The spinal cord begins at the foramen magnum and ends opposite the disc between the first and second lumbar vertebrae. It is approximately 45 cms. long in males and 42 cms. long in females. The spinal cord has the same meningeal coverings as the brain.
The cord is nearly cylindrical in shape but its diameter increases considerably in some two regions giving rise to the large nerves to the limbs -the cervical and lumbar swellings.
In cross section the cord can be seen to consist of an internal core of grey matter and an external layer of white matter. The grey matter is roughly H-shaped, with the horizontal bar formed by the grey commissure.
The cord is partly bisected by an anterior median fissure and a posterior median septum. The central canal is a narrow canal running through the grey commissure. It is continuous above with the central canal of the medulla and ends blindly below in the filum terminale.
The amount of grey matter varies greatly at different levels and is proportional to the volume of tissue supplied by spinal nerves at that level. Thus, where the large nerves to the limbs originate in the cervical and lumbo-sacral regions of the cord there will be large amounts of grey matter.
The white matter forms a thick coating around the grey matter and is composed of longitudinally running nerve fibres, both ascending and descending. The volume of white matter increases steadily from below upwards and is greatest in the cervical region. This is because there is sequential addition of afferent axons to the cord as it ascends, and because there are fewer descending axons as it approaches the sacral cord.
Organisation of Grey Matter
The anterior and posterior limbs of the H-shaped grey matter form the anterior and posterior horns . Since the horns form continuous ridges throughout the length of the spinal cord, they are also known as the anterior and posterior grey columns. (Note that the term ‘posterior column’ however is used to refer to the ascending white matter located between the posterior horns).
The dorsal roots of spinal nerves enter the posterior horns and the ventral roots arise in the anterior horns. In the thoracic region there is a small extension of the lateral aspect of the grey matter – the lateral horn. This is concerned with the sympathetic outflow in this area.
The cell bodies in the grey matter are grouped into clusters of nuclei, sometimes called laminae, which extend throughout the long axis of the cord. Rexed has produced a scheme describing 10 laminae : I-VI in posterior horn, VII intermediate, VIII and IX in anterior horn and X around the central canal. Some of these produce clearly delineated columns e.g. lamina II – substantia gelatinosa, laminae III and IV – chief or proper sensory nucleus, and VII – Clarke’s nucleus (dorsalis). The substantia gelatinosa is the relay nucleus for incoming pain and temperature fibres, and Clarke’s column is the origin of the dorsal spinocerebellar tract.
Types of Neurones in Spinal Gray Matter
Neurones may be categorised into 3 main groups. The smallest and most numerous are the inter-neurones. Many receive afferents from dorsal root fibres and descending tracts and terminate on motor cells or tract cells. Motor cells (or root neurones) have cell bodies in the CNS but axons in the periphery e.g. alpha and gamma motor neurones.
Tract cells are those with cell bodies mainly in the dorsal horn and whose axons make up the ascending tracts of the lateral and anterior white columns.
This consists of ascending and descending nerve fibres. Many of the fibres are gathered together into discrete pathways known as tracts or fasciculi.
The white matter can be divided into 3 main columns or funiculi on each side of the spinal cord – anterior, posterior and lateral .
In the column there is a septum dividing it into a medial and lateral fasciculus cuneatus.
4 Ascending tracts are important clinically:
1. The posterior (dorsal) columns
This system transmits sensations of discriminatory touch, vibration and proprioception. The axons ascend on the side of entry and cross to the other side in the medulla after synapsing in the nuclei gracilis and cuneatus. The gracile tract carries input from the sacral and lumbar segments and the cuneate tract carries cervical and thoracic fibres.
2. Lateral spinothalamic tracts
These convey sensations of pain temperature and cross the midline around the level of entry.
3. Anterior spinothalamic tracts
This is a crossed pathway for simple touch-pressure. However many fibres ascend for some distance before crossing over, so the tract is essentially bilateral.
4. Spinocerebellar tracts
These provide information about the position of the body in space and relative body positions. (The posterior tract is uncrossed and the anterior one is crossed).
There are several descending pathways, many of them being -extra-pyramidal tracts which originate in the brain stem.
However, the only descending tracts of major clinical importance are the corticospinal tracts. These originate in the motor cortex, descend through the internal capsule, cerebral peduncles and anterior pons to the pyramids of the medulla where 85-90% of axons cross the midline and continue into the spinal cord as the lateral corticospinal tract. The other 10-15% continue uncrossed as the anterior corticospinal tract – but many of these fibres may also cross at their destination.
Approximately 55% of fibres in the corticospinal tracts terminate in the cervical cord (because of the fine control over musculature of the upper limb), 20% in the thoracic region and 25% in the lumbosacral segments.
In all there are 31 pairs of spinal nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal.
All the nerves are constructed in a similar way: each is attached by an anterior and a posterior root to the spinal cord. The anterior root is composed of motor fibres and the dorsal root of sensory fibres. The dorsal root also has a ganglion containing cell bodies of the sensory neurones. The 2 roots unite in the intervertebral foramen to form a common trunk – the mixed spinal nerve and from this point on the nerves are mixed. Almost immediately the trunk divides into a large ventral ramus and a smaller dorsal ramus.
The portion of spinal cord to which any one pair of spinal nerves are attached is called a spinal segment – thus there are also 31 spinal segments.
The size of the roots varies and is related to the volume of tissue supplied – so lower lumbar roots are large but thoracic roots are small. Similarly the roots vary in length and direction. This is primarily because the spinal cord is considerably shorter than the vertebral canal. The L4 nerve roots, for example, still leave between L4 and L5 although the cord ends at the Ll/L2 disc, and achieve this by elongating the roots in the subarachnoid space. Thus the upper cervical roots are short (2 cm.) and horizontal, whereas the lower are elongated (up to 25 cm.) and increasingly oblique. The lumbar sacral and coccygeal roots descending in the subarachnoid space make up the cauda equina.
The shortening of the spinal cord also results in displacement of spinal segments from their corresponding vertebral levels.
The upper 4 cervical nerves supply the neck; the lower 4 cervical nerves and first thoracic (ventral rami) pleat together to innervate the upper limb, the skin of the trunk is supplied by T2 to LT inclusive and the lower sacral nerves; the skin of the lower limb from Spinal segments L1 to S3.
Segmental Innervation of Muscles
Each spinal nerve originally supplies the musculature derived from the myotome of the same segment. Derivatives of adjoining myotomes often fuse, so that the clear segmental pattern of innervation is to some extent disrupted. For this reason sensory abnormalities are often more useful for localisation of lesions than motor ones.
Nevertheless there is an association between spinal segments and muscle innervation as Table 1 shows.
Innervation of Voluntary Muscles by Spinal Roots
|Spinal Root||Muscles Innervated||Reflex|
|T1- 6||Thoracic musculature|
|T6- 12||Abdominal musculature|
|S1 -||Gastrocnemius, plantar Flexors||Ankle jerk|