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Neurons in the Pontomedullary Reticular Formation Signal Posture and Movement Both as an Integrated Behavior and Independently

¹Unité de physiologie et biomécanique de la locomotion, Département d'éducation physique et de réadaptation, Université catholique de Louvain, Louvain-la-Neuve, Belgium; ²Balance and Voluntary Movement Laboratory, Department of Kinesiology and Physical Education, McGill University; and ³Groupe de Recherche sur le Système Nerveux Central, Département de physiologie, Faculté de médecine, Université de Montréal, Montreal, Quebec, Canada

Submitted 21 December 2007; accepted in final form 16 July 2008

We have previously suggested that the discharge characteristics of some neurons in the pontomedullary reticular formation (PMRF) are contingent on the simultaneous requirement for activity in both ipsilateral flexor muscles and contralateral extensors. To test this hypothesis we trained cats to stand on four force platforms and to perform a task in which they were required to reach forward with one forelimb or the other and depress a lever. As such the task required the cat to make a flexion movement followed by an extension in the reaching limb while maintaining postural support by increasing extensor muscle tonus in the supporting limbs. We recorded the activity of 131 neurons from the PMRF of three cats during left, ipsilateral reach. Of these, 86/131 (66%) showed a change in discharge frequency prior to the onset of activity in one of the prime flexor muscles and 43/86 (50%) showed a bimodal pattern of discharge in which activity decreased during the lever press. Among the remaining cells, 28/86 (33%) showed maintained activity throughout the reach and the lever press. Most cells showed a broadly similar pattern of discharge during reaches with the right, contralateral limb. We suggest these results support the view that a population of neurons within the PMRF contributes to the control of movement in one forelimb and the control of posture in the other forelimb as a coordinated unit. Another population of neurons contributes to the control of postural support independently of the nature of the activity in the reaching limb.

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