The trigeminal pathways.

The role of the trigeminal system in facial and dural sensitivity has been recognized for a long time. More recently, the trigeminal system has also been considered a prominent actor in brain nociceptive innervation. It is the anatomical substrate of several frequent conditions, such as primary or secondary headaches, trigeminal neuralgia, and other orofacial pains. Appreciation of the delicate anatomical arrangement of the trigeminal pathway is one of the keys to understanding these conditions' pathophysiology and to proposing innovative treatments. This review provides a structured presentation of existing knowledge about the trigeminal system, from classical anatomical data to the most recent literature. First, we present the organization of the trigeminal pathway from the trigeminal divisions, nerve, and nuclei to the thalamus and somatosensory cortex. We describe the neurons and fibers' repartition at each level, depending on the location (somatotopic organization) and the type of receptors (modal organization). Such a dual somatotopic-modal arrangement of the trigeminal fibers is especially clear for the juxtapontine segment of the trigeminal nerve and the trigeminal nuclei of the brainstem. It has significant clinical consequences both for diagnosis and treatment. Second, we explore how the trigeminal system is modulated and involved in reflexes, for instance the trigemino-cardiac and the trigemino-autonomic reflexes, which could play an essential role in the autonomic symptoms observed in cluster headache. Finally, we present how to interact with this complex system to relieve pain mediated by the trigeminal system. This section covers both neuromodulatory and lesional approaches.