Anatomy of the Spine

The spinal col­umn is the body’s main sup­port struc­ture. Its thir­ty-three bones, called ver­te­brae, are divid­ed into five regions: cer­vi­cal, tho­racic, lum­bar, sacral and coc­cygeal. The cer­vi­cal region con­sists of sev­en ver­te­brae labeled C1 to C7. The first cer­vi­cal ver­te­bra is called the atlas. The sec­ond is called the axis. Togeth­er, the atlas and axis form the joint that con­nects the spine to the skull and allows the head to swiv­el and nod. The tho­racic region, locat­ed in the mid-back, con­sists of twelve ver­te­brae labeled T1 to T12. These ver­te­brae serve as attach­ment points for the ribcage. The lum­bar region, com­mon­ly called the low­er back, con­sists of five ver­te­brae labeled L1 to L5. This is the main weight-bear­ing sec­tion of the spinal col­umn. The sacral region con­sists of five fused ver­te­brae labeled S1 to S5. These ver­te­brae form a sol­id mass of bone, called the sacrum, which pro­vides the attach­ment point for the pelvis. The coc­cygeal region, com­mon­ly called the tail­bone, con­sists of four small ver­te­brae. These tiny bones may be fused or sep­a­rate. Togeth­er they form the coc­cyx, an attach­ment point for var­i­ous mus­cles, ten­dons and lig­a­ments. The coc­cyx also helps sup­port the body when a per­son is sitting.

Alto­geth­er, the ver­te­brae of the spine’s five regions sup­port the weight of the body and pro­tect the spinal cord and nerve roots. Each indi­vid­ual ver­te­bra has a com­plex set of struc­tures nec­es­sary to the over­all func­tion of the spine. The main struc­ture of a ver­te­bra is the ver­te­bral body — a cylin­der-shaped sec­tion of bone at the front of the ver­te­bra. It is the main weight-bear­ing sec­tion of the ver­te­bra. Behind the ver­te­bral body is the ver­te­bral canal. The spinal cord trav­els through this chan­nel. The spinal cord is the main bun­dle of nerve fibers con­nect­ing the brain to the rest of the body. The spinal cord ends near the L1 and L2 ver­te­brae, where it divides into bun­dles of nerve roots called the cau­da equina. Exit­ing the sides of the spine are nerve roots, thick nerve branch­es that trans­mit sig­nals between the spinal cord and the oth­er parts of the body. On either side of the ver­te­bral canal are pedi­cle bones, which con­nect the ver­te­bral body to the lam­i­na. The lam­i­na cre­ates the out­er wall of the ver­te­bral canal, cov­er­ing and pro­tect­ing the spinal cord.

Pro­trud­ing from the back of the lam­i­na is the spin­ous process. It pro­vides an attach­ment point for mus­cles and lig­a­ments that move and sta­bi­lize the ver­te­brae. Trans­verse process­es pro­trude from the sides of each ver­te­bra. Mus­cles and lig­a­ments that move and sta­bi­lize the ver­te­brae attach to the trans­verse process­es. The artic­u­lar facets form the joints where each ver­te­bra con­nects with the ver­te­brae above and below it. Each ver­te­bra has four facets (two supe­ri­or facets and two infe­ri­or facets). The facet joints have a cov­er­ing of car­ti­lage, which allows move­ment. Between the ver­te­bral bod­ies are the tough, elas­tic spinal discs. They pro­vide a flex­i­ble cush­ion, allow­ing the ver­te­brae to bend and twist. Each disc has a tough out­er wall called the annu­lus fibro­sus and a soft inte­ri­or called the nucle­us pulposus.

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