Spinothalamic tract

Spinal cord

The spinothalamic tract utilises three neurons in order to transmit the sensory information from the skin to the primary sensory cortex. This begins with the pseudounipolar neurons located within the dorsal root ganglion. These neurons extend from the skin to the posterior (dorsal horn) of the spinal cord at that segmental level. These neurons are of two types:

• A delta fibres are large diameter axons that transmit (as fast as 6 milliseconds!) fast immediate pain. This type of pain is rapidly localized (stepping on a Lego piece, will certainly leave you in no doubt about the location of the injury!), and travels in the lateral spinothalamic tract.
• C fibres will make several connections in the dorsal horn before ascending. They transmit slow, aching pain that may result from inflammation. The slow type is pain is poorly localized. A dermatome map is a guide to which nerves innervate which areas of skin.

Once the neurons enter the spinal cord, they either descend or ascend a few vertebral levels. This is achieved by travelling via Lissauer’s tract (named after the nineteenth century German neurologist), which is a collection of descending and ascending collaterals of the primary neurons. Next the neurons will synapse with the secondary neurons in one of two areas of the spinal cord (the substantia gelatinosa or the nucleus proprius).

The substantia gelatinosa is a grey gelatinous mass of neuroglia and nerve cells that is located in a cap like formation at the apex of the spinal cord posterior grey matter. It extends the entire length of the spinal cord to the medulla. It contains Rexed lamina II.

The nucleus proprius is located just adjacent to the substantia gelatinosa and is a similar neural structure. Both these structures are grey due to the low concentration of myelin surrounding the nerve cells. Both the substantia gelatinosa and the nucleus proprius are referred to as ‘tract cells’. These tract cells travel one or two vertebral levels higher than the point of entry. From here the axons of the 2nd order neurons cross obliquely in the anterior gray and white commissure and ascend contralaterally in the anterolateral section of the spinal cord. 

From here they travel upwards to reach the rostral ventrolateral medulla. The fibres of the lateral spinothalamic tract ascend through the medulla, and lies between the inferior olivary nucleus and the trigeminal nerve spinal tract nucleus. The anterior spinothalamic tract now joins it, and together they form a structure called the spinal lemniscus.

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Brain

This lemniscus now ascends through the dorsal part of the pons, and reaches the midbrain. These neurons will continue to ascend and will synapse with the third order neurons that lie within various thalamic nuclei (these are the ventral posterior lateral, the ventral medial posterior and the medial dorsal). The lateral spinothalamic tract has many fibres that synapse in the ventral posterolateral nucleus of the thalamus. Crude pain, as well as temperature sensation will initiate an emotional reaction by synapsing here.

The third order neurons (ventrolateral nucleus fibres) will now travel further upwards, and pass through the posterior limb of the internal capsule, then the corona radiata to reach the primary sensory cortex i.e. postcentral gyrus. They will also synapse with the insular cortex and reach other high cortical centres i.e. the primary somatosensory cortex (postcentral gyrus), and the cingulate cortex.