The spinal cord is that part of the central nervous system that is enclosed in and protected by the vertebral column. It consists of nerve cells and their connections (axons and dendrites), with both gray matter and white matter, the gray surrounded by the white. The cord conveys the 31 spinal nerve pairs of the peripheral nervous system.
The vertebral column consists of vertebrae described as belonging to 5 groups (called segments). These segments are (in order from top to bottom): the cervical, thoracic, and lumbar vertebrae, and the sacrum and coccyx.
Embryology of the human spinal cord
In the human fetus, the spinal cord extends all the way down to the sacral vertebrae. As a person matures, the spinal cord shortens relative to the rest of the body, so at adulthood, the spinal cord only reaches down to around the level of L1 (the first, i.e. highest, lumbar vertebra), where it terminates and the cauda equina begin - this is why lumbar punctures are usually carried out on an adult at the (lower) level of L3/L4.
Anatomy of the human spinal cord
It originates in the inferior end of the medulla oblongata, exiting the skull via the foramen magnum. It is wrapped in three layers of membranes, called meninges.
The spinal cord carries sensory signals and motor innervation to most of the skeletal muscles in the body. Just about every voluntary muscle in the body below the head depends on the spinal cord for control. Similarly, most cutaneous sensation below the neck is transmitted via the spinal cord. Most of the sympathetic pathways and the lower (i.e. non-vagal) parasympathetic pathways also go through the spinal cord.
Diseases of the spinal cord
Damage to nerves of the spinal cord, called myelopathy, can result in paraplegia or quadriplegia, depending on the level within the spinal cord of the damage.