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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 neurons that make up the spinothalamic tract are located principally within the dorsal horn of the spinal cord. These neurons receive synaptic inputs from sensory fibers which 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 in animals) transmits touch.
The types of sensory information transmitted via the STT 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) and Indirect (for affective and arousal impact of pain).
Indirect projections include the Spino-Reticulo-Thalamo-Cortical (part of the Ascending Reticular Arousal System, aka ARAS) and the 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.
Traveling up the brainstem, the tract moves dorsally, as the neurons ultimately synapse with the thalamus.
Unilateral lesion usually causes contralateral analgesia (loss of pain) and temperature. Analgesia will normally begin 1-2 segments below the level of lesion, affecting all caudal body areas. This is clinically tested by using pin pricks.
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