Neurophysiological aspects of the trigeminal sensory system: an update

Abstract

<jats:title>Abstract</jats:title><jats:p>The trigeminal system is one of the most complex cranial nerve systems of the human body. Research on it has vastly grown in recent years and concentrated more and more on molecular mechanisms and pathophysiology, but thorough reviews on this topic are lacking, certainly on the normal physiology of the trigeminal sensory system. Here we review the current literature on neurophysiology of the trigeminal nerve from peripheral receptors up to its central projections toward the somatosensory cortex. We focus on the most recent scientific discoveries and describe historical relevant research to substantiate further. One chapter on new insights of the pathophysiology of pain at the level of the trigeminal system is added. A database search of Medline, Embase and Cochrane was conducted with the search terms ‘animal study’, ‘neurophysiology’, ‘trigeminal’, ‘oral’ and ‘sensory’. Articles were manually selected after reading the abstract and where needed the article. Reference lists also served to include relevant research articles. Fifty-six articles were included after critical appraisal. Physiological aspects on mechanoreceptors, trigeminal afferents, trigeminal ganglion and central projections are reviewed in light of reference works. Embryologic and anatomic insights are cited where needed. A brief description of pathophysiology of pain pathways in the trigeminal area and recent advances in dental stem cell research are also discussed. Neurophysiology at the level of the central nervous system is not reviewed. The current body of knowledge is mainly based on animal and cadaveric studies, but recent advancements in functional imaging and molecular neuroscience are elucidating the pathways and functioning of this mixed nerve system. Extrapolation of animal studies or functioning of peripheral nerves should be warranted.</jats:p>

Sensory interaction theory: revision of the craniofacial region

“…The unilateral stimulation of the tongue with sodium chlorate in patients with an injury in the contralateral chorda timpani nerve generated bilateral nuclear activation at the brainstem [65]. Gustative impairment after trigeminal surgery indicates the existence of central and peripheral sensorial interaction, as supported by animal studies [21,66]. Injury to the lingual nerve, which has both trigeminal nerve and facial nerve fibres, leads to the faster regeneration of large fibres to the detriment of the smaller ones (pain, temperature and gustation), and corresponds to the symptoms of pain and dysgeusia in patients [67].…”

“…The trigeminal system is the largest and most complex somatosensory system in the human body. There is an intense convergence in it of inputs from the oral and nasal mucosa, cornea, facial skin, lips, teeth, nose, dura mater, tongue, deep tissues and part of the auditory canal, which are processed in the central nervous system along with adjacent somatosensory inputs mediated by other cranial nerves (VII, IX, X) [18][19][20][21]. Its complexity is closely related to the evolutionary importance of this body area in survival and interaction with the environment and other beings [22].…”

“…vision, odours, oral sensations that coordinate chewing and swallowing with breathing) and interoceptive sensory inputs (e.g. levels of glucose, O 2 , CO 2 in the blood flow) [18,21].…”

“…They generate precise responses determined by anatomic circuits that are potentiated according to the exposition to sensory stimuli, stored as memories [91,92]. Consistency of perception depends on sensory integration according to the size of receptive fields of neurons, the correct inhibition of undesired stimuli and the convergence of data on cortical areas of association, which are more functional than topographic [21,46,93]. To understand craniofacial perception, it is essential to comprehend its multisensorial functions [94].…”

An improved animal model for herpesvirus encephalitis in humans

“…These neurons are synaptically connected to three trigeminal nuclei present in the myelencephalon (spinal trigeminal nucleus, Sp5), MET (principal sensory trigeminal nucleus, Pr5) and mesencephalon (MES) (mesencephalic trigeminal nucleus, Me5). Second order neurons from the trigeminal nuclei project to the ventral posteromedial nucleus of the TH [31] whereas third order neurons originating from TH eventually reach the primary somatosensory cortex (SSC) in the parietal lobe (PL) [32]. For PrV, invasion of the CNS is also facilitated through sympathetic and parasympathetic nerve endings including their autonomic ganglia as well as through the facial nerve [27][28][29].…”