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Anatomy of Back Pain
Introduction to the Nervous System

In the previous section, we learnt that compression of the spine (via the facet joints) damages the nervous system which, in turn, leads to back pain.

However, back pain is often accompanied by many other symptoms – ranging from organ dysfunction to referred pain – all of which are caused by damage to the nervous system. Hence, to understand these symptoms, we first need to understand the nervous system in more depth.

The human nervous system is made up of the following components:
  • Central Nervous System
  • Peripheral Nervous System

The central nervous system (CNS) comprises the brain and the spinal cord, and controls the vast majority of bodily functions. The peripheral nervous system (PNS) is connected to the CNS, but lies beyond it – it provides a nerve supply to the rest of the body, and relays messages to and from the CNS.

In total, there are 43 main nerves (actually nerve pairs) that branch off the CNS to the peripheral nervous system: 12 cranial nerves that originate in the brain, and 31 spinal nerves that originate (rather unsurprisingly) in the spinal cord.

The cranial nerves, labelled CN I (1) - CN XII (12), are so called because they exit the brain through holes in the skull (or cranium).

They provide the nerve supply (for example) to the nose via the olfactory nerve (CN I), and to the eyes via the optic nerve (CN II).
 Note
Technically speaking, the spinal cord does not run the entire length of the spinal column (that is, from the brain to the base of the spine) – it ends at the junction between the twelfth thoracic vertebra, and the first lumbar vertebra (T12-L1) – the so called thoracolumbar junction.

After this, the cord splits into multiple, separate nerves that continue down into the vertebral column – the anatomical name for this feature is the cauda equina, which is Latin for ‘horse’s tail’.

The spinal nerves branch off the spinal cord, exiting through holes in the vertebral column (the intervertebral foramina). A diagram of the spinal nerve pairs is shown opposite.

The nerves that exit the spinal column are labelled according to the level of the hole from which they emerge.

This can get slightly confusing, however, because there are eight cervical nerves (see table below) – as a result, the first seven nerves (C1-C7) exit from the holes above the first seven vertebrae (C1-C7); however, the eighth nerve (C8) emerges from the hole below C7. All of the remaining nerves exit from the hole below their corresponding vertebra.

Level No. Vertebrae Labels Nerve Pairs Labels
Cervical 7 C1 - C7 8 C1 - C8
Thoracic 12 T1 - T12 12 T1 - T12
Lumbar 5 L1 - L5 5 L1 - L5
Sacral 5 S1 - S5 5 S1 - S5
Coccygeal 4 Co1 - Co4 1 Co1


Each of the 43 nerves can be compared to a tree – it has a large trunk, or root, that splits into successively smaller branches, each of which eventually innervates (or provides a nerve supply to) a particular part of the body.

The Peripheral Nervous System

The peripheral nervous system comprises the following sub-components (each of the 43 nerves of the PNS belongs to one, or more, of them):

  • Somatic Nervous System

  • Autonomic Nervous System

  • Enteric Nervous System

The somatic nervous system (SNS) is responsible for all of our voluntary movements – movements that require conscious thought before they happen e.g. running (somatic means of, or relating to, the body).

The autonomic nervous system (ANS) is responsible, as its name suggests, for involuntary, or subconscious, nervous activity (autonomous means self-governing). The ANS, for example, controls heartbeat and the function of most bodily organs – it comprises the following, sub-components:

  • Sympathetic Nervous System

  • Parasympathetic Nervous System

The enteric nervous system (ENS) is associated with the gastrointestinal (GI) tract (or alimentary canal) that runs from the mouth to the anus (enteric means ‘of the intestines’). It is linked to the ANS, but, unlike the rest of the human body, it is largely (though not completely) independent of the brain.

Together, these systems account for the 31 nerve-pairs that exit the spinal column (and the 12 that exit the head). Each spinal nerve consists of both motor and sensory units and, for this reason, they are referred to as mixed nerves. The motor units relay information away from the spinal cord, passing instructions from the brain to the muscle or organ in question; the sensory units relay information (from the organ, or muscle) to the brain about various sensations, including temperature, pain and pressure.

As a result, nerve root compression (which, in the spine, is caused by compression of the facet joints) can lead to a wide variety of effects, depending on:

  • which of the 3 systems (somatic, autonomic, enteric) the nerve belongs to
  • whether the sensory component, motor component, or both are affected
  • the severity of the compression

So, we have 31 nerves that exit the spinal column, each of which can belong to one or more of the three peripheral systems (and each of which contains both sensory and motor components). Hence, if we can determine the exit points of the three peripheral systems (somatic, autonomic, enteric), and we know which muscle, or region of the body, each individual nerve supplies, then we can determine the exact effect of nerve root compression.

In the following sections, we discuss each of the peripheral systems in more detail. We then provide a map of the relevant, spinal exit points, and discuss the effects of facet joint compression, as they apply to each system.


Next >>> The Somatic Nervous System

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Anatomy of Back Pain

Spinal Column

Vertebrae

Facet Joints

Nervous System
Introduction
Somatic
Autonomic
Enteric