~ Pergamon Archs oral Biol. Vol. 41, No. 6, pp. 607-611, 1996 Copyright © 1996 Elsevier Science Ltd. All rights reserved Printed in Great Britain PII: S0003-9969(96)0001B41 0003-9969/96 $15.00 + 0.00 A METHOD FOR THE STUDY OF SINGLE JAW ADDUCTOR MUSCLE MOTOR UNITS IN THE RAT ROBERT E. DRUZINSKY Department of Physiology, Northwestern University Medical School, Chicago, IL 6061I, U.S.A. (Accepted 28 December 1995) Summary--Single motor units of the superficial masseter muscle were stimulated by impaling axons in the third division of the trigeminal nerve. Glass micropipette electrodes were driven into the nerve as it runs along the floor of the cranial cavity while passing small current pulses through the electrode to activate axons. Stable penetrations were maintained for as long as 45 min and twitch tensions were recorded for motor units that generated as little as 159 mg (1.6 mN) of force within a superficialmasseter muscle capable of producing 20 g of twitch force. These are the first measurements of single motor-unit forces measured directly from the tendon of a whole jaw adductor muscle. Copyright © 1996 Elsevier Science Ltd. Key words: jaw adductor muscles, rat, rodents, single motor unit, superficial masseter, trigeminal nerve. INTRODUCTION Little is known about the properties of motor units of the jaw adductor muscles (e.g. Herring, 1994). Properties of individual motor units have been examined extensively in many postcranial skeletal muscles of vertebrates (reviewed in Burke, 1981, 1990), but it has become clear that the properties of motor units in one muscle cannot be inferred from the properties of motor units in other muscles. For example, motor units of extraocular muscles which, like the jaw adductors, are innervated by brainstem nuclei, have extraordinary properties (reviewed in Goldberg, 1990) unlike those of any postcranial muscle studied to date. Although complex muscle architectures and the presence of intriguing fibre types, including one (IIM, Bosley and Rowlerson, 1980) found only in jaw adductors (e.g. Rowlerson, 1990), make studies of single motor units of jaw adductor muscles a fruitful area for research, technical difficulties encountered when attempting to record from and stimulate single motor units in jaw muscles have limited the number of such studies of single-unit properties (see Discussion). Several features of chordate anatomy have been crucial to the great success of studies of motor units in postcranial muscles. These include (1) the location of the motoneurones of axial muscles in ventral horn nuclei, (2) the presence of ventral or motor rootlets, and (3) long peripheral nerves. 'Classically', single motor units have been studied by teasing filaments from ventral roots (e.g. Henneman, Somjen and Carpenter, 1965a, b; Edstr6m and Kugelberg, 1968) or impaling cell bodies of motoneurones with microelectrodes (e.g. Burke et al., 1973). Filament techniques are particularly useful for preparations that require the study of one or several motor units for hours at a time, but severely limit the number of motor units that can be studied in a single experiment. In contrast, many motor units can be sampled in a single experiment by impaling cell bodies in the ventral horns of the spinal cord using glass microelectrodes. Although long-term stability is not quite as good as with filament preparations, in successful experiments the movements of the cord are minimized and a given motor unit can be studied for an hour or more. In other studies, axons of motoneurones have been impaled outside of the central nervous system in ventral roots as they leave the spinal cord (Botterman and Cope, 1988a, b; Cope and Clark, 1991) and in peripheral nerves. The ventral roots are almost pure populations of motor axons, which makes the identification and impalement of axons from a given muscle relatively straightforward and surgical access to the ventral roots is, with practice, routine. Dissec- tions of peripheral nerves are usually relatively simple, and the long lengths of these mixed nerves permit investigators to free a nerve from surrounding tissues and mount it on a platform for impalement by glass micropipettes. Typically, penetrations of axons by micropipettes are extremely stable and can often be maintained for hours. Unfortunately, most of the anatomical features that have facilitated studies of single motor units in postcranial muscles are not present in the trigeminal system. The cell bodies of trigeminal motoneurones are located in the brainstem rather than the spinal cord, and to maintain stable, long-term penetrations of cell bodies in the brainstem is extremely difficult. The motor root of the trigeminal nerve is small and not comparable to a ventral root of the spinal cord. 607