ELSEVIER Neuroscience Letters 215 (1996) 99-102 Stretching of an isolated part of the sciatic nerve of the frog (Rana rindibunda) causes excitation of its sensory nerve fibres G. Theophilidis*, K. Kiartzis Laboratoly of Animal Physiology, Department of Zoology, School of Biology, Aristotle Universi~', Thessaloniki 54006, Macedonia, Greece Received 8 July 1996; revised version received 31 July 1996; accepted 31 July 1996 Abstract The stretching of the isolated distal part of the sciatic nerve of the frog causes the excitation of a number of its nerve fibres. The excitation lasts as long as the force is applied, indicating a slow adapting mechanoreceptor system. The fact that in experiments in situ stretching of the sciatic nerve causes the reflex activation of the femoral muscles is a clear indication that the activated nerve fibres belong to primary afferent fibres. According to the results of this study either some of the primary afferent fibres in the sciatic nerve of the frog also have the potential of stimulus transduction (mechanical tension) or the sciatic nerve is equipped with nerve terminals which are sensitive to stimulus energy. The implication of this sensory mechanism in the proprioception of the leg is discussed further. Keywords." Frog; Isolated; Sciatic nerve; Stretching; Action potentials; Mechanoreception The effect of tissue stretching on peripheral nerves has become a subject of intense investigation the past few years [5,7,8,10]. If a nerve could be elongated without significant damage then the additional length could be used to overcome a nerve gap without recourse to a nerve graft [4,10]. The effects of elongation on a variety of peripheral nerves were studied using physiological and morphological methods. Most of the physiological studies were concerned mainly with the conduction properties (e.g. conduction velocity) of the nerve fibres during or after stretching [5,8]. However, during any kind of nerve stretching studies inevitably one could ask whether the nerve has the ability to inform the central nervous system about the forces applied to cause its elongation. It is a surprising fact that while muscles, bones and joints are equipped with a massive number of proprioceptors [2,6], large peripheral nerves like the sciatic nerve which are also exposed to stretch and pressure [1] are not equipped with any kind of mechanoreceptor mechanism. So far mechan- oreceptors with no specialised encapsulated structures (free endings) have been classified as cutaneous endings, alimentary receptors, urinary mechanoreceptors, receptors * Corresponding author. Tel.: +30 31 996861/998288; fax: +30 31 998269/206138; e-mail: theofilidis@olymp.ccf.auth.gr of the respiratory tract and vascular receptors [2], but none of the known categories of mechanoreceptors [2] contains a group of receptors with free ending on the peripheral nerves. The purpose of this work is to investigate whether action potentials could be generated during the stretching of the peripheral part of the sciatic nerve of the frog. The frogs (Rana rindibunda), weighing between 100 and 140 g, were stunned, decapitated and pithed. The per- ipheral part of the sciatic nerve, 40 + 1.3 mm long starting from the knee, was dissected and placed in a Petri dish with oxygenated saline (bubbled with 100% 02). The com- position of the saline was (in mM 1-l) 111 NaCI, 2.41 KC1, 10 HEPES, 2 CaC12, 10 glucose (pH 7.2) and the experi- ments were performed at room temperature (21 + I°C). For the in vitro experiments the proximal end of the isolated nerve was clamped, 1 mm above the saline level of the Petri dish, using a surgical haemostatic clamp (see hc in Fig. 1A). The distal end was attached to an isometric tension transducer (Grass F03B, displacement rate 1 ram/ kg), mounted on a micromanipulator, using thick surgical thread (see tt and micr in Fig. 1A). The tension transducer was driven manually by the micromanipulator, so for any lengthening of the nerve it was possible to monitor the force required for this stretching. The lengthening of the nerve during stretching was measured accurately using a 0304-3940/96/$12.00 © 1996 Elsevier Science Ireland Ltd. All rights reserved PII S0304-3t)40(96)12953-0