Nervous System The Spinal Cord

The spinal cord in the adult human is approximately 45 cm long. It is said to be cylindrical in shape, although flattened in the antero-posterior diameter. Above the spinal cord is continuous with the brain stem and below it tapers into the conus medullaris which is attached to the coccyx by the filum terminale. The cord usually ends at the level of L1-2 although there is great variation in this.

Macroscopically the cord has slight grooves on its anterior and posterior surfaces, the anterior median fissure and posterior median sulcus respectively. In transverse section a central canal can be seen with an H shaped area of grey matter composed of nerve cells surrounded by white matter composed of nerve fibres. The grey matter is divided by virtue of its H shape into two anterior horns (carrying the anterior columns) and two posterior horns (carrying the posterior columns) the two being cross linked by the transverse comissure which corresponds to the cross bar of the H shape. The white matter is largely composed of medullated cells in a longitudinal orientation, which are classified according to their relationship to the grey matter into posterior, anterior and lateral white columns. See Figure CA.17.

Ascending tracts of the cord:

1) The posterior white column transmits fine touch sensation and proprioception. The two fasciculi (medial and lateral) connect to their respective cuneate and gracile nuclei in the medulla. The fibres then cross in the medullary decussation to reach the sensory cortex via the thalamus.

2) The lateral white column carries the lateral spinothalamic tract, which conveys pain and temperature sensation. The cell bodies lie in the posterior horn of the opposite side and fibres then cross in the anterior white comissure to ascend to the thalamus.

3) The anterior and posterior spinocerebellar tracts ascend in the lateral column. Proprioception sensation is transmitted through these tracts, without crossing, to the cerebellum.

Descending tracts of the cord:

1) The pyramidal tract is the major motor pathway of the cord. It lies in the posterior part of the lateral white column. The tract arises from the pyramidal cells in the motor cortex to cross in the medulla and descend in the pyramidal tract of the contralateral side. At each segmental level fibres pass to the anterior horn of the same side to make the link between upper and lower motor neurones.

Figure CA.18 A typical spinal nerve

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Phrenic

Figure CA.19 Cervical plexus

2) The direct pyramidal tract is a minor tract running close to the anterior median fissure. Fibres descend from the motor cortex without crossing, in contrast to the pyramidal tract. At each segmental level fibres cross to the opposite anterior horn.

Blood Supply

The spinal cord receives arterial supply from the anterior and posterior spinal arteries with additional contributions from spinal branches of the vertebral, intercostal, lumbar and sacral arteries. Radicular arteries serve to reinforce the anterior spinal arterial supply and in the low thoracic or high lumbar level one of these is large and supplies most of the lower two-thirds of the cord, this is the arteria radicularis magna. Venous drainage is by way of a complex of anterior and posterior spinal veins which drain into segmental veins and thence to the azygos, lumbar and sacral veins according to level.

The Spinal Meninges

There are three layers covering the spinal cord, the dura, arachnoid and pia mater. Dura Mater

The dura mater consists of two layers. On the surface of the brain the layers are separated by the cerebral venous sinuses. On the surface of the spinal cord the inner dural layer is densely fibrous and the outer layer less so. The outer or endosteal layer ends at the foramen magnum where it blends with the periosteum of the skull. Most commonly the dural sac ends within the sacrum at the level of S2. It may however, terminate as high as L5. The attachments of the dura are as follows:

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1) Superiorly: the edges of the foramen magnum, bodies of C2 and C3 vertebrae

2) Anteriorly: posterior longitudinal ligament

3) Laterally: a sheath along the dorsal and ventral nerve roots

4) Inferiorly: the filum terminale

The anatomy of the epidural space is detailed on pages 142-143. Arachnoid Mater

The arachnoid mater is a delicate membrane closely applied to the dura mater. In some areas the arachnoid herniates the dura mater to form the arachnoid villi which have a major role in the circulation of CSF. The cerebral layer of arachnoid loosely invests the brain and only dips into the main longitudinal fissure between the cerebral hemispheres.

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