Spinal cord

The spinal cord is the portion of the central nervous system that lies below the brain. At the foramen magnum, the opening in the skull base, it is continuous with the medulla oblongata, and it runs down the vertebral canal to about the first or second lumbar vertebra in adults, where it tapers to a point. About a centimeter across, it is protected by the vertebrae, three layers of meninges and cerebrospinal fluid. Thirty-one pairs of spinal nerves leave it, and the nerve roots trailing below its tip form the cauda equina, or horse's tail.

In cross-section, a butterfly-shaped core of gray matter is surrounded by white matter. The posterior horns of the gray matter receive sensory input, the anterior horns hold the motor neurons that drive skeletal muscles, and lateral horns in the thoracic and upper lumbar cord contain sympathetic neurons. The white matter carries ascending tracts for sensation and descending tracts for motor commands. The cord also handles reflexes, such as withdrawing a hand from heat, before the brain is even aware.

Because the cord ends high in the lower back, a lumbar puncture to sample cerebrospinal fluid is done below it. Cord injuries cause loss of movement and sensation below the damaged level: high neck injuries can affect all four limbs (tetraplegia), lower ones the legs (paraplegia).