An Overview of the White Matter of the Spinal Cord : Ascending and Descending Tracts
Names of Tracts in the Spinal Cord
The White Matter on the outside of the spinal cord is composed of thousands of axons whose function is to conduct electrical signals rapidly from one site in the CNS to another.
The axons are arranged in bundles (or Tracts) that pass from the spinal cord to structures in the brain (the ascending tracts or pathways); or from the brain to the spinal cord.
Some neuronal cell bodies in the spinal cord have axons that terminate in the thalamus, and are therefore known as spino-thalamic neurones. Similarly, some cortical cells have axons that descend into the spinal cord, and are therefore known as cortico-spinal axons because they originate in the cortex and terminate in the cord.
Ascending Pathways to the Thalamus and Cerebral Cortex
Touch and Vibration.
Information about light touch and vibration pass through the dorsal columns to reach the dorsal column nuclei on the same side of the medulla. Here there is a relay and the second order neurones cross over to the opposite side of the neuraxis in the medulla to reach the thalamus. This is sometimes called the 'sensory decussation'.
Pain and Temperature.
Information about injurious stimuli and temperature are relayed in the dorsal horn of the same side of the body. Within a few segments, the axons of these neurones then pass to the opposite (contralateral) lateral columns and project to the thalamus in the spinothalamic tract, which is the 'classical' pain pathway. Other ascending tracts carrying information about nociception pass rostrally in the antero-lateral system and other smaller pathways.
The dorsal columns contain the axon collaterals of neurones that sense touch, and project rostrally to the dorsal column nuclei at the caudal end of the medulla. In lower segments of the cord the dorsal columns are thin, and these fibres remain in a medial position. Axons from higher segments of the cord are added to the lateral edge of the dorsal columns. In the cervical region there are two distinct components of the dorsal columns - the fasciculus gracilis (medial) and fasciculus cuneatus (lateral).
In some segments the dorsal columns also contain proprioceptive afferents, e.g in the lower segments of the cord, where these afferents project to Clarke's Column. And in the cervical cord where they project to the accessory cuneate nucleus.
Clarke's Column (Nucleus Proprius) exists in the ventral part of the dorsal horn and is the origin of the Spino-Cerebellar Tracts.
The corticospinal tract starts in the cerebral cortex and travels through the brain and brainstem to enter the opposite side of the spinal cord. The following describes the position of the corticospinal tract within the spinal cord.
In the spinal cord the majority of the corticospinal fibres travel down the cord in the lateral columns and innervate motneurones on the same ventral horn. These motoneurones innervate the muscles of the limbs.
A minor proportion travel in the anterior columns, before crossing over to the opposite vental horn at a segmental level. Many of these fibres make contact with motoneurones innervating the axial muscles of the trunk.
As a result, within the spinal cord, these two pathways are known as the lateral and anterior corticospinal tracts.
Syringomyelia A cyst occurs within the central canal of the spinal cord, and expands causing damage to the axons in the immediate area.
Brown-Sequard Syndrome. This is the result of a lesion which transects one side of the spinal cord. The damage is partly segmental, but has consequences for the motor and sensory innervation of the body below the level of the lesion
Tractotomy (Cordotomy). Neurosurgeons sometimes section the spino-thalamic tract in an attempt to relieve chronic intractable pain by dividing the axons in the spinal cord.
This procedure can be successful, but breakthrough pain sensation can occur after a year or so, and one reason for this may be that there are alternative ascending pain pathways in the anterolateral system and other smaller pathways..