疼痛与痒神经机制的异同:感受、传导与调控

Neural similarities and differences between pain and itch:Reception,transmission and modulation

疼痛和痒是人体最重要的两种保护性躯体感觉,它们会导致个体产生不同的保护性反射行为.疼痛和痒在感觉信息传导过程中存在一些相似之处,但两者是否共享神经通路,目前还存在很大争议.本文以近些年神经影像和电生理学领域的研究结果为主,从躯体感觉在外周和中枢神经系统之间上行传导和下行调控的角度系统总结和比较了疼痛和痒在神经机制方面的异同.首先,本文从外周神经系统、脊髓水平以及大脑水平3个层面阐述了疼痛与痒的神经编码机制及其异同点;其次介绍了两者中枢调控系统的异同,并从感觉缓解与奖赏系统激活的角度阐述两者神经机制的关系.现有研究结果表明,疼痛和痒之间并不是单纯的独立、重叠或拮抗关系,而是存在着复杂的交互作用,并且疼痛与痒的缓解也与奖赏系统存在双向调控关系.

Both pain and itch are important protective sensations of the human body,which could induce reflective behaviors such as withdraw and scratch when facing danger.Pain and itch have some similarities in the process of sensory information transmission and modulation,but whether the two sensations share the same neural pathway is still controversial.Most of the existing research focuses on a single level(e.g.,neurons,fibers,or brain regions)to explore the relationship between two sensations.Here,we discussed the similarities and differences of neural mechanisms between pain and itch perceptions from bottom-up and top-down pathways of the peripheral and central nervous system based on recent neuroimaging and electrophysiological studies.First,we reviewed the neural coding mechanisms of pain and itch on the peripheral nervous system and central nervous system.Existing research based on the peripheral nervous system shows that the itch reception and nociception induced by different stimuli,such as heat pain,mechanically pain,histamine dependent and independent itch,selectively transmit through different primary afferent nerves,including Aδ and C fibers.Furthermore,we summarized the roles of the spinothalamic tract and the spinoparabrachial pathway in the sensory information transmission of pain and itch sensations separately at the spinal cord level.Then we compared the specific brain regions related to these two sensations,including the insula,amygdala,cingulate cortex,primary and secondary sensory cortex,which composed a"matrix"responsible for producing the perception of itch and pain but with different features.Second,we introduced the descending pain modulation system with critical regions of the periaqueductal gray(PAG)and the rostral ventromedial medulla(RVM),and we compared the different modulation mechanisms of pain and itch perceptions in the central nervous system.Meanwhile,we explained the pain and itch analgesic mechanism in the view of bidirectional modulation involved the reward system,which usually considered merely the opposite of aversive somatosensory system.The existing research conclusion showed that the neural mechanisms of pain and itch are not purely independent,overlapping,or antagonistic,but have very complex interactions.To reveal the underlying mechanisms of complex interactions,more precise neurological techniques(e.g.,high-resolution head-neck combined imaging)are needed for future studies.To distinguish the mechanisms of pain and itch sensations are very important to the targeted clinical treatment.Our review summarized their similarities and differences,which could provide some new insights to explore the useful treatment directions.