Physiological properties of unmyelinated low-threshold tactile (CT) afferents in the human hairy skin

Abstract

For more than a decade it has been known that human skin is supplied with a system of slowly conducting unmyelinated afferents that respond strongly to innocuous skin deformation (Nordin, 1990; Vallbo et al., 1993). Using the microneurography technique, nerve impulses from single unmyelinated, low-threshold (CT) mechanoafferents in the hairy skin were recorded in healthy human subjects. Single afferents were identified by their sensitivity to innocuous skin deformation, low mechanical threshold, and slow conduction velocities, î1m/s. To characterize the physiological properties of this afferent system, the receptive fields of the identified afferents were subjected to different types of mechanical, thermal and chemical stimuli. The receptive fields were mapped with a computer-controlled lightweight probe. It was found that the organization of the receptive terminals was highly non-uniform, with an area of 1-34 mm2. Fields were roughly round or oval without any preferred orientation.The response properties of CT-afferents to controlled mechanical stimulation were studied. It was found that the response to a change in the indentation amplitude increased up to 3 mm, along a decelerating curve. A pronounced receptor fatigue was observed when series of identical stimulations were delivered. Fatigue increased with an increasing number of stimulations and with shorter inter-stimulus time intervals. During long-lasting indentations an initial prominent response during a few seconds was followed by sustained but irregular activity at low rates in most afferents. This suggests that CT-afferents are poorly designed for coding discriminative aspects of touch.The CT-afferents showed little or no response to pure cooling or warming, but afterdischarge was frequently observed with combined mechano-thermal cooling suggesting that cooling may importantly influence the CT-afferents response to mechanical stimulation.The CT-afferents showed low or negligible activity in response to topical application of capsaicin (2 %). This supports the interpretation that these afferents constitute a separate type of mechanosensitive C-afferents distinct from high-threshold nociceptors.