Interactions between Neural Oscillators and their Functional Hierarchical Structures

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Animals were decerebrated at the precollicular-postmammillary level under halothane anesthesia. The animal was then fixed to a stereotaxic frame, and the limbs were placed on a moving treadmill belt. The glass-insulated tungsten microelectrode was inserted into the midbrain to identify the mesencephalic locomotor region (MLR), stimulation of which induces locomotion on a treadmill. Tonic electrical stimulation (50 pulses/s) was delivered to the MLR. Electromyograms were recorded by implantation of bipolar electrode into the bilateral gastrocnemius muscle and the diaphragm to evaluate the stepping rhythm and the respiratory rhythm, respectively. The cardiac rhythm was assessed by recording of electrocardiogram (ECG). The heart beat periods were transformed into another pulse train in which the pulses were arranged in equal intervals with the pulse height representing the heart beat periods. The transformed heart beat signal was designated as the heart beat fluctuation. At rest, the card … More iac rhythm was strongly modulated by the respiratory rhythm (Respiratory Sinus Arrhythmia, RSA). The extent of such respiratory modulation of heart beat became small during treadmill locomotion. This result suggested that the coupling strength between the cardiac and the respiratory rhythms became weak during locomotion. The animal was then vagotomized bilaterally under halothane anesthesia. At rest, the extent of the resiratory modulation of heart beat markedly reduced after vagotomy. However, the strength of the cardiorespiratory coupling became greater during locomotion than that at rest. The animal was then paralyzed with gallamine triethiodide (4 mg/Kg i.v.), and artificially ventilated. Pneumothorax was also performed under anesthesia. In such a preparation, tonic electrical stimulation was delivered to the MLR to elicit fictive locomotion. The coherence between the heart beat fluctuation and the efferent discharges of the hindlimb muscle nerve was used to evaluate the coupling between cardiac and locomotor rhythms during MLR-elicited fictive locomotion. Heart beat was apparently modulated by the centrally generated stepping rhythm. This result demonstrated that there was a locomotor-cardiