, 2004). Moreover, molecular profiling suggests that Merkel cells express the machinery capable of sending both excitatory and modulatory signals to sensory neurons ( Haeberle et al., 2004). However, mechanical stimulation of isolated Merkel cells does not generate mechanically gated currents. These findings, collectively, point to a modulatory
role for Merkel cells during the transmission of mechanical forces onto associated Aβ SAI-LTMR endings ( Diamond et al., 1986, Haeberle et al., 2004 and Yamashita et al., 1992). SAII-LTMRs. SAII-LTMRs, like SAI-LTMRs, yield a sustained response to skin indentation but differ in their interspike intervals, which are much more uniform than those of SAI afferents (Table 1). Like SAI-LTMRs, SAII afferent conduction velocities fall within the Aβ range (20–100 m/s), although this can be quite varied across species. SAII-LTMRs U0126 in vitro innervate the skin less densely than SAIs, and selleck compound their receptive fields are about five times larger, with one central low-threshold spot
on the skin for each SAII fiber (Johansson and Vallbo, 1980). SAIIs are one-sixth as sensitive as SAIs to skin indentation but two to four times more sensitive to skin stretch and changes in hand and finger shape (Edin, 1992 and Johnson et al., 2000). Interestingly, SAII-LTMRs transmit information about skin stretch with little interference from other textural aspects of an object held in the hand. Psychophysical and microneurography Endonuclease studies suggest two major functions of SAII afferents in touch perception, both resulting from their sensitivity to skin stretch. The first is detecting hand shape and finger conformation, or proprioception, which is likely integrated with information conveyed from muscle spindles and joint afferents. In this regard, it is interesting that SAII-LTMRs share certain physiological characteristics with proprioceptors. A second potential role for SAII-LTMRs is in the detection of object motion and velocity when the direction of object movement produces
skin stretch. Unlike SAI-LTMRs, considerable controversy surrounds SAII afferents. First, although reported regularly in microneurography studies of the human hand, neurophysiological evidence of their presence has not been observed in studies of the monkey hand (Blake et al., 1997a, Blake et al., 1997b, Connor et al., 1990, Goodwin et al., 1997 and Johnson and Lamb, 1981), and only recently have neurons with SAII properties been reported in the mouse (Wellnitz et al., 2010 and Woodbury and Koerber, 2003). Second, the morphology of SAII mechanoreceptors remains elusive. Unlike the well-established Merkel cell-neurite complex corresponding to SAI-LTMRs, SAII responses have only been postulated to arise from Ruffini endings, though direct evidence to support this idea is lacking (Chambers et al., 1972) (Figure 1A).
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