It takes two: intranasal trigeminal chemosensation and its role in odor processing

Odor perception is a complex, multimodal experience mainly shaped by the interaction between the olfactory and trigeminal systems. Descriptors such as warm, fresh, or spicy reflect the contribution of chemosensory input from the trigeminal nerve, which adds thermal and tactile dimensions to odor perception. The trigeminal nerve innervates the head, including the nasal cavity; its fibers express several transient receptor potential channels to which odorant molecules can bind. Despite its sensory function and its putative impact on olfactory processing, the chemosensory ability of the trigeminal system has received comparatively little attention. This review examines the molecular and physiological foundations of trigeminal chemosensation, highlighting transient receptor potential channels broad sensitivity, their perceptual roles, and their interactions with the olfactory system. Assessing nasal trigeminal chemosensory function presents several methodological challenges. Here, we explore the tools available for studying the complexity of trigeminal chemosensory encoding ex vivo and in vivo in animal and human models. These techniques have demonstrated that, although the trigeminal and olfactory systems are distinct sensory modalities, they converge at multiple processing stages within the nervous system, including the olfactory epithelium (OE), the olfactory bulb, and other brain regions. In humans, this convergence leads to the activation of overlapping brain regions, resulting in perceptual modulation where information from the trigeminal system enhances or suppresses the response of the olfactory system. As a consequence of this intimate connection, olfactory dysfunction is often accompanied by reduced trigeminal sensitivity. Therefore, we examine the involvement of the trigeminal system in conditions of olfactory dysfunction.