Structures

• Vertebral column
  
• Meninges
  
• Spinal cord

Vertebral Column

The vertebral column is made up of seven cervical, 12 thoracic, five lumbar, five fused sacral and four fused coccygeal vertebrae (33 in total) separated by intervertebral discs. The primary and secondary curvatures give the classical sinusoidal pattern with cervical and lumbar lordosis, thoracic and pelvic kyphosis.

A typical vertebra has a vertebral body situated anteriorly and the vertebral arch posterolaterally thereby enclosing the vertebral canal containing the spinal cord.

• Vertebral body – anterior and weight bearing
  
• Vertebral arch made of
  
  
  
  • Two transverse processes – posterolateral projections
    
  • Two pedicles connecting the body to the transverse process
    
  • Single spinous process posteriorly
    
  • Two laminae – between transverse process and spinous process
  
  
• The intervertebral foramina are present between the successive pedicles and transmit the spinal nerve and radicular vessels
  
• Superior and inferior articular processes with their articular facets connect adjacent vertebral arches

The array of ligaments present posteriorly adjoining the vertebral arch, which are pierced by the needle during spinal anaesthetic, as shown in Figure 4.1, include

• Supraspinous ligament – connects the tips of spinous processes
  
• Interspinous ligament – connects the facing borders of the adjacent spinous processes
  
• Ligamentum flavum – connects the facing borders of adjacent laminae

Vertebra

There are 33 vertebrae in the human body – seven cervical, 12 thoracic, five lumbar, five sacral and four coccygeal. They are distinct in sizes and shapes, and the anatomy of a typical cervical, thoracic and lumbar vertebrae is summarised below. (See also Figure 4.2).

Comparison of cervical, thoracic and lumbar vertebrae

Cervical spines – C1 and C2

C1 (Atlas) has no vertebral body or spinous process. It has lateral masses bound by the posterior and anterior arch. The anterior arch contains a facet which articulates with the dens of C2 (axis) and is supported by the transverse ligament which connects the two lateral masses. The atlanto-dens interval (predendate space) is the distance between the odontoid process and the anterior arch of the atlas. A distance of >3 mm in adults (>7 mm in children) signifies atlanto-axial subluxation and hence the patient is at risk of cervical cord injury if manipulated.

Possible clinical application questions…

1. 1. How do you clear C-spines in a multi-trauma patient?
   
2. 2. What physiological changes happen with spinal cord injury?
   
3. 3. How can you prevent secondary injury?

Intervertebral discs

The intervertebral discs lie between the vertebral bodies and are responsible for 25% of total height of the vertebral column. They are important in providing structural support and helping with movement.

Discs are bounded anteriorly and posteriorly by the anterior and posterior longitudinal ligaments, respectively.

The intervertebral disc comprises of

• An outer annulus fibrosus – made of type I and type II collagen produced by fibroblast cells, organised as lamellae
  
• An inner nucleus pulposus – formed of type II collagen and proteoglycans produced by the chondrocyte-like cells, holds water within the disc
  
• Two vertebral endplates – also formed by proteoglycans and type II collagen, produce a layer of the hyaline cartilage between discs and the adjacent vertebral bodies

Blood supply

• In adults, most parts of the disc are avascular. The peripheral annulus receives its nutrients from segmental arteries, which are branches of the aorta.

Nerve supply

• Recurrent sinuvertebral nerve, which arises in the dorsal root ganglion

Spinal Cord and Tracts

The spinal cord is the specialised nerve tissue continuous with the medulla oblongata, enclosed circumferentially by the spinal meninges and suspended in the cerebrospinal fluid. It travels the vertebral column in the neural arch and terminates at L1/2 vertebral level in adults (L3/4 at birth) and measures about 45–50 cm in adults (Figure 4.3).

• Conus medullaris: tapered end of spinal cord at its termination
  
• Cauda equina (horse’s tail): bunch of spinal nerves at the conus medullaris (contains lumbar, sacral and coccygeal spinal nerves)
  
• Filum terminale: a thin strand of pia mater with no neural tissue that connects the conus medullaris to the coccyx.

Cross section of spinal cord

It is oval in shape with a deep anterior fissure and a shallow posterior septum. Anterior and posterior nerve roots arise on the lateral surface and combine to form the spinal nerves at the intervertebral foramen. They soon divide into the anterior (sensory and motor innervation to the front of the body) and posterior (sensory and motor supply to the back) primary rami. The central canal or the ependymal canal is a CSF filled space which is a continuation of the fourth ventricle and runs the entire length of the spinal cord.

