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A great deal has been written about the human spine. Googling the word will return a million different links to all sorts of articles and discussions on the topic. Certainly too much to read it all, and that’s assuming you even should. Unfortunately, the Internet as a whole lacks peer review (and most other kinds of review), allowing inaccurate or even deliberately false information to spread, often by multiple outlets picking up the same poor source without checking the facts.
In this article, physical therapist Mgr. Kateřina Klimešová reviews key information on the anatomy, physiology, and function of the human spine.
Elementary school teaches us the spine is the axial skeleton of the torso and consists of vertebrae. The vertebrae are 33 or 34 in total, with the difference being individual, but always found in the coccyx. This is in humans a solid bone, but developmentally it is the fusion of the coccygeal vertebrae, which may be four or five in number (labeled from top to bottom as Co1 through Co5), depending on the person. The shape and curvature of the coccyx are also rather individual.
The coccyx is the very bottom of the spine. Going up, next in line are the five sacral vertebrae (S1 through S5), which are also developmentally fused into a bone – in this case, the sacrum, which is part of the pelvis. Above these are five lumbar (L1 – L5), twelve thoracic (Th1 – Th12), and finally seven cervical (C1 – C7) vertebrae.
Most people probably imagine a vertebra much like it’s shown in the picture. This is a lumbar vertebra, which has the characteristic bulky body, side processes, and a lumen (hole) for the spinal cord.
Less well known is the fact that not all vertebrae look like this. For example, C1 (the topmost cervical vertebra, or the “atlas”) looks very different from a lumbar vertebra. The C1 has no body and its spinous process is quite indistinct; its “custom” shape is intended to allow a flexible joining of the head to the body.
The atlas only needs to support the weight of the head. The lower we go down the spine, the more body mass needs to be supported (in the upright standing posture the spine is built for): arms, chest, organs, muscle. The weight of all this equipment eventually ends up loaded onto the spine (which is the only bone formation connected to the legs, which stand on the ground, pushing against gravity), which gradually becomes more massive in its lower reaches to accommodate it.
Another signature feature of the spine is its double-S curvature. This evolved after our ancestors decided two legs are better than four for walking, completely changing their posture and the mechanical requirements on the skeleton. The curved shape is essentially a spring damper. If the spine were straight as an I-beam, all the shocks caused by walking, running, and jumping around would have to be absorbed by the vertebrae themselves. A vertebra can take quite a lot, but the materials it’s made of couldn’t bear such forces alone without the S-bend, and would eventually deform and crack.
As it is, the spinal curvature dampens the gross shocks, while the remaining difference to a smooth ride is delivered by the spinal discs, which serve as flexible, shock-absorbing pads between individual vertebrae. Indeed, automotive enthusiasts may note a spine is not unlike a car suspension strut, with the spinal curvature roughly analogous to the spring, while the discs form a sort of composite damper.
The spine has three key functions: support, protection, and mobility. Its support function consists of holding up the torso, as well as providing attachment points for certain muscles.
The protective function relates to the spinal cord, the body’s major nerve bundle, damage to which can cause permanent paralysis. The bodies and arches of the vertebrae form the spinal canal, essentially a bone-armored conduit for the spinal cord.
Finally, the spine allows mobility in the torso, partly by compression of spinal discs and partly in the joints between vertebrae. The spine can bend forward, back, and sideways, as well as compress and extend like a spring. The total range of motion is the sum of the (relatively small) ranges of the individual vertebrae.
The cervical spine is the most mobile, allowing up to a 90-degree tilt forward and back and 70 degrees of rotation for the head. About half of the rotational range is in the uniquely structured joint between the first and second cervical vertebrae.
The thoracic spine is fairly immobile, as it is connected to the ribcage, which in turn is held together in front by the sternum. Its greatest range of motion, relatively speaking, is in rotation.
In the lumbar spine, rotation is nearly impossible due to the shape of the vertebrae. Leaning forward is also very limited, as the powerful ligaments supporting the lumbar lordosis are quite tight in this direction. Backward tilt is possible in about the same range as in the thoracic spine.
The mobility ranges of the various segments of the spine are important for understanding the origin of various spinal problems, such as how rib blockages arise from a sedentary lifestyle (link in Czech). That, however, is largely beyond the scope of this article.
Hopefully, by this point you have the basic facts needed for a solid overview of the anatomy, physiology, and function of the spine. If you want to read more about the musculoskeletal system and its conditions, feel free to check out our other articles.
I am a physical therapist with over 10 years of experience, specializing in spine and spinal cord injuries. I am also a Nordic walking instructor accredited by the Czech Ministry of Education, and personal physical therapist to the current Czech champion and Czech indoor record holder in disability shot put.
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