Spinal Column
Anatomy Of The Spine
The spine is a flexible column, composed of a stack of individual bones. Each bone is called a
vertebra. There are seven vertebrae in the neck (cervical vertebrae) twelve in the thoracic
region (thoracic vertebrae) and five in the lumbar region (lumbar vertebrae). At the top, the skull
rests on the atlas vertebra (the first cervical vertebra). This is called the occipito-cervical
junction. At the bottom, the fifth lumbar vertebra rests on the sacrum (a large triangular bone)
which consists of five fused vertebral elements. The sacrum forms part of the pelvis. Below the
sacrum, there is a small string of bones, the residual tail, called the coccyx. This is composed of
up to six or seven segments.
The vertebrae are separated from each other by joints. At the front, there is a large
fibro-cartilaginous joint, called a disc. At the back, there are facet joints. The bones are bound
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together by ligaments. The most important of these are the anterior longitudinal ligament, the
posterior longitudinal ligament and the ligamentum flavum (see diagrams).
Down the centre of the spine there is a canal – the vertebral or spinal canal. In this canal, we
find the spinal cord and spinal nerves.
The Spinal Canal
From the outside in, there are a number of layers. First, there is a layer of fat and large veins,
found in the epidural space (epi means around – hence epidural means around the dura). The
next layer is the dura mater. As its name implies, it is a very tough fibrous layer, which is
resistant to penetration and ensheaths the nerves as they leave the spinal column. Just inside
the dura, there is a layer of a thin silvery membrane, that looks a bit like a spider’s web. This is
called the arachnoid. Inside the arachnoid layer, we find cerebro-spinal fluid, which is
continuous with the cerebro-spinal fluid that bathes and supports the brain. Closely applied to
the spinal cord and nerves is the pia mater, which in the spine, serves as a membrane that
binds the spinal cord and the nerves into anatomical entities.
The spinal nerves leave the spinal canal to the sides, via gaps between the bones known as
foramina. The spinal nerves then go out into the neck, arm, trunk and limbs, bundled together
as peripheral nerves. Each spinal nerve contains motor fibres, which innervate the muscles
(and signal to them that the need to move). There are sensory nerves, which transmit sensation
from the skin, joints and deeper organs back to the spinal cord. Then, there are sympathetic
nerve fibres that run along with the peripheral nerves, which control a large number of
peripheral functions, particularly sweating and the diameter of the blood vessels of the skin.
Finally, in the sacral region there are para-sympathetic nerves, which innervate the lower part of
the gut (the hind gut) and the visceral structures of the pelvis, including the bladder, the rectum,
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the internal anal sphincter and provide erectile function.
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The spinal nerve roots are numbered very precisely. The first cervical spinal nerve root emerges
between the occiput and the back of the C1 vertebra. This is a very small sensory nerve, which
does not appear to have much in the way of representation on the skin surface. The next nerve
root is much larger – and emerges between the lamina of the C1 vertebra and C2. This is the
C2 nerve root, which goes on to become the major constituent of the occipital nerves. Emerging
through the C2/C3 foramen, we have the first of the neck cervical spinal nerves – C3. Because
there are 7 cervical vertebrae, but 8 cervical nerves, the nerve that emerges between the 7
cervical vertebra and the first thoracic vertebra and known as “C8”. The first thoracic nerve
emerges between the first thoracic vertebra and the second (known as the T1 root). The next
twelve roots emerge serially, so that the nerve that emerges between the T12 vertebra and the
L1 vertebra is the T12 nerve root. The nerve that emerges between the first and second lumbar
vertebrae is known at the L1 nerve root. The nerve roots are then numbered serially until we get
to the nerve that emerges between the fifth lumbar vertebra and the sacrum – this is the L5
nerve root. The first sacral nerve root emerges through the first sacral foramen – and so on until
we get to “S5”.
The different body structures innervated by the spinal nerve roots may be summarised as
follows. The first four cervical nerve roots innervate the strap muscles of the neck and C3, 4 and
5 give off a very important nerve – the phrenic nerve – which innervates the diaphragm. From
C5 downwards (with some contribution from C4) to T1, the spinal nerves unite to form divisions,
trunks and then cords – creating a structure known as the brachial plexus. This brachial plexus
is a structure much beloved of anatomy examiners and testing the knowledge of this structure
has been used to torture medical students for centuries. In essence, the brachial plexus is
responsible for the innervation of the upper limb. This innervation is roughly serial – so that the
more proximal muscles (around the shoulder joint) are innervated by the upper parts of the
brachial plexus, whereas the intrinsic hand muscles (the little muscles within the palm) are
innervated by the first thoracic nerve (or T1). The remaining thoracic nerves innervate the
segmental musculature of the trunk. When we get to the leg, we find that the innervation to the
muscles comes from L2 – S2. The pattern of innervation (the myotomes) is given in the table.
One simple way of remembering the innervation in the lower limbs is that the leg is twice the
size of the arm and so each muscle receives its innervation from at least two spinal nerves. The
only exception seems to be the dorsiflexors of the foot, which are remarkably sensitive to loss of
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function within the L5 nerve root.
Stability
The spinal column depends upon a number of factors for its stability. The notion of “stability” is
one that is debated amongst practitioners of spinal medicine and surgery. However, a useful
working definition would be one as follows.
"A spine may be regarded as stable if it allows a normal, physiological range of motion, without
neurological compromise; and for growth in the spine to occur, without spinal deformity".
Pathophysiology Of The Spine
- Wear and tear in Discs
- Spondylolysis
- Spondylolisthesis
- Nerve root compromise
- Spinal cord compression
- Tumours
- Tuberculosis (patient on ward NOW)
- Epidural abscess - empyema
Mechanism of damage - all ischaemia. Stops the machine and wrecks the machinery.
General rule - the longer and harder the nervous system is squeezed, the worse the outcome.
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