Complex sentence: The ascending pain and temperature fibers of the spinothalamic tract also send information to the PAG via the spinomesencephalic tract (so-named because the fibers originate in the spine and terminate in the PAG, in the mesencephalon or midbrain).
Split into simpler sentences: The ascending pain and temperature fibers of the spinothalamic tract also send information to the PAG via the "spinomesencephalic tract". The spinomesencephalic tract is so-named because the fibers originate in the spine and terminate in the PAG, in the mesencephalon (midbrain).