Springer Handbook of

Abstract

The trigeminal nerve is the fifth and thickest cranial nerve and not only it is responsible for facial sensation and motor functioning, but it is also responsible for chemosensory perception. On top of innervating the skin of the face, the trigeminal nerve also innervates the mucosa of the nose and mouth. Here, chemosensory perception starts with the activation of different receptors, the most important and best known ones being the transient receptor potential (TRPtransient receptor potential (TRP)) channels which give rise to sensations such as burning, warmth, coolness, coldness, and pain. From the mucosa, the trigeminal chemosensory information is conveyed through the trigeminal ganglion to the thalamic nuclei in the brain stem; from here fibers project to both, the somatosensory cortex and chemosensory areas of the brain. Most odors stimulate the trigeminal system, in addition to the olfactory system, especially in higher concentrations. However, overlaps between both sensory systems are not limited to the stimulus level, as they interact with each other on peripheral (mucosa) and central (brain) levels. As a consequence, subjects with a lacking olfactory system show lower trigeminal sensitivity and subjects with a lack of trigeminal sensitivity show lower olfactory activations. Different techniques are available to assess the state and the functionality of the trigeminal system. Such techniques include behavioral assessment by testing participants lacking a sense of smell – in order to exclude olfactory interference – as well as administrating different types of stimuli (pure odorants, pure trigeminal, or a mix of both). More objective measures include electrophysiological methods that evaluate the peripheral and central activations via the negative mucosal potentials (NMPs) and the trigeminal event-related potentials (ERPevent-related potential (ERP)s), respectively, as well as functional magnetic resonance imaging, and to a lesser extent, positron emission tomography (PETpositron emission tomography (PET)).