord, and peripheral nerve send axons (nerve fibers) up and down the spinal cord in spinal tracts. These spinal tracts are called white matter because axons are coated with a membrane called myelin and myelin appears white. In the spinal cord, white matter is usually situated close to the surface of the cord, arranged into several columns called the anterior, posterior, and lateral columns. The spinal cord contains neurons located in the middle part of the spinal cord. The areas of the spinal cord that contain neurons is called gray matter. The gray matter is most abundant in the parts of the spinal cord that connect to the arms and legs, called the cervical and lumbosacral enlargements.
The spinal cord transmits signals for sensations and to control movement, as well as breathing, bladder, bowel, sweating, blood pressure, sexual, and other essential functions of the body. The spinal cord contains neuronal circuitry for reflexes that control all these functions. Over 20 million axons ascend and descend in the human spinal cord, organized into spinal tracts named according to their source and destination. For example, the spinal tract that sends axons from the cerebral cortex to the spinal cord is called the corticospinal tract. Likewise, the tract that sends axons from the red nucleus in the midbrain to the spinal cord is called the rubrospinal tract. The sensory tract that transmits pain and temperature sensation from the spinal cord to the thalamus is called the spinothalamic tract. Some tracts, however, are named by their position. For example, the posterior column transmits sensory information from the spinal roots to the brainstem.
Neurons that send axons to muscles are called motoneurons while neurons that send axons to other neurons are called interneurons. Motoneurons and interneurons receive information from descending axons and sensory axons. When you activate sensory input to the spinal cord by tapping a tendon, the activity turns on motoneurons that cause the muscle of that tendon to contract. This is called a monosynaptic reflex. To signal the muscles to move, the brain sends information directly to motoneurons or indirectly through interneurons that can either excite or inhibit other neurons.
Sensory neurons send axons from the spinal cord to the brain. Some sensory axons go from peripheral nerve neurons in posterior sensory ganglia located just outside of the spinal column. Posterior sensory ganglion neurons send an T-shaped axon to the body where it collects information like touch and movement while the other end goes into the spinal cord and branches. One branch goes into the gray matter where it activates motoneurons and the other end goes up the posterior column all the way to the brainstem.
