Afferent excitation of human motor cortex as revealed by enhancement of direct cortico-spinal actions on motoneurones |
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| Institution: | 1. Departamento de Tecnologia Bioquímico-Farmacêutica, Faculdade de Ciências Farmacêuticas, Universidade de São Paulo, São Paulo, Brazil;2. Department of Chemical Engineering, Faculty of Engineering and Sciences, University of La Frontera, Temuco, Chile;3. Department of Parasitology, Institute of Biomedical Sciences, University of São Paulo, São Paulo, Brazil;1. Centro studi e ricerche in Neuroscienze Cognitive, Dipartimento di Psicologia, Università di Bologna, 47521, Cesena, Italy;2. IRCCS Fondazione Santa Lucia, 00185 Rome, Italy;3. Department of Psychology, Faculty of Science and Technology, University of Westminster, London W1B 2HW, UK;4. Centre for Brain Science, Department of Psychology, University of Essex, Colchester, CO4 3SQ, UK;1. Department of Neurorehabilitation, Neurological Area, IRCCS San Raffaele Hospital, Milan, Italy;2. Experimental Neurophysiology Unit, Division of Neuroscience, Institute of Experimental Neurology (INSPE), San Raffaele Scientific Institute, Milan, Italy;3. University Vita-Salute San Raffaele, Milan, Italy;1. Department of Neurosurgery, Washington University School of Medicine, St. Louis, MO, USA;2. Division of Hand and Microsurgery, Department of Orthopedic Surgery, Washington University School of Medicine, St. Louis, MO, USA;1. IRCCS San Camillo Hospital, Venice, Italy;2. Department of Movement Sciences, Research Center for Motor Control and Neuroplasticity, KU Leuven, Belgium;3. Department of Human Neurosciences, Sapienza, University of Rome, Rome, Italy |
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| Abstract: | Changes in motor cortex excitability induced by somatosensory afferences were evaluated in 5 subjects by testing how the short-latency cortico-spinal effects evoked by transcranial magnetic stimulation in flexor carpi radialis (FCR) motoneurones were influenced by volleys in median nerve afferent fibres. Transcranial magnetic stimulation induced two facilitatory peaks on FCR H reflex, the first at a conditioning-test interval of about ?3 msec and the second at 0 msec, separated by a phase of inhibition. If an electric shock to the median nerve at the wrist, 0.8-1 × motor threshold (MT) for thenar muscles, preceded the cortical stimulus by 18–25 msec, an increase in size of both facilitatory peaks was observed. The increase was partly due to a direct action of the median nerve volley on motoneurones. When this contribution was subtracted, two peaks of additional facilitation resulted as the effect of combined conditioning. Additional facilitation was present even during the short-lasting phase ascribed to monosynaptic cortico-spinal excitation of motoneurones, i.e., the first millisecond of the earliest facilitatory peak. This result indicates that cortical responsiveness to magnetic stimulation had been enhanced by the peripheral stimulus. The time course of the excitability changes in motor cortex was compared with the cortical somatosensory evoked potentials (SEPs) induced by the same peripheral stimulus. Additional facilitation was present immediately after the N20 peak of SEPs and lasted 8–10 msec. Additional facilitation had the same threshold as N20 (0.6 × MT) and grew in parallel with it when grading the afferent stimulus up to 1 MT. |
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