Pleasure
In recent years a growing body of evidence has been accumulating, from anatomical, psychophysical, electrophysiological, and neuroimaging studies, that a further submodality of afferent, slowly conducting, unmyelinated C‐fibers exists in human hairy skin that are neither nociceptive nor pruritic, but that respond preferentially to low force, slowly moving mechanical stimuli. These nerve fibers have been classified as C‐tactile afferents (CT‐afferents) and were first described by Nordin [18] and Johansson et al. [19]. Evidence of a more general distribution of CT‐afferents have subsequently been found in the arm and the leg, but never in glabrous skin sites such as the palms of the hands or the soles of the feet [20]. It is well known that mechanoreceptive innervation of the skin of many mammals is subserved by A and C afferents, but until the observations of Nordin and Vallbo C‐mechanoreceptive afferents in human skin appeared to be lacking entirely. The functional role of CT‐afferents is not fully known, but their neurophysiological response properties, fiber class, and slow conduction velocities preclude their role in any rapid mechanical discriminative or cognitive tasks, and point to a more limbic function, particularly the emotional aspects of tactile perception [21]. However, the central neural identification of low‐threshold C mechanoreceptors, responding specifically to light touch, and the assignment of a functional role in human skin has only recently been achieved. In a study on a unique patient lacking large myelinated Ab‐fibers, it was discovered that activation of CT‐afferents produced a faint sensation of pleasant touch, and functional neuroimaging showed activation in the insular cortex but no activation the primary sensory cortex, identifying CT‐afferents as a system for limbic touch that might underlie emotional, hormonal, and affiliative responses to skin–skin contacts between individuals engaged in grooming and bonding behaviors – pleasant touch [22]. If pain is elicited via sensory C‐ and Aδ‐fibers then it is reasonable to speculate that the same system may be alternatively modulated to deliver a sensation of pleasure. A study employing the pan‐neuronal marker PGP9.5 and confocal laser microscopy has identified a population of free nerve endings in the epidermis that may be the putative anatomic substrate for this submodality [23].
In recent years a growing body of evidence has been accumulating, from anatomical, psychophysical, electrophysiological, and neuroimaging studies, that a further submodality of afferent, slowly conducting, unmyelinated C‐fibers exists in human hairy skin that are neither nociceptive nor pruritic, but that respond preferentially to low force, slowly moving mechanical stimuli. These nerve fibers have been classified as C‐tactile afferents (CT‐afferents) and were first described by Nordin [18] and Johansson et al. [19]. Evidence of a more general distribution of CT‐afferents have subsequently been found in the arm and the leg, but never in glabrous skin sites such as the palms of the hands or the soles of the feet [20]. It is well known that mechanoreceptive innervation of the skin of many mammals is subserved by A and C afferents, but until the observations of Nordin and Vallbo C‐mechanoreceptive afferents in human skin appeared to be lacking entirely. The functional role of CT‐afferents is not fully known, but their neurophysiological response properties, fiber class, and slow conduction velocities preclude their role in any rapid mechanical discriminative or cognitive tasks, and point to a more limbic function, particularly the emotional aspects of tactile perception [21]. However, the central neural identification of low‐threshold C mechanoreceptors, responding specifically to light touch, and the assignment of a functional role in human skin has only recently been achieved. In a study on a unique patient lacking large myelinated Ab‐fibers, it was discovered that activation of CT‐afferents produced a faint sensation of pleasant touch, and functional neuroimaging showed activation in the insular cortex but no activation the primary sensory cortex, identifying CT‐afferents as a system for limbic touch that might underlie emotional, hormonal, and affiliative responses to skin–skin contacts between individuals engaged in grooming and bonding behaviors – pleasant touch [22]. If pain is elicited via sensory C‐ and Aδ‐fibers then it is reasonable to speculate that the same system may be alternatively modulated to deliver a sensation of pleasure. A study employing the pan‐neuronal marker PGP9.5 and confocal laser microscopy has identified a population of free nerve endings in the epidermis that may be the putative anatomic substrate for this submodality [23].
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