The spine from top to bottom

The spine is at once a column, a hinge and a shield. It holds the body upright, lets it bend and twist, and protects the spinal cord that carries messages between the brain and the rest of the body. This page is a short tour from the base of the skull to the tailbone.

Regions of the spine

Anatomists divide the spine into regions, each named in Latin. The National Institute of Arthritis and Musculoskeletal and Skin Diseases lists them in its overview of back pain:

• Cervical spine — the neck, from the base of the skull to the top of the shoulders.
• Thoracic spine — the mid and upper back, where the ribs attach.
• Lumbar spine — the lower back, between the ribs and the pelvis.
• Sacrum and coccyx — the fused bones at the back of the pelvis and the small tailbone below them.

Seen from the side, a healthy spine is not straight. MedlinePlus's guide to good posture describes natural curves at the neck, mid back and low back, and explains that good posture keeps those curves without exaggerating them. The curves act a little like a spring, spreading the load of the head and upper body rather than stacking it in a rigid line.

Vertebrae

The building blocks of the spine are the vertebrae: small bones stacked on top of one another. Each has a solid, roughly drum-shaped body at the front, which bears weight, and a ring of bone at the back, which surrounds and protects the spinal cord. Bony projections stick out from the ring to the sides and the back; they are the bumps you can feel down the middle of someone's back, and they act as anchor points for muscles and ligaments. The vertebrae grow larger from the neck down to the lower back, matching the increasing load they carry.

Discs

Between the vertebral bodies sit the intervertebral discs, which NIAMS describes as cushion-like pads that act like shock absorbers and spacers for the spine. They allow the spine to bend and twist and give the nerves room to leave the spinal canal. The discs and sciatica page explains what happens as discs age.

Joints

Each vertebra meets its neighbours at several points. At the front, the vertebral bodies are joined by the disc. At the back, a pair of small joints, called facet joints, link each vertebra to the ones above and below. Like other joints, they have smooth cartilage surfaces and a lining that produces lubricating fluid. The shape and angle of the facet joints differ from region to region, which is part of why the neck turns easily, the mid back rotates a little, and the lower back bends forward and back more than it twists. At the base of the spine, the sacrum meets the pelvis at the sacroiliac joints.

Ligaments, tendons and muscles

Ligaments are short bands of tough, flexible tissue that hold the vertebrae together; long ligaments also run the length of the spine at the front and back. Tendons connect muscles to bone. The muscles of the back and trunk come in layers, from small deep muscles that link neighbouring vertebrae to long muscles that run beside the spine, and the large muscles of the abdomen, hips and shoulders. Together they hold the spine steady and move it.

How the regions move

Each region of the spine has its own job, and that shows in how it moves. The neck is built for range: it nods, tilts and turns so that the eyes and ears can follow the world. The thoracic spine, braced by the ribs, moves less and holds the upper body steady, though it still rotates and bends a little with each breath and each reach. The lumbar spine carries the heaviest load and bends forwards and backwards more than it twists, which is why turning is best done by moving the feet and hips. The sacrum hardly moves at all; its job is to pass weight between the spine and the pelvis. When one region is stiff, its neighbours often take up more of the movement, which is one reason an ache in one part of the back can seem to spread to another.

The spinal cord and nerves

The spinal cord is a long bundle of nerves running down through the canal formed by the rings of the vertebrae. Nerve roots branch out between each pair of vertebrae and travel to the arms, the trunk and the legs, carrying messages to and from the brain. Because the nerves leave the spine so close to the discs and joints, a problem in one of those structures can be felt far away, as pain, tingling or numbness in an arm or a leg. The pages on neck anatomy and lower back anatomy look at each region in closer detail.