They are divided into four regions:
- seven cervical or neck vertebrae (labeled C1-C7)
- twelve thoracic or upper back vertebrae (labeled T1-T12)
- five lumbar vertebrae (labeled L1-L5)
- the sacrum and coccyx (fused bones at the base of the spine)
Vertebrae are linked by ligaments, tendons, and muscles. Back pain can occur due to injury, for example, improper lifting technique can cause a sprain, pull, strain, or spasm in one of these muscles or ligaments in the back. Between the vertebrae are round, spongy pads of cartilage called discs that act like shock absorbers.
Degeneration or pressure from overexertion can cause a disc to shift or protrude and bulge, causing pressure on a nerve and resultant pain. When this happens, the condition is called a slipped, bulging, herniated, or ruptured disc, sometimes resulting in permanent nerve damage.
The column-like spinal cord is divided into segments similar to the corresponding vertebrae: cervical, thoracic, lumbar, sacral, and coccygeal. The cord also has nerve roots and rootlets which form branch-like appendages. Along the dorsal root are the cells of the dorsal root ganglia, critical in the transmission of "pain" messages from the cord to the brain.
NIH Publication No. 01-2406