The first step in any somatosensory perception involves the activation of primary sensory neurons whose cell bodies reside within dorsal root ganglia (DRG) and cranial sensory ganglia. The somatosensory system serves three major functions exteroreceptive and interoceptive, for our perception and reaction to stimuli originating outside and inside of the body, respectively, and proprioceptive functions, for the perception and control of body position and balance. Understanding how we perceive and react to the physical world is rooted in our understanding of the sensory neurons of touch. Indeed, sensations emanating from a cadre of touch receptors, the sensory neurons that innervate our skin, can be qualitatively different. However, it was Johannes Muller who, in 1842, introduced the concept of sensory modalities ( Müller, 1842), prompting us to ask whether nerves that convey different qualities of touch exhibit unique characteristics. How do we recognize and interpret the myriad of tactile stimuli to perceive the richness of the physical world? Aristotle classified touch, along with vision, hearing, smell, and taste, as one of the five main senses. The flutter of an insect’s wings, a warm breeze, a blunt object, raindrops, and a mother’s gentle caress impose mechanical forces upon the skin, and yet we encounter no difficulty in telling them apart and react differently to each. Our tactile world is rich, if not infinite. An integrative model in which ensembles of impulses arising from physiologically distinct LTMRs are integrated and processed in somatotopically aligned mechanosensory columns of the spinal cord dorsal horn underlies the nervous system’s enormous capacity for perceiving the richness of the tactile world. We discuss the spinal cord neurophysiological representation of complex mechanical forces acting upon the skin and current views of how tactile information is processed and conveyed from the spinal cord to the brain. Here, we review the properties and functions of LTMRs, emphasizing the unique tuning properties of LTMR subtypes and the organizational logic of their peripheral and central axonal projections. The first step leading to perception of innocuous touch is activation of cutaneous sensory neurons called low-threshold mechanoreceptors (LTMRs). The somatosensory system decodes a wide range of tactile stimuli and thus endows us with a remarkable capacity for object recognition, texture discrimination, sensory-motor feedback and social exchange.
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