88 Brain Research, 517 (1990) 88-98 Elsevier BRES 15476 The motor output of the Mauthner cell, a reticulospinal command neuron Jonathan Nissanov*, Robert C. Eaton and Randolf DiDomenico Behavioral Neuroscience Group, Department of Biology, University of Colorado at Boulder, CO 80309 (U.S.A.) (Accepted 24 October 1989) Key words: Mauthner neuron; Reticulospinal system; Escape response; Command neuron; C-start; Spinal cord; Electrical stimulation; Motor system We electrically stimulated individual Mauthner (M-) cells to determine their motor contribution to C-starts of swimming goldfish. In comparison with sensory-evoked C-starts, M-reflexes triggered by electrical stimulation of single M-cells were significantly weaker and less variable. Stage 1 turns were both longer in duration and smaller in angle for the M-reflex when compared with the sensory-evoked C-start. This translates to an average reduction of 22% in angular velocity during stage 1. Likewise, during stage 2, the distance moved by the fish was reduced by 15% and the absolute value of stage 2 turning angle was reduced by 47%. In addition, the normal mechanical or neural coupling between stages 1 and 2 appeared to be altered for the M-reflex. From this and our other recent studies, we conclude that there must be two primary groups of reticulospinal neurons in the escape triggering network. The first group includes the M-cell and determines the initial left-right direction of the response and the extent of stage 1 angle. From previous EMG recordings we know that the second group of neurons can fire within 5-15 ms (average, 9 ms) after the stage 1 cells. These determine the onset time and direction of stage 2. Together the coupling of the two primary groups results in the full propulsive force and turning flexibility of the C-start. INTRODUCTION The reticulospinal (RS) system is integral to compre- hensive movements of limb and body 3°'36. For example, rhesus monkeys with bilateral transections of the medial brainstem pathways (at the level of the Vlth nucleus) lack orienting movements when approached with food and can follow it only with their eyes 31. However, if only the pyramidal tract is transected, fine control of the digits is lacking but the animals can still turn their bodies and pick up food by opening and closing the hand on an outstretched arm. Despite its importance in vertebrate motor control, little is known about the functional organization of the RS system or how its neurons interact 35. In the present paper we show the effect on movement production due to the activation of a single RS neuron, the Mauthner cell (M-cell) of teleost fish. We use this preparation as a model for studying mechanisms of reticulospinal movement control. Brainstem nuclei in general, and the RS system in particular, are thought to be highly conserved across the vertebrate classes 28"34"39. Moreover, M-cell physiology is well characterized 1'3' 4,11,18,19,2o,22,29,4o,48 and the functional and anatomical features of the M-ceU are not only preserved across the distantly related vertebrates-lamprey 5, teleost fishes 47 and amphibians 44 -- but M-cells also share specific similarities with the medial RS tract neurons of mam- mals. As we have previously reviewed 35, both the mamma- lian medial RS tract neurons and the M-cells receive major inputs from vestibular and tectobulbar fibers; both have single axons that course caudally in the medial longitudinal fasciculus; both make mono- and di-synaptic connections with motoneurons all along the spinal cord; and both are involved in coordination of complex, large-scale movement patterns. Thus, the M-cells are most likely homologous to mammalian medial RS tract fibers and their general features can be used for a comparative understanding of the role of the medial RS system. The M-cell plays a central role in the initiation of the C-start escape response shown in Fig. 1A 13. This behav- ior is used for predator avoidance 27'42. During the C-start both spinal and supra-spinal motor pools are recruited so that much of the animal's motor system is subsumed. This includes muscles controlling not only the trunk but also the eyes, jaw, operculum and fins 6'12. In the present *Present address: Image Processing Center, Electrical and Computer Engineering Department, Drexel University, Philadelphia, PA 19104, U.S.A. Correspondence: R.C. Eaton, Department of Biology, EPO Box 334, University of Colorado at Boulder, CO 80309, U.S.A. 0006-8993/90/$03.50 t~) 1990 Elsevier Science Publishers B.V. (Biomedical Division)