The spinothalamic tract is an ascending pathway of the spinal cord . It is responsible for the transmission of pain, temperature, and crude touch to the somatosensory region of the thalamus. In this article we will discuss the anatomy of the tract, its location, functions as well as its clinical relevance.
The spinothalamic tract is also known as the ventrolateral system or anterolateral system. It is a sensory tract that transmits information from the skin to the thalamus in the brain. The anterolateral system is composed of:
- The anterior and lateral spinothalamic tracts. The former helps localize crude touch and pressure, the latter painful or temperature sensation.
- The spinoreticular tract, which is responsible for increasing our level of arousal/alertness in response to the pain or temperature. The fibres ascend from the muscles, joints and skin to synapse in the reticular formation.
- The spinotectal tract, which enables us to orient our eyes and move our head toward the relevant stimulus. The fibres ascend to synapse in the superior colliculi of the midbrain.
Once the sensory information has been transmitted to the thalamus, it is sent to the postcentral gyrus , or primary sensory cortex. This is accurately represented on the cortex of the brain in various diagrams, as a homunculus. The most sensitive parts of our body (lips, hands) have larger areas representing them. The legs and genitalia lie on the medial surface of the brain (the anterior most gyrus of the parietal lobe), and the face, hands, arms etc. lie on the lateral surface.
The spinothalamic tract is divided into two further tracts. These are the lateral and anterior spinothalamic tracts. The lateral pathway transmits both temperature and pain information. The anterior tract transmits crude touch and pressure information. This tract (unlike the corticospinal tract, or the dorsal column pathway/medial lemniscus pathway) decussates (crosses) at the level of the spinal cord, rather than at the brainstem. The tract is organised into a somatotopic map, i.e. the cervical section is most medial, and the sacral segment most lateral (with the thoracic and lumbar components in between).
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.
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.
The sensory information that the spinothalamic tract transmits is designed to initiate an action in the person. This is referred to as ‘affective sensation’ e.g. a hot object causes us to withdraw, a sharp object causes us to withdraw, an itch causes us to scratch.
There are two subsystems within the tract that have slightly different roles. These are the direct and indirect systems. The indirect system is further subdivided into the spino-reticulo-thalamo-cortical pathway (this is a part of the reticular arousal system which maintains consciousness), and the spino-mesencephalic-limbic pathway (main role is to cause the pain to have an impact and cause a reaction to it).
Lesions of the Spinothalamic tract
We can contrast the spinothalamic tract with the dorsal column/medial lemniscus pathway. The former crosses/decussates at the level of the spinl nerve, where as the latter crosses at the level of the medulla. If a lesion occurs in the brainstem or higher, the patient presents with loss of pain perception, crude touch and temperature sensation contralateral (other side) to the lesion. However with spinal cord lesions, the loss of is ipsilateral.
If we compare this to the dorsal column/medial lemniscus pathway (the dorsal column/medial lemniscus pathway transmits proprioceptive and vibration information) a spinal cord lesion will cause an ipsilateral (same side) loss of proprioception and vibration sense. Lesions in the medulla or above will cause contralateral loss due to the higher decussation. This pattern of sensory loss is called ‘dissociated sensory loss’.
Chronic pain can be caused by a variety of factors, such as:
- injury to the nerve endings
- scar formation in the nerves
- aberrant reinnervation following nerve damage, causing the person to interpret touch as pain (allodynia)
Repeated stimulation of the C fibres e.g. through chronic joint inflammation, will cause repeated release of glutamate. In turn, this can cause a ‘winding up’ effect and ultimately make the individual more prone to that pain.