, 2011, Millard and Woolf, 1988 and Woodbury et al , 2001) (Figur

, 2011, Millard and Woolf, 1988 and Woodbury et al., 2001) (Figure 1B). Viewed in cross-section, each palisade of the longitudinal lanceolate ending is partially surrounded by processes of a terminal Schwann cell, with the side adjacent to hair shaft keratinocytes often devoid of a glial

covering. The shape and configuration of the palisades and their associated glial cells suggests a mechanism by which Aβ RA-LTMRs are exquisitely sensitive to hair follicle deflection, with putative sites for mechanotransduction located between the nerve fiber and the hair follicle keratinocytes (Halata, 1993 and Takahashi-Iwanaga, DNA Damage inhibitor 2000). With the recent development of mouse genetic tools, anatomical features of LTMRs, such as receptive fields, can now be defined by the number of hair follicles that they associate with. We now appreciate the existence of a variety of anatomical peripheral receptive fields formed by Aβ hair follicle afferents, which can range from single hair follicles to clusters of adjacent hair follicles (Li et al., 2011, Suzuki et al., 2012 and Wu et al., 2012). Aδ-LTMRs. A second major group of hair follicle-associated LTMRs are classified as Aδ-LTMRs according to their intermediate

conduction velocities (Table 1). Hair follicle-specific Aδ-LTMRs were originally described as D-Hair units, meant to reflect their specific response to movements of small sinus and down hairs in the cat and rabbit. Aδ-LTMR-like responses are also found in humans, though not

almost always correlated to hair follicle movement and it remains unclear how, or even if, Aδ-LTMR units influence touch perception (Adriaensen et al., 1983). The unique physiological properties of Aδ-LTMR BMN673 responses have been uncovered through in vivo and in vitro studies of model organisms. Most notably, studies in the cat and mouse reveal that Aδ-LTMR responses exhibit some of the lowest mechanical thresholds and highest dynamic sensitivity of any other LTMR, making Aδ-LTMRs the most sensitive mechanoreceptor in skin (Brown and Iggo, 1967, Burgess and Perl, 1967 and Koltzenburg et al., 1997). Aδ-LTMR physiological profiles are remarkably consistent and uniform within a given animal both in terms of their conduction velocity, which falls within the Aδ range, and their physiological receptive fields, which exhibit little variability from proximal to distal hairy skin. In addition, Aδ-LTMRs are sensitive to rapid cooling, but not warming, of the skin (Adriaensen et al., 1983, Brown and Iggo, 1967 and Li et al., 2011). As with Aβ RA-LTMRs, Aδ-LTMR responses are rapidly adapting and silent in the absence of tactile stimulation (Table 1). During the decades in which Aδ-LTMRs were originally described and subsequently thoroughly characterized, the anatomy of Aδ-LTMRs remained largely unknown, though their sensitivity to down hair movement, in particular air-jet stimulation of hair follicles, led to speculation that they form close associations with hair follicles.

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