The pathway of the protopathic sensibility originates from small nerve cells in the spinal ganglia, called protopathic neurons. They send thin, poorly myelinated or unmyelinated nerve fibers for the senses of pain and temperature. Their centripetal axons enter the spinal cord through the lateral part of the posterior root. They bifurcate in Lissauer’s tract and terminate in the dorsal border region of the substantia gelatinosa and in the posterior horn of the grey substance of spinal cord. The secondary fibers cross to the opposite side and ascend in the anterolateral funiculus as lateral spinothalamic tract (2nd neuron). The tract does not form a discrete fiber bundle but consists of loosely arranged fibers that are mixed with fibers of other systems. The fibers entering at various root levels join ventromedially. Thus, the sacral fibers lie at the surface, and the cervical fibers that joined last lie in the inner part of the anterolateral funiculus.
In the medulla oblongata, the lateral spinothalamic tract (spinal lemniscus) is located at its lateral margin above the olive and gives off numerous collaterals to the reticular formation. Here, too, a considerable portion of the fibers (spinoreticular tract) terminate. The reticular formation is part of the ascending activation system, the stimulation of which puts the organism into a state of alertness. Hence, the impulses transmitted via the pain pathway not only cause a conscious sensation but also increase the attention via the reticular formation. By contrast, the pathway of the epicritic sensibility runs through the brain stem without giving off any collaterals.