Disc Structure & Function

Our intervertebral disc (Discus intervertebralis) has a round-elliptical shape. It is located between all vertebrae from C2-C3 to L5-S1. In young people, it appears as a white, gelatinous, and translucent structure. Over the years, like all collagen structures, it adopts a more yellow-brown color and gradually loses its elasticity, flexibility, and resilience.

Task and Function

The tasks of the intervertebral disc are varied:

• First, it absorbs compressional and shock forces acting on the spine.

• It also facilitates movement between individual vertebrae. The thickness of the disc seems to determine the extent of movement between two vertebrae, while the facet joints influence how that movement occurs.

• Finally, the intervertebral disc keeps the spine's ligaments taut. This increases the stability of the spine.

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

Structure

An intervertebral disc changes significantly over the course of our growth and aging. It is divided into a fibrous outer layer (Annulus fibrosus) and a watery inner core (Nucleus pulposus). At birth, the disc consists half of the nucleus with few collagen fibers. However, the outer half is structured with many collagen fibers in different rings. Over time, this clear separation of nucleus and annulus disappears. The disc evolves into a more homogeneous fibrocartilaginous structure.

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

Biomechanically, the disc experiences more tensile stress in its outer regions, while the inner region is subjected to more pressure and compression loads.

The disc is in contact with the end plates of the adjacent vertebrae on both sides. Initially, these end plates are composed of hyaline cartilage. As the years pass, they begin to calcify and harden into bone, starting from the vertebral bodies. There is ongoing debate about whether the end plates belong to the disc or the vertebral body. However, in cases of spinal injury, it often becomes evident that the end plates form a stronger bond with the disc than with the vertebrae.

The disc's ability to bind water is greatly enhanced by the negative charge of proteoglycans and glycosaminoglycans in its ground substance, maintaining the collagen network under tension and providing exceptional stability and resistance to deformation.

Notably, the ground substance cannot fully maximize its water uptake capacity, being restricted by the collagen network. We will revisit this point later.

What Our Disc Needs

As mentioned, only a small part of the disc's outer area is vascularized. However, recent studies reveal that diffusion and osmosis adequately supply it with oxygen and nutrients. Thus, it is capable of regeneration and healing throughout.

Changes in position (under the influence of gravity), movement, or training support and enhance these transport mechanisms: when gravity no longer compresses the disc (e.g., while lying down), it fills with fluid (hydration). Once gravity acts again, fluid is expelled from the disc (dehydration). Similar processes occur with increased loading followed by unloading!

With the increasing ossification of the end plates with age, these transport processes (diffusion and osmosis) become less efficient, resulting in less nutrient supply, especially in areas with poorer blood circulation.

Another challenge is the very long turnover time of collagen, ranging from 300 to 500 days, making regeneration into high-quality disc tissue a lengthy and sometimes arduous process.

As we age, the ability to bind water diminishes. This is why a size difference is often observed in young individuals in the morning after sleeping, as their discs have expanded slightly.

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

What Effect Does Training Have

Alternating stress and relief create a constant change in electrical tension within the disc. This charge alteration causes piezoelectric activity (Piezoelectricity: change in electrical polarization and thus the occurrence of an electrical voltage in solids when elastically deformed, Wikipedia). This piezoelectric tension triggers synthetic activity in the cells to produce more ground substance material (see also our Blog on Connective Tissue). Additionally, diffusion and osmosis transport mechanisms mentioned before optimally ensure that the disc receives necessary building blocks (amino acids, glucose, etc.) and removes waste products.

The forces acting on collagen structures and fibers during movement also improve their quality and resilience.

Thus, movement and training have a positive impact on the physiological functions of our intervertebral discs—just as with most functions in the human body.

We will discuss common disc problems and treatment options in a future blog...

Once again, the motto is: Life is movement!

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Title Image Credit

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