Somatic sensory cranial nerve pathways are homologous in many ways to the medial lemniscal and spino-thalamic pathways of spinal nerves. The trigeminal nerve, or cranial nerve V, is the primary sensory nerve involved in transmitting touch and pain information from the face and neck. It contains fibers from mechanoreceptors, thermoreceptors, and nociceptors for the face, oral mucosa, distal part of the tongue, portions of the dura mater covering of the cerebral cortex, and tooth pulp. First-order neuronal cell bodies are located in the trigeminal ganglia adjacent to the brain stem. Central fibers enter the pons and separate into two main tracts. Axons that carry information about light touch and proprioception synapse in the principal or main sensory nucleus of cranial nerve V, which in turn sends fibers across the midline to ascend to the ventral posterior medial nucleus (VPM) of the thalamus. This pathway is analogous to the medial lemniscal pathway ofthe spinal cord. First-order neurons carrying information about pain and temperature send fibers to the spinal trigeminal nucleus located in the lower medulla. Second-order axons cross the midline and ascend to the VPM and intralaminar thalamic nuclei in a manner similar to the spinothalamic pathway of spinal nerves.
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