Trigeminal nerve injury is one of the causes of chronic orofacial pain. Patients suffering from this condition have a significantly reduced quality of life. The currently available management modalities are associated with limited success. This article reviews some of the common causes and clinical features associated with post-traumatic trigeminal neuropathic pain (PTNP). A cascade of events in the peripheral and central nervous system function is involved in the pathophysiology of pain following nerve injuries. Central and peripheral processes occur in tandem and may often be co-dependent. Due to the complexity of central mechanisms, only peripheral events contributing to the pathophysiology have been reviewed in this article. Future investigations will hopefully help gain insight into trigeminal-specific events in the pathophysiology of the development and maintenance of neuropathic pain secondary to nerve injury and enable the development of new therapeutic modalities.
O lfaction is the sense of smell. It is one of the chemical senses, involving the detection of chemical stimuli and conversion of these stimuli into electrical energy for perception via the central nervous system. 1 Apart from playing a role in the determination of the flavor of food and beverages, olfaction has a role in acting as an early warning system to detect hazards. Reduction of olfactory function has been shown to be associated with loss of appetite, consumption of bad food, and, in many people, problems with cooking. 2 It has been hypothesized that olfaction aids in possible avoidance of food poisoning. 3 Olfaction also plays a significant role in the process of enjoyment of food. Most information regarding the flavor of food is thought to come from olfaction. 4 Lack of proper olfaction also has been associated with weight loss and weight gain. 5,6 The sense of smell is reported in the literature as connected to emotions, either positive or negative. 7 Quality of life is reduced significantly in patients with olfactory disorders. 8 Olfactory disorders have been reported as prominent features that can be possible early signs of neurodegenerative (ND) diseases. 9 Loss of this sensation has been attributed to be one of the first manifesting symptoms in COVID-19. 10 In our article, we highlight the basic principles underlying the physiology and pathophysiology of olfaction and its possible relationship with disease entities. We also look at the significance of olfaction as it relates to dentistry and orofacial pain. PHYSIOLOGY OF OLFACTIONThe chemosensation of olfaction is mediated via the cranial nerves CN I (olfactory) and CN V (trigeminal). 11 CN I is responsible for olfaction, whereas CN V mediates general sensory innervation including chemosensation. The olfactory epithelium present in the superior part of the nostrils contains olfactory cells, which are the receptor cells for olfaction. 12 An action potential is generated when the odorant molecule binds to the olfactory receptor. 13 The action potential is carried via the axons of these primary afferent neurons to the olfactory bulb, where the synapsing with second-order neurons occurs. 14 Anatomically, the olfactory bulb is positioned over the cribriform plate of the ethmoid bone. These second-order neurons form the olfactory tract carrying signals to the higher centers in the brain. 15 These centers include the primary and secondary olfactory cortices. 16 Figure 1 shows the gross structures involved in olfaction and olfactory pathways. DISORDERS OF OLFACTIONOlfaction disorders may be classified as congenital or acquired. Being born with an olfactory disorder is rare. 8 Quantitatively, olfactory disorders can be divided into anosmia, hyposmia, and hyperosmia. Anosmia is the inability to perceive odors. It includes total anosmia, which is an inability to perceive all odors, and partial anosmia, which is an inability to perceive some but not all odors. Reduced ability to smell is termed hyposmia. Enhanced ability to smell is termed hyperosmia, whic...
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