Aδ-fiber low threshold mechanoreceptors innervating mammalian hairy skin: A review of their receptive, electrophysiological and cytochemical properties in relation to Aδ-fiber high threshold mechanoreceptors

• The belief that all Aδ-fiber neurons are nociceptors is not empirically based.
• The evidence that Aδ-LTMs exist in various species including humans is provided.
• The evidence that Aδ-LTMs differ from Aδ-HTMs (nociceptors) in various ways is provided.

The sensation of gentle touch of the mammalian hairy skin is mediated by morphologically and physiologically distinct classes of low-threshold mechanoreceptors (LTMs) which are classified, according to their axonal conduction velocities, into Aβ-, Aδ- and C-LTMs. Although Aδ-LTMs (D-hair cells) were first described about five decades ago, and have been found in hairy skin of every species examined including humans, it is commonly assumed that all Aδ-fiber neurons are nociceptors. This view is endorsed by many textbooks. This article reviews the evidence that Aδ-LTMs exist in substantial proportions in different species, and that their peripheral and central axonal endings, molecular markers, receptive, electrophysiological and cytochemical properties are distinct from those of Aδ-high-threshold mechanoreceptors (Aδ-HTMs). A brief overview of some of the ion channels and markers that are expressed by the two populations of primary afferent neurons is also provided. Failure to recognize the existence and properties of Aδ-LTMs might lead/have led to misinterpretations of data. Aβ-LTMs and C-LTMs have been reviewed elsewhere and are not subject of this review.