Journal of applied physiology
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Comparative Study
Corticospinal-evoked responses in lower limb muscles during voluntary contractions at varying strengths.
This study investigated corticospinal-evoked responses in lower limb muscles during voluntary contractions at varying strengths. Similar investigations have been made on upper limb muscles, where evoked responses have been shown to increase up to approximately 50% of maximal force and then decline. We elicited motor-evoked potentials (MEPs) and cervicomedullary motor-evoked potentials (CMEPs) in the soleus (Sol) and medial gastrocnemius (MG) muscles using magnetic stimulation over the motor cortex and cervicomedullary junction during voluntary plantar flexions with the torque ranging from 0 to 100% of a maximal voluntary contraction. ⋯ In both Sol and MG, MEP and CMEP amplitudes [normalized to maximal M wave (Mmax)] showed an increase, followed by a plateau, over the greater part of the contraction range with responses increasing from approximately 0.2 to approximately 6% of Mmax for Sol and from approximately 0.3 to approximately 10% of Mmax for MG. Because both MEPs and CMEPs changed in a similar manner, the observed increase and lack of decrease at high force levels are likely related to underlying changes occurring at the spinal level. The evoked responses in the Sol and MG increase over a greater range of contraction strengths than for upper limb muscles, probably due to differences in the pattern of motor unit recruitment and rate coding for these muscles and the strength of the corticospinal input.
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The present study stems from our recent demonstration (Moreau-Bussiere F, Samson N, St-Hilaire M, Reix P, Lafond JR, Nsegbe E, Praud JP. J Appl Physiol 102: 2149-2157, 2007) that a progressive increase in nasal intermittent positive pressure ventilation (nIPPV) leads to active glottal closure in nonsedated, newborn lambs. The aim of the study was to determine whether the mechanisms involved in this glottal narrowing during nIPPV originate from upper airway receptors and/or from bronchopulmonary receptors. ⋯ A few days later, polysomnographic recordings were performed to assess glottal muscle electromyography during step increases in nIPPV (volume control mode). Results show that active glottal narrowing does not develop when nIPPV is applied on the upper airways only, and that this narrowing is prevented by bilateral vagotomy when nIPPV is applied on intact airways. In conclusion, active glottal narrowing in response to increasing nIPPV originates from bronchopulmonary receptors.