How does the spinal cord work? Neurons (i.e. nerve cells) located in the brain, spinal cord and peripheral nerves send out axons (i.e. nerve fibers) that travel up and down the spinal cord to form spinal cord tracts. The spinal cord bundles are called white matter because the axons are wrapped in a white myelin membrane. The white matter is usually close to the surface of the spinal cord and is divided into columns called the anterior, posterior, and lateral columns. The neurons of the spinal cord are located in the middle part of the spinal cord. The area of the spinal cord that contains the neurons is called the gray matter. The gray matter is most abundant in the spinal cord connecting the arms and legs, and these spinal cord regions are then called the cervical and lumbosacral expansions, respectively. The spinal cord transmits sensation and controls movement, as well as other basic bodily functions such as breathing, bladder, bowel, sweating, blood pressure, and sexual function. The spinal cord contains nerve reflex circuits that control all of these functions. More than 20 million axons travel up and down the human spinal cord, forming spinal tracts that are often named according to their origin and destination. For example, the spinal cord tracts that send axons from the cerebral cortex to the spinal cord are called corticospinal tracts. Similarly, the spinal cord bundle that emits axons from the red nucleus of the midbrain to the spinal cord is called the red nucleus tractus solitarii. The sensory tracts that transmit pain and temperature sensations from the spinal cord to the thalamus are called the spinal thalamic tracts. However, some spinal cord tracts are named according to their location, such as the posterior column that transmits sensory information from the spinal roots to the brainstem. The neurons that send axons to the muscles are called motor neurons. Neurons that send axons to other nerve cells are called interneurons. Motor neurons and interneurons receive information from the descending axons and sensory axons. When you tap a tendon and send sensory input to the spinal cord, this activity activates the motor neuron and causes the muscle connected to that tendon to contract. This is called a monosynaptic reflex. When sending motor signals to muscles, the brain can send signals directly to motor neurons, or it can send signals indirectly by stimulating or inhibiting other neurons through interneurons. Sensory neurons send axons from the spinal cord to the brain. Some sensory axons emanate from peripheral neurons in the posterior sensory ganglia, which are located outside the spinal column. Neurons in the posterior sensory ganglia send T-shaped axons that collect bodily information such as touch and movement at one end and enter the spinal cord and its branches at the other end. One branch enters the gray matter and activates motor neurons; the other end travels up the posterior column to the brainstem.