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Spinal cord

Created: 10/12/2004
 

The spinal cord and meninges lie in the vertebral canal. In the 3-month fetus, the spinal cord and vertebral canal are the same length but the vertebral canal grows more rapidly so that in the adult the cord extends from the foramen magnum to the level of the first or second lumbar vertebra. Thus, the adult spinal cord occupies only the upper two-thirds of the vertebral canal. It is roughly cylindrical in shape being slightly flattened antero-posteriorly and possesses cervical and lumbar enlargements from which originate the nerves supplying the upper limb (C4–T1) and the lower limb (L1–S3) plexuses. These enlargements lie opposite the lower cervical and lower thoracic vertebrae, respectively. 31 pairs of spinal nerves emerge from the cord and, since the adult spinal cord is shorter than the vertebral canal, these nerve roots descend in the canal, with increasing obliquity and length, to reach their appropriate intervertebral foramina, through which they exit the canal. The lumbar and coccygeal nerves are the longest and descend beyond the end of the spinal cord as the cauda equina (Figure 1). The inferior end of the spinal cord tapers (conus medullaris) and from its end a thin strand of pia mater, the filum terminale, descends through the cauda equina to be attached to the coccyx.

Figure 1

Spinal cord in vertebral canal

Structure of the spinal cord

A cross-section of the cord (Figure 2) shows a central canal around which is the H-shaped grey matter, the anterior horns of which contain mainly sensory cells. The grey matter is surrounded by white matter containing the fibres of the ascending and descending tracts. A median fissure partially divides the white matter anterior to the central canal. The anterior and posterior cerebellar tracts ascend the ipsilateral side of the cord to the cerebellum, the lateral and anterior spinothalamic tracts convey pain and temperature sensation. Their fibres ascend the cord in the posterior horn for a few segments before crossing to continue their ascent in the opposite spinothalamic tracts. The posterior columns convey fine touch and proprioception uncrossed to the medulla and then, after synapsing, to the sensory cortex and the cerebellum. The descending tracts are motor in function; the crossed pyramidal tract conveys the fibres from cells in the opposite motor cortex that have crossed in the medulla to reach the contralateral cord. These fibres synapse in each segment with motor cells in the anterior horn. The direct pyramidal tract descends from the motor cortex without decussation. Its fibres synapse at each segment with cells in the anterior horn of the opposite side.

Figure 2

Cross section of the spinal cord

Spinal meninges and cerebrospinal fluid (CSF)

The spinal meninges comprise the three membranes surrounding and supporting the spinal cord; from without inwards, the dura, arachnoid and pia mater. The subarachnoid space, between the arachnoid and pia mater contains CSF, a clear, slightly alkaline fluid. This space is widest in the lowest part of the vertebral canal and it is here, the lumbar cistern, that the CSF surrounds the cauda equina and filum terminale. CSF is formed by the choroid plexuses of the lateral, third and fourth ventricles of the brain.

The dura mater
is strong, formed of elastic and fibrous tissue. It is the outermost covering of the spinal cord and provides the external layer of the dural sac, the tubular sheath that lies free in the vertebral canal. Superiorly it adheres to the margin of the foramen magnum where it is in continuity with the cranial dura mater; inferiorly it is anchored to the coccyx by the filum terminale. Within the vertebral canal, the dural sheath is separated from the periosteum of the vertebrae by the extradural or epidural space (described in more detail on page 148) but it extends along the dorsal and ventral nerve roots as far as the intervertebral foraminae to form dural root sleeves.

The arachnoid mater
lines the dural sac, separated from it by a potential space, the subdural space. The arachnoid mater is a thin avascular membrane that also ensheathes the spinal cord and the spinal nerve roots. It is connected to the underlying pia mater by delicate strands of connective tissue, the arachnoid trabeculae. Between the arachnoid and the pia mater lies the subarachnoid space, which contains the CSF (Figure 3).

Figure 3

Spinal cord

The pia mater is the innermost membrane covering the spinal cord and adheres closely to it. The pia mater ensheathes the spinal nerve roots and covers the spinal blood vessels.

The spinal cord is suspended in the dural sac by pairs of segmental denticulate ligaments, composed of pia mater. There are 21 pairs of these ligaments, each arising from the side of the spinal cord midway between the dorsal and ventral nerve roots.

Blood supply of the spinal cord:
Figure 4 shows the vertical distribution and Figure 5 the horizontal distribution of the blood supply. Branches of the vertebral, deep cervical, intercostal, and lumbar arteries contribute to three arteries that run the length of the spinal cord; the anterior spinal and the two posterior spinal arteries. The anterior spinal artery arises at the level of the foramen magnum by the junction of two branches, one from each vertebral artery. Each posterior spinal artery arises from the posterior inferior cerebellar artery at the same level. 21 pairs of segmental radicular arteries supply the nerve roots and about half of them contribute to the spinal arteries. Of these larger branches, the largest is the great anterior radicular artery of Adamkiewicz (radicularis magna), which supplies the lower thoracic and upper lumbar parts of the cord. It usually arises from a lower intercostal or a high lumbar artery but may arise as low as L4 or as high as T8 (Figure 4). Since it makes a major contribution to the spinal cord blood supply, spinal injury or aortic surgery may compromise the blood supply of the lower part of the spinal cord. Though the other segmental radicular arteries are small their contributions to the anterior and posterior spinal arteries are important.

Figure 4

Blood supply to the spinal cord

Figure 5

Blood supply to the spinal cord (ii)

Venous drainage of the cord is usually by three anterior and three posterior spinal veins, which run longitudinally and receive the radicular veins. The spinal veins join the veins draining the vertebral bodies to form the internal vertebral venous plexus. This plexus surrounds the dura mater, lying in the epidural space (Figure 6) (the epidural space is described in more detail on page 148). It communicates superiorly with the venous sinuses of the cranium and drains by multiple communications into the vertebral and ascending lumbar veins and thence into the azygos and hemiazygos veins.

Figure 6

Arterial supply and venous drainage of the spinal cord

Clinical relevance

Spinal injuries may, by damaging the radicular arteries, seriously compromise the function of the spinal cord and cause weakness and paralysis of muscles. The areas that are most vulnerable to deprivation of blood supply are T1–3, T5 and L1. Blockage of a posterior spinal artery may have little effect owing to the extensive anastomoses, but occlusion of an anterior spinal artery often produces ischaemia of the anterior central part of the cord causing flaccid paralysis and loss of pain and temperature sensation. Thrombosis of the great radicular artery of Adamkiewicz may produce a paraplegia because it makes a major contribution to the blood supply of the lower two-thirds of the spinal cord.

Copyright © 2004 The Medicine Publishing Company Ltd


ArticleDate:20041210
SiteSection: Article
 
   
    
                                            
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