Disc Structure & Function

Our intervertebral disc (Discus intervertebralis) has a round, elliptical structure. It is located between all vertebral bodies from C2-C3 to L5-S1. In young people, it is a white, gelatinous, and translucent structure. Over the years, like all collagen structures, it takes on a more yellowish-brown color and also increasingly loses its elasticity, flexibility, and resilience.

Task and Function

The task of the intervertebral disc is multi-faceted:

• First of all, it absorbs compression and impact forces acting on the spine.

• Furthermore, it also enables movement between the individual vertebrae. In this process, the thickness of the disc appears to determine the extent of movement between two vertebrae, whereas the facet joints govern how the movement progresses.

• And last but not least, the disc also keeps the spinal ligaments under tension. This ensures and increases the stability of the spine.

Harrygouvas at Greek Wikipedia, Facet Joints Motion, CC BY-SA 3.0

Structure

An intervertebral disc changes markedly and very significantly over the years, our growth, and our age. It is divided into a fibrous outer side (Annulus fibrosus) and a watery inner core (Nucleus pulposus). At birth, the disc still consists about half of the nucleus with few collagen fibers. However, the outer half is structured with many collagen fibers in varying rings. This clear separation of nucleus and annulus disappears as we grow. The disc develops into a more homogeneous fibrocartilaginous structure.

Henry Vandyke Carter Henry Gray, Gray66, marked as public domain, details on Wikimedia Commons

Biomechanically, the outer areas of the disc are subject to more tensile stress, while the inner area is exposed to more pressure and compression loads.

The intervertebral disc is in contact with the end plates of its adjacent vertebral bodies on both sides. Initially, these end plates consist of hyaline cartilage. Over the years, however, they begin to calcify and ossify, starting from the vertebral bodies. There is still a debate as to whether the end plates belong to the disc or to the vertebral body. In the case of spinal injuries, however, it is often shown that the end plates form a stronger bond with the disc than with the vertebral bodies.

The disc's ability to bind water is enormous due to the negative charge of proteoglycans and glycosaminoglycans in its ground substance. Thanks to this strong water binding, the collagen network is kept under tension, resulting in outstanding stability and resistance to deformation.

It is also remarkable that the ground substance cannot fully exhaust its maximum capacity for absorbing water, as this is prevented by the collagen network. We will return to this point later

What Our Disc Needs

As already mentioned, only a small part of the outer area of the intervertebral disc is supplied with blood. However, recent studies show that it is also fully supplied with oxygen and nutrients via diffusion and osmosis. Consequently, it is capable of regeneration and healing throughout its entire structure!

Through changes in position (under the influence of gravity), movement, or exercise, the transport mechanisms are supported and promoted: when compression from gravity is absent (e.g., when lying down), the disc fills with fluid (hydration). When gravity acts again, fluid is squeezed out of the disc (dehydration). A similar process naturally occurs during increased load followed by subsequent relief!

Due to the increasing ossification of the end plates with age, however, the transport processes (diffusion and osmosis processes) for nourishment increasingly decline. As a result, the disc receives fewer and fewer nutrients, especially in the less vascularized areas.

Another challenge is the extremely long turnover time of collagen, which is around 300 to 500 days. Regeneration to a high-quality disc tissue is therefore a long and sometimes arduous journey.

With increasing age, however, the body loses the ability to bind water. This is why a difference in height can be observed in young people in the morning after sleeping, as the discs have expanded or grown slightly.

Henry Vandyke Carter Henry Gray, GA111, marked as public domain, details on Wikimedia Commons

What Effect Does Training Have

Alternating between loading and unloading creates a constant change in electrical voltage within the disc. This change of charge causes piezoelectric activity (piezoelectricity: change in electrical polarization and thus the occurrence of electrical voltage on solid bodies when they are elastically deformed, Wikipedia). This piezoelectric tension is a triggering stimulus for the cells' synthetic activity to produce more ground substance material (see also our blog on Connective Tissue). In addition, the transport mechanisms of diffusion and osmosis, created by the constant shift between loading and unloading, are optimal, supplying the disc with the necessary building blocks (amino acids, glucose, etc.) and transporting waste products away.

The forces acting on the collagen structures and fibers during exercise also make them higher in quality and more resilient.

Movement and training therefore have – as is usually the case in the human body – a highly positive influence on the physiological functions of our intervertebral discs.

Common problems with the intervertebral disc and therapy options are discussed in another blog post.

Once again, it's true: life is movement!

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Cover Image Credits

Henry Vandyke Carter Henry Gray, Gray66, marked as public domain, details on Wikimedia Commons