Single unit responses of human cutaneous mechanoreceptors to air-puff stimulation Keiko Mizobuchi a, * , Satoshi Kuwabara a , Shinobu Toma b , Yoshio Nakajima b , Kazue Ogawara a , Takamichi Hattori a a Department of Neurology, School of Medicine, Chiba University, Chiba, Japan b Department of Physiology, School of Medicine, Chiba University, Chiba, Japan Accepted 31 May 2000 Abstract Objective: To investigate responsiveness of human cutaneous mechanoreceptor to selective tactile stimuli produced by brief air-puff stimulation. Methods: Using percutaneous microneurography, activities of single sensory units innervating glabrous skin of the hand were recorded, and air-puff stimuli with a short rise time (0.5 ms), generated by a high-speed air control system, were applied to the receptive ®eld. Receptor activation time was estimated as the latency difference between electrically and air-puff evoked responses. Results: Thirty units were analyzed: all 4 kinds of mechanoreceptors of human glabrous skin (fast adapting type 1 [FA I, n  7], fast adapting type 2 [FA II, n  4], slowly adapting 1 [SA I, n  5] and slowly adapting 2 [SA II, n  14]) were activated by air-puff stimulation. Estimated receptor activation times were 0.6±6.2 ms (mean 2.2 ms). FA II units occasionally responded twice or more to a single air-puff stimulus. Conclusions: Brief air-puffs can activate all 4 human cutaneous mechanoreceptors, and the receptor transduction time is estimated as approximately 2.0 ms. Properties of human mechanoreceptors can be studied using air-puff stimulation and microneurography. q 2000 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Human; Air-puff stimulation; Mechanoreceptors; Receptor transduction; Terminal axon; Microneurography 1. Introduction Human glabrous skin has 4 types of low threshold mechanoreceptors; fast adapting types I (FA I) and II (FA II) and slowly adapting types I (SA I) and II (SA II) accord- ing to the size of the receptive ®eld and receptor activity adaptation pattern to sustained skin indentation (Vallbo and Johansson, 1984). These mechanoreceptors are highly differentiated speci®c organs of the distal nerve terminals of primary sensory neuron. In peripheral neuropathies such as toxic, diabetic, or other metabolic neuropathies, axons degenerate in a dying-back manner, which receptor would be affected earlier than in the intermediate nerve segments. Whereas measurement of amplitude or conduction velocity of sensory nerve action potentials (SNAP) is useful to esti- mate function of the sensory axon and myelin, a new kind of electrodiagnostic test is necessary to clarify pathophysiol- ogy of the sensory receptor. To investigate the cortical projection of the human mechanoreceptors, mechanical stimulation such as manual tapping with a hammer (Larsson and Prevec, 1970) or tapping with an electromagnetically driven stimulator (Ishiko et al., 1980), has been used to stimulate the skin mechanoreceptors naturally. However, such stimuli could activate not only cutaneous mechanoreceptors but also underlying joint, tendon or muscle receptors. Air-puffs has been considered to stimulate selectively cutaneous mechan- oreceptors. Hashimoto (1987, 1988) developed a novel air- puff stimulator with a high-speed air-control system that enables to make a constant tactile stimulation to the skin: (1) with constant pressure to the skin surface; (2) with short rise and fall times; and (3) for a small stimulation area innervated by only a few mechanoreceptors. Although this type of air-puff stimulator was developed to allow record- ings of SNAPs and somatosensry evoked potentials (SEPs) to selective activation of cutaneous mechanoreceptors, it has not been con®rmed whether all 4 types of mechanore- Clinical Neurophysiology 111 (2000) 1577±1581 1388-2457/00/$ - see front matter q 2000 Elsevier Science Ireland Ltd. All rights reserved. PII: S1388-2457(00)00368-0 www.elsevier.com/locate/clinph CLINPH 200042 * Corresponding author. Department of Neurology, Chiba University School of Medicine, 1-8-1 Inohana, Chuo-ku, Chiba, 260-8670, Japan. Tel.: 181-043-222-7171 ext. 5414; fax: 181-043-226-2160. E-mail address: mizobuchik@tim.hi-ho.ne.jp (K. Mizobuchi).