The spine includes nerves, bones, muscles and discs. The spine is divided into five regions: cervical (neck bones), thoracic (in the chest), lumbar (low back), sacral (attached to the pelvis), and coccygeal (tail bone).
Each region has a number of vertebral bones, typically there are 7 cervical vertebral bones, 12 thoracic vertebral bones, and 5 lumbar vertebral bones. The sacrum is a single, large fused bone. The coccyx is made of one or two small bones (occasionally, an individual may have one extra level or one bone less than normal. E.g. It is not unusual to see four or six lumbar levels).
C1 refers to the top cervical bone and L5 generally refers to the lowest lumbar level.
The vertebrae, the bones of the spine/vertebral bodies, are separated from each other by a disc. The discs are in the anterior (front) of the spine. Discs allow for movement shock absorption.
Nucleus Pulposus: is the interior of the vertebral disc. It is a jelly-like substance that helps absorb the axial load (i.e. forces pushing down on your body when you are sitting upright or standing).
Annulus Fibrosus: The annulus is a set of thicker/tougher/collagen-rich bands that surrounds and contains the nucleus pulposus in multiple concentric circles. Abnormalities in the annulus may cause pain because the annulus is supplied with pain-carrying nerves.