The grey matter is H-shaped and contains cell bodies of interneurons and motor neurons, as well as neuroglial cells and unmyelinated axons. The white matter consists of bundles of myelinated axons which form the various ascending and descending tracts (Figure 4.4).

Descending tracts (motor)

• Lateral corticospinal tract (crossed pyramidal tract): fibres carry voluntary motor activity from cortex, decussate in the medulla and descend the spinal cord on the contralateral side.
  
• Anterior corticospinal tract (uncrossed pyramidal tract): voluntary motor activity from the cortex reaches the spinal cord without decussation.
  
• Tecto spinal tract: this extrapyramidal tract causes movement of the head in response to visual and auditory stimuli from the midbrain tectum to the contralateral spinal cord.
  
• Rubro spinal tract: this extrapyramidal tract regulates voluntary movements and reflexes. Fibres originate from the red nucleus of midbrain, cross to the opposite midbrain and then descend down the spinal cord.

Ascending tracts (sensory)

• Anterior and posterior spinocerebellar tracts: posterior spinocerebellar tract ascends on the ipsilateral side of the spinal cord to enter the cerebellum whilst the anterior tract does a ‘double cross’, meaning it crosses to the contralateral side of the spinal cord initially but then crosses back to the ipsilateral cerebellum. They both carry proprioception, fine touch and vibration modality from the golgi tendon and muscle spindles.
  
• Anterior and lateral spinothalamic tracts: fibres carrying touch, pain and temperature sensation ascend for a few segments and then cross to the contralateral side of the spinal cord to reach the thalamus and cortex.
  
• Posterior columns: the medial tract of Goll (fasciculus gracilis – transmits information from the lower parts of the body – legs and trunk) and the lateral tract of Burdach (fasciculus cuneatus – transmits information from the upper parts of the body – neck, trunk and arms) carry the sensation of fine touch and position sense and reach the medulla uncrossed, but then decussate to reach the thalamus and cortex.

What are the neurological features that occur in various types of spinal cord injury?

This depends on the cause and type of injury (Table 4.1).

Table 4.1 Features in Various Types of Spinal Cord Injury

Meningeal Layers

Meninges are the connective tissue layers surrounding the spinal cord. They are

• Duramater: a tough, outer layer that envelops the spinal cord. Superiorly it attaches to the foramen magnum and continues as the cranial dura; inferiorly it ends at S2.
  
• Arachnoid: a delicate tissue which is avascular and terminates at S2
  
• Piamater: the innermost layer which is adherent to cord and ends with the cord at L1/2

The presence of these layers gives rise to three spaces

• Epidural space: situated between the vertebral canal and the dura
  
• Subdural space: potential space at the dura-arachnoid interface which is pressed against the dura due to CSF pressure
  
• Subarachnoid space: between arachnoid and pia and contains the CSF

Cerebrospinal fluid

The cerebrospinal fluid (CSF) is produced by the epithelium of the choroid plexus in the lateral, third and fourth ventricles. From the third ventricle, the CSF flows to the fourth ventricle through the aqueduct of Sylvius. It then drains into the subarachnoid space through the foramens of Magendie (medial) and Luschka (lateral) before being reabsorbed by the arachnoid villi in the dural sinuses.

The total volume is around 150 ml of which 25 ml flushes the spinal theca and the daily production is around 600 ml. The CSF pressure measures 5–10 cmH₂O in the lateral/supine posture which increases to 30–40 cmH₂O in the sitting position.

How does the composition of CSF differ from that of plasma?

Table 4.2 Differences between CSF and Plasma

*Because of the substantially low protein, CSF has less buffering capacity.

Circulation

• Spinal cord blood supply

Spinal Cord Circulation

The spinal cord derives its blood supply from a single anterior spinal artery (ASA), paired posterior spinal arteries (PSA) and by the communicating segmental arteries and the pial plexus.

• ASA: single artery formed at the foramen magnum by the union of each vertebral artery. Blood flows centrifugally supplying the anterior two-thirds of the spinal cord in front of the posterior grey column.
  
• PSA: derived from the posterior inferior cerebellar artery (PICA) or vertebral artery, with blood flowing centripetally in this arterial system. The arteries lie along the posterolateral surface of the spinal cord medial to the posterior nerve roots.
  
• Pial arterial plexus: surface vessels branch from the ASA and PSA forming an anastomosing network that penetrates and supplies the outer portion of the spinal cord.
  
• Segmental branches: segmental or radicular branches arise from vertebral, deep cervical, costo-cervical, aorta and the pelvic vessels. There are 20–40 pairs of radicular arteries all of which are important but the one below is the largest.

Arteria radicularis magna

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