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, gel-like, translucent structure. Over the years, like all collagen structures, it takes on a more yellow-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. The thickness of the intervertebral disc seems to co-determine the extent of movement between two vertebrae, whereas the facet joints dictate how the movement occurs.

• And last but not least, the intervertebral disc also keeps the ligaments of the spine under tension. In this way, it provides and increases the stability of the spine.

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

Structure

An intervertebral disc changes strikingly and very clearly over the course of the years, our growth, and age. It is divided into a fibrous outer layer (annulus fibrosus) and a watery inner core (nucleus pulposus). At birth, half of the intervertebral disc consists of the nucleus with few collagen fibers. The outer half, however, is structured with many collagen fibers in different rings. However, this clear separation between nucleus and annulus disappears in the course of growth. 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 intervertebral disc is increasingly subjected to tensile stresses in the outer areas, while the inner area is subjected to more pressure and compressive stresses.

The intervertebral disc is in contact with the endplates of its adjacent vertebral bodies on both sides. Initially, these endplates consist of hyaline cartilage. Over the years, however, they begin to calcify and ossify, starting from the vertebral bodies. There is still discussion as to whether the endplates should be counted as part of the intervertebral disc or the vertebral body. In injuries of the spine, however, it is often shown that the endplates form a firmer connection with the intervertebral disc than with the vertebral bodies.

The ability of the intervertebral disc to bind water is enormous due to the negative charge of the proteoglycans and glycosaminoglycans in its ground substance. The strong water binding keeps the collagen network under tension, creating great stability and resistance to deformation.

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

What Our Intervertebral Disc Needs

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

By changing positions (under the influence of gravity), movement, or training, the transport mechanisms are supported and promoted: if compression due to gravity is absent (e.g. when lying down), the intervertebral disc fills with fluid (hydration). When gravity acts again, fluid is squeezed out of the intervertebral disc (dehydration). The same happens, of course, with increased load followed by relief!

Due to the increasing ossification of the endplates in old age, however, the transport processes (diffusion and osmosis processes) for nutrition become increasingly poor. As a result, the intervertebral disc receives fewer and fewer nutrients, especially in the less well-perfused areas.

Another problem is the very long turnover time of collagen, which lies between 300 and 500 days. Regeneration to a high-quality intervertebral 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 intervertebral discs have expanded or enlarged 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 of electrical tension within the intervertebral disc. This change of charge causes piezoelectric activity (piezoelectricity: change of electrical polarization and thus the appearance of electrical tension on solids 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 Connective Tissue). In addition, the transport mechanisms of diffusion and osmosis, driven by the constant change between loading and unloading, are put to optimal use, supplying the intervertebral disc with the necessary building blocks (amino acids, glucose, etc.) and removing waste products.

The forces acting on the collagen structures and fibers during movement also make them qualitatively better and more resilient.

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

Common problems with the intervertebral disc and treatment options will be discussed in another blog.

Once again, the rule is: life is movement!

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Cover Image Credits

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