Journal of Neuroscience Methods, 39 (1991 ) 103-107 103 © 1991 Elsevier Science Publishers B.V. All rights reserved 0165-0270/91/$03.50 NSM 111274 Threshold depolarization measurements in resting human motoneurones K.S. TiJrker and T.S. Miles Department of Physiology, The Unit ersity of Adelaide, AdelaMe, South Australia 5000 (Australia) (Received 12 February 1991) (Revised version received 22 May 1991) (Accepted 23 May 1991) Key words: Threshold depolarization; Human motoneurones; Ia EPSP; Soleus muscle; Single motor units Threshold depolarization is defined as the amplitude of the depolarization that will just bring a neurone to its firing threshold. Here, a novel technique is described for the measurement of threshold depolarization in resting human motoneurones. The tibial nerve was stimulated at an intensity that was just sufficient for the resulting Ia afferent volley to activate a single motor unit in the soleus muscle. The same motor unit was then recruited by voluntary activation, and the amplitude of the EPSP elicited by this stimulus was determined by a manner described earlier. The value found for the threshold depolarization was around 5 mV for all 6 motoneurones that were successfully tested. Introduction It is generally accepted that motoneurones are recruited in an orderly sequence in a graded voluntary contraction. This orderly recruitment is believed to be determined by cell factors, some of which are related to the physical size of the motoneurone (Henneman et al., 1965). Other fac- tors such as synaptic density and initial segment threshold depolarization also contribute to the functional recruitment threshold of individual motor neurones (reviewed in Burke, 1981; Gustafsson and Pinter, 1985). In spite of its importance, the measurement of initial segment threshold depolarization neces- sary for the recruitment of human motor neu- rones has not been previously possible due to Correspondence: Dr. K.S. Tiirker, Department of Physiology, University of Adelaide, G.P.O. Box 498, Adelaide SA 5(/01, Australia. Tel.: 61-618-2285108. Fax.: 61-618-2240464. technical difficulties. The only available data come from reduced animal preparations (Pinter et al., 1983; Gustafsson and Pinter, 1984; Hult- born et al., 19881. The aim of the present experiments was, therefore, to determine the so-called threshold depolarization in the motoneurones of intact hu- man subjects. Threshold depolarization refers to the amount by which the membrane potential of a quiescent motoneurone must be depolarized in order to bring it to its firing threshold (Pinter et al., 1983). It should be noted at the outset that there are some discrepancies in the nomenclature used by different investigators for describing the various aspects of the membrane potential during tonic compared with "single-spike" firing. In this re- port, we have adopted the definition for afterhy- perpolarization (AHP) proposed by Calvin and Schwindt (19721, namely: that "afterhyperpolari- zation is the negative-going afterpotential seen in single-spike experiments .... Relative to the "rest-