Nuclei pulposi

At a glance

English

Nuclei pulposi

Latin (TA2)

Nuclei pulposi

System

Joint

Region

Back

In order

From the nucleus pulposus c2-c3 to the nucleus pulposus l5-s1

About the nuclei pulposi

Each intervertebral disc has two parts: a tough outer ring, the anulus fibrosus, and a soft central core, the nucleus pulposus. The nuclei pulposi form a series that runs the length of the movable spine, one in every disc from the joint between the second and third cervical vertebrae down to the junction of the fifth lumbar vertebra and the sacrum. There is no disc and so no nucleus between the skull and atlas or between the atlas and axis.

All members share the same structure and job. The nucleus is a gel made mostly of water held by proteoglycans in a loose network of fine collagen, with scattered cartilage-like cells. It develops from the notochord, and in young people it is very wet and almost fluid. It sits a little behind the centre of the disc, enclosed by the lamellae of the anulus around it and by the cartilaginous end plates of the vertebral bodies above and below. When the spine is loaded the nucleus cannot be compressed, so it pushes outwards in all directions and the anulus fibres are tensed, spreading the force evenly across the end plates. As the spine bends, the nucleus shifts slightly towards the side being stretched. Under steady load through the day the nucleus slowly loses some water, so the spine is slightly shorter by evening than on waking.

The series changes along its length. The discs, and their nuclei, are smallest and thinnest in the thoracic region, where the ribs limit movement, and largest in the lumbar region, which carries most weight. In the neck and lower back the discs are thicker in front, helping to create the forward curves (lordoses) of those regions. The nucleus also lies further back in the lumbar discs. The lowest lumbar levels, L4-L5 and L5-S1, take the greatest loads and movement and are where degeneration and prolapse are most common; in the neck, C5-C6 and C6-C7 are most often affected.

With age the nucleus loses water and proteoglycan, becomes more fibrous and less able to spread load, and the disc loses height. Its blood supply is absent in adults, so repair is poor. If the anulus cracks, nuclear material can bulge or escape backwards and sideways, usually just to one side of the posterior longitudinal ligament, and press on a nerve root, causing sciatica or arm pain. A large central prolapse in the lower back can compress the cauda equina. Material may also break through the end plate into the vertebral body, forming a Schmorl node.