The spinal cord lies within the vertebral canal which is formed by the vertebrae of the spinal column.
The spinal cord starts superiorly at the level of the foramen magnum and terminates inferiorly at the conus medullaris. This is at the level of L1/2 in the adult and L4 in the neonate. A thin strand of pia mater (the filum terminale) runs from the conus medullaris to the coccyx.
There is a total of 31 pairs of spinal nerves made up of 8 cervical; 12 thoracic, 5 lumbar, 5 sacral and 1 coccygeal nerve
- Each spinal nerve leaves the vertebral canal through the intervertebral canal
- The first cervical nerve emerges between the occiput and the arch of the atlas (C1)
- C2-C7 emerge above their respective vertebrae
- C8 emerges between C7 and T1
- Below this, each spinal nerve emerges below its corresponding vertebra
- The lumbar, sacral and coccygeal nerves descend beyond the end of the spinal cord as the cauda equine
A transverse section through the spinal cord displays the following structures
- The central canal filled with cerebrospinal fluid is at its ‘core’
- Surrounding the central canal is the ‘butterfly- shaped’ grey mattercontaining neuronal cell nuclei and demyelinated nerve fibres
- Surrounding this is the white matter, containing ascending and descending myelinated fibres
The sensory neurons enter the spinal cord via the dorsal horn and synapse, and the motor nerves emerge from the ventral horn of the spinal cord. Interneurons lying within the grey matter provide the connection between the sensory and motor neurones in the case of a reflex arc.
Spinal nerves are termed ‘mixed nerves’ because they contain both sensory and motor fibres. Prolapsed intervertebral discs impinge on the spinal nerve and cause both motor and sensory symptoms.
- The major descending (motor) pathway is the lateral corticospinal tract which is also known as the pyramidal tract. There are two major ascending (sensory) pathways; the spinothalamic tract and the dorsal (posterior) columns.
- Each of the ascending pathways carry different sensory modalities
It is important to understand the distribution of motor and sensory pathways as it facilitates the identification of symptoms and signs associated with a variety of spinal cord diseases.
A hemisection of the spinal cord produces the Brown-Séquard syndrome.
Ipsilateral: paralysis, loss of joint position, pressure, light touch and vibration sensation.
Contralateral: loss of pain, temperature and crude touch sensation.
Major Ascending Pathways
There are two major ascending (sensory) pathways – the spinothalamic tract and the dorsal (posterior) columns.
Knowledge of the distribution of the motor and sensory pathways allows an understanding of the symptoms and signs of various spinal cord diseases.
A hemisection of the spinal cord produces the Brown-Séquard syndrome – ipsilateral paralysis, loss of joint position, light touch, pressure and vibration sensation with loss of contralateral loss of pain, crude touch and temperature sensation.
- Marks the course of the corticospinal tract from motor cortex as it descends through the internal capsule (x). In the medulla (m) the tract decussates with the majority of its fibres descending as the lateral corticospinal tract. Infarction in the region of the internal capsule therefore results in contralateral paralysis.
- (b) Marks the course of the spinothalamic tract. First order neurones enter the spinal cord via the dorsal horn and synapse there. Second order neurones decussate within a few spinal levels and ascend to the ventroposterior nucleus (vp) of the thalamus where they synapse with the third order neurones which transmit to the somatosensory cortex.
- Marks the course of the dorsal column. First order neurones again enter the spinal cord via the dorsal horn. They ascend in the ipsilateral fasciculus cuneatus and gracilis. They synapse in the medulla and the second order neurones decussate and ascend to the VP nucleus of the thalamus. Here they synapse with the third order neurones which pass to the somatosensory cortex
The conus medullaris in the infant is at the level of L3, and by the age of 1 year is at the adult level of L1-2. The dural sac terminates at S2 in neonates, and S1-S2 at 1 year, although this can still be at S3 in some individuals.
Thus, a caudal injection using a needle or catheter in a neonate should avoid the spinal cord itself, but can still hit the dural sac and lead to a dural tap. At 1 year of age an epidural at L3-4 should avoid the spinal cord.
The sacrum is flatter and narrower in the neonate, so that caudal catheters are easier to thread upwards to achieve a higher level of block, if required.