The Nerve Roots, Nerves, and Pain Receptors

The nerves of our body form a complete back and forth circuit as the travel from our brain to spinal cord, to the nerves in our tissues and parenchyma of organs and back again. Not all nerves that reach the tissues have the same function, and not all nerves are designed to send and receive signals for pain. Another factor which makes the connection between nerve damage/nerve root irritation difficult to understand fully is the fact that as nerves extend away and branch from their nerve roots, they sometimes branch back together and combine with branches from other nerve roots. This complex configurations where nerves branch back together and combine with branches from other nerve roots is one of the reasons that the function of certain muscles and body parts are affected by multiple nerve roots. This is one of the reasons that L4, L5, and S1 all may affect our ability to dorsiflex our feet (when we dorsiflex our feet, we turn our foot upwards, and decrease the angle between our shin and long axis of the foot).

If you look at a diagram of the spinal cord and nerve root, you will be able to see the various nerve roots and key locations, including the brachial plexus, conus medularis (where the spinal cord ends), and cauda equina. Because the spinal cord ends at L1, the nerves that eventually exit from between the lower lumbar and sacral vertebrae must run down the length of the spine for longer and longer distances as they make their way to the lower lumbar and sacral regions. The strands of nerves from below the conus medularis that begin to spread outwards and downwards through the spinal canal resemble the shape of a horse's tail, parted down the middle. For this reason the region occupied by these first branches of nerve roots is called the cauda equina. Cauda equine, translated from Latin, means "horses tail".

Some, but not all, of these nerve endings have pain receptors. It is only relatively recently that doctors and neuroscientists have begun to map the various nerve roots and individual branches of nerves towards a more comprehensive understanding of the nervous system as it pertains to pain. Today, our understanding of which nerve endings have pain receptors and which don't is better understood. For example, we know that there are pain receptors that branch to the ligaments of the spine ,the blood vessels which supply the muscles of the back, the dura mater, and the facet joints. However, we know that there are no pain receptors in the nerves that terminate in the nucleus pulposus (interior of disc), and none in the disc annulus (outer covering of disc) except at the location where the annulus is attached to the posterior longitudinal ligament.

Among the anatomical structures where there are pain receptors, some are more sensitive to pain signals than others. The overall interaction between the specific nerves involved in pain, and how our brain perceives and interprets these pain signals may also be complicated by our personality and ability to respond to physical pain on a cognitive level.