DIRECT PATHWAY OF PAIN AND TEMPERATURE (AND REMAINING FIBERS OF TOUCH) SENSES
The central branches of neurons carrying impulses from pain and temperature (and some touch—crude touch) sense organs (first-order neurons) synapse with neurons located in the dorsal horn. The axons of the latter (second-order neurons) cross the midline and ascend in the anterolateral part of the spinal cord as the ventral (anterior) and lateral spinothalamictracts (Figure 5.34B). Some fibers end in a specificregion in the thalamus where they synapse with third-order neurons that convey the impulses to the cerebral cortex. Other fibers synapse with the reticu-lar formation to maintain alertness. The anterior spinothalamic tract primarily carries impulses for itch, tickle, pressure, and crude touch sensations, and the lateral spinothalamic tract carries impulses for pain and temperature.
Although a specific pathway has been described for pain sensations, the perception of pain can be modified in various ways; hence, the major differ-ences seen in pain perception between individuals. Definition, pain theory, pain mechanism and re-sponse, type of pain.