Chapter 50 – The Spinal Cord

Abstract

The spinal cord is part of the central nervous system (CNS), located within the spinal canal of the vertebral column. The spinal cord begins at the foramen magnum, where it is continuous with the medulla oblongata. The spinal cord is much shorter than the vertebral column, ending at a vertebral level of L1/2 in adults, but at a lower level of around L3 in neonates.

Chapter 50 The Spinal Cord

Describe the anatomy of the spinal cord

Like the brain, the spinal cord is enveloped in three layers of the meninges: pia, arachnoid and dura mater. Cerebrospinal fluid (CSF) surrounds the spinal cord in the subarachnoid space and extends inferiorly within the dural sac to approximately the S2 level. After the spinal cord terminates, the pia and dura merge to form the filum terminale, which tethers the cord to the coccyx.

The spinal cord is divided into 31 segments, each emitting a pair of spinal nerves. There are:

Eight cervical segments. Note: there is one more pair of cervical nerves emitted than there are cervical vertebrae.
Twelve thoracic segments.
Five lumbar segments.
Five sacral segments.
One coccygeal segment.

With the exception of C1 and C2, the spinal nerves exit the spinal canal through the intervertebral foramina.

The spinal cord enlarges in two regions:

The cervical enlargement at C4–T1, corresponding to the brachial plexus, which innervates the upper limbs;
The lumbar enlargement at L2–S3, corresponding to the lumbar plexus, which innervates the lower limbs.

At the terminal end of the spinal cord:

The conus medullaris is the tapered terminal portion of the cord.
The cauda equina is the collection of spinal nerves that continue inferiorly in the spinal canal after the cord has ended, until they reach their respective intervertebral foramina.

Describe the cross-sectional anatomy of the spinal cord

In cross-section, the spinal cord is approximately oval, with a deep anterior median sulcus and a shallow posterior median sulcus. The centre of the cord contains an approximately ‘H’-shaped area of grey matter, surrounded by white matter:

The grey matter contains unmyelinated axons and the cell bodies of interneurons and motor neurons. Located in the centre of the grey matter is the CSF-containing central canal. The points of the ‘H’ correspond to the dorsal and ventral (posterior and anterior) horns. There are also lateral horns in the thoracic region of the cord, which correspond to pre-ganglionic sympathetic neurons.
The white matter contains columns of myelinated axons, called tracts. These tracts are organised into:
1. – Ascending tracts, containing sensory axons;
2. – Descending tracts, containing motor axons.

The most important ascending tracts are shown in Figure 50.1:

The dorsal (posterior) columns contain axons of nerves concerned with proprioception (position sense), vibration and two-point discrimination (fine touch).
The anterior and lateral spinothalamic tracts carry sensory information about pain, temperature, crude touch and pressure.
The anterior and posterior spinocerebellar tracts carry proprioceptive information from the muscles and joints to the cerebellum.

The most important descending tracts are (Figure 50.1):

The anterior and lateral corticospinal tracts, also known as the pyramidal tracts, carry the axons of upper motor neurons. In the ventral horn of the spinal cord, these axons relay to α-motor neurons (or lower motor neurons) that innervate muscle.
The extrapyramidal tracts: rubrospinal, tectospinal, vestibulospinal, olivospinal and reticulospinal tracts. The extrapyramidal neurons originate at brainstem nuclei and do not pass through the medullary pyramids. Their primary role is in the control of posture and muscle tone.

Figure 50.1 Cross-section of the spinal cord (extrapyramidal tracts not shown).

Describe the blood supply to the spinal cord

The spinal cord is supplied by three arteries, derived from the posterior circulation of circle of Willis (see Chapter 45). However, the blood flow through these vessels is insufficient to perfuse the cord below the cervical region – an additional contribution from radicular arteries is essential. The three spinal arteries are:

One anterior spinal artery, which arises from branches of the right and left vertebral artery (see Chapter 45, Figure 45.1). The anterior spinal artery descends in the anterior median sulcus and supplies the anterior two-thirds of the spinal cord – essentially all of the structures, with the exception of the dorsal columns. The anterior spinal artery is replenished along its length by several radicular arteries, the largest of which is called the artery of Adamkiewicz. The location of this vessel is variable, but is most commonly found on the left between T8 and L1.
Two posterior spinal arteries, which arise from the posterior inferior cerebellar arteries (see Chapter 45, Figure 45.1). The posterior spinal arteries are located just medial to the dorsal roots and supply the posterior third of the cord. Again, the posterior spinal arteries are replenished by radicular arteries.

Blood from the spinal cord is drained via three anterior and three posterior spinal veins located in the pia mater, which anastomose to form a tortuous venous plexus. Blood from this plexus drains into the epidural venous plexus.

Clinical relevance: anterior spinal artery syndrome

The artery of Adamkiewicz most commonly arises from the left posterior intercostal artery, a branch of the aorta. Damage or obstruction of the artery can occur through atherosclerotic disease, aortic dissection or surgical clamping during aortic aneurysm repair. As the anterior spinal artery supplies the anterior two-thirds of the spinal cord, cessation of blood flow can have profound consequences (Figure 50.4).

Signs and symptoms of anterior spinal artery syndrome are:

Paraplegia, as a result of involvement of α-motor neurons within the anterior horn of the cord (i.e. a lower motor neuron deficit at the level of the lesion) and the corticospinal tracts carrying the axons of upper motor neurons (i.e. an upper motor neuron deficit below the level of the lesion).
Loss of pain and temperature sensation due to involvement of the spinothalamic tracts.
Autonomic dysfunction involving the bladder or bowel due to disruption of the sacral parasympathetic neurons.

Crucially, proprioception and vibration sensation remain intact. These sensory modalities are carried in the dorsal columns, which are supplied by the posterior spinal arteries and thus remain unaffected.

Describe the main sensory afferent pathways

The somatosensory nervous system consists of:

Sensory receptors, which encode stimuli by repetitive firing of action potentials. The different sensory receptor types are specific to their sensory modalities: proprioceptors, nociceptors, thermoreceptors and mechanoreceptors relay sensory information concerning limb position, tissue damage (potentially causing pain), temperature and touch, respectively. The perception of the stimulus is dependent upon the neuronal pathway rather than the sensory receptor itself. For example, pressing on the eye activates the optic nerve and gives the impression of light, despite the stimulus being pressure rather than photons.
First-order neurons transmit action potentials from sensory receptors to the spinal cord, where they synapse with second-order neurons. These neurons are pseudounipolar, with their cell bodies located in the dorsal root ganglion, a swelling of the dorsal root just outside the spinal cord.
Second-order neurons conduct action potentials to the thalamus, where they synapse with third-order neurons.
Third-order neurons relay action potentials to the cerebral cortex via the internal capsule.
The primary somatosensory cortex is the area of the cerebral cortex that receives and performs an initial processing of the sensory information. It is organised in a somatotropic way with specific areas of cortex dedicated to specific areas of the body, known as the sensory homunculus. Of note: the hands and lips make up a major component, reflecting their tactile importance.

There are two major pathways by which sensory information ascends in the spinal cord:

The dorsal column–medial lemniscal (DCML) pathway carries sensory information about two-point discrimination, vibration and proprioception (Figure 50.2a). The name of the pathway comes from the two structures through which the sensory signals pass: the dorsal columns of the spinal cord and the medial lemniscus in the brainstem:
1. – The first-order neuron is extremely long. It enters the dorsal root of the spinal cord and ascends in the dorsal columns on the same side (ipsilateral). Sensory neurons from the lower body travel in the medial gracile tract and synapse in the gracile nucleus in the medulla oblongata, whilst sensory neurons from the upper body travel in the lateral cuneate tract and synapse in the cuneate nucleus.¹
2. – In the medulla, first-order neurons synapse with second-order neurons, which then cross over to the contralateral side and ascend to the thalamus. After this sensory decussation, the fibres ascend through the brainstem in a tract called the medial lemniscus.
The spinothalamic tract carries sensory information about crude touch, pressure, temperature and pain (Figure 50.2b). In contrast to the DCML pathway, the spinothalamic tract crosses the midline at the level of the spinal cord rather than the medulla:
1. – The first-order neurons enter the dorsal root of the spinal cord and may ascend or descend one or two vertebral levels (along Lissauer’s tract) before synapsing with second-order neurons in the dorsal horn.
2. – The axons of the second-order neurons decussate anterior to the central canal of the spinal cord, in an area called the anterior commissure, before ascending to the thalamus in the contralateral spinothalamic tract.