Some pathways concerned with transmission of impulses activated by peripheral painful and thermal stimuli

(A) Entering peripheral nerve fibers ascend and descend several segments and then cross over to ascend to the thalamus on the opposite side of entry (L: lower extremity; U: upper extremity; F: face). (B) Many fibers terminate in the mesencephalon. Layering of fibers in the lateral spinothalamic tract is shown in (C) and details of synapses in (D). Pain and temperature fibers from the face descend from the midpontine level to the upper cervical levels before postsynaptic fibers synapse and cross to the opposite side.