The spinothalamic tract is a sensory pathway originating in the spinal cord that transmits information about pain, temperature, itch and crude touch to the thalamus. The pathway decussates at the level of the spinal cord, rather than in the brainstem like the posterior column-medial lemniscus pathway and corticospinal tract.
The cell bodies of neurons that make up the spinothalamic tract are located principally within the dorsal horn of the spinal cord. These neurons receive input from sensory fibers that innervate the skin and internal organs.
There are two main parts of the spinothalamic tract (STT):
- The lateral spinothalamic tract transmits pain and temperature.
- The anterior spinothalamic tract (or ventral spinothalamic tract) transmits touch.
The types of sensory information transmitted via the spinothalamic tract are described as affective sensation. This means that the sensation is accompanied by a compulsion to act. For instance, an itch is accompanied by a need to scratch, and a painful stimulus makes us want to withdraw from the pain.
There are two sub-systems identified:
- Direct (for direct conscious appreciation of pain)
- Indirect (for affective and arousal impact of pain). Indirect projections include
- Spino-Reticulo-Thalamo-Cortical (part of the ascending reticular arousal system, aka ARAS)
- Spino-Mesencephalo-Limbic (for affective impact of pain).
Path of sensation
The Spinothalamic Tract, like the Dorsal Column-Medial Lemniscus Tract, use three neurons to convey sensory information from the periphery to conscious level at the cerebral cortex.
Pseudounipolar neurons (those with only one long process) in the dorsal root ganglion have axons that lead from the skin into the dorsal spinal cord where they synapse with secondary neurons in the marginal nucleus. These secondary neurons are called tract cells.
The axons of the tract cells cross over to the other side of the spinal cord via the anterior white commissure, and to the anterolateral corner of the spinal cord (hence the spinothalamic tract being part of the anterolateral system). The axons travel up the length of the spinal cord into the brainstem. Pain travels through spinothalamic tracts, and crosses over in the spinal cord at the point it enters, and then travels up contralaterally.
Traveling up the brainstem, the tract moves dorsally. The neurons ultimately synapse with third-order neurons in several nuclei of the thalamus -- including the medial dorsal, ventral posterior lateral, and ventral medial posterior nuclei. From there, signals go to the cingulate cortex, the primary somatosensory cortex, and insular cortex respectively.
Unilateral lesion usually causes contralateral anaesthesia (loss of pain) and temperature. Anaesthesia will normally begin 1-2 segments below the level of lesion, affecting all caudal body areas. This is clinically tested by using pin pricks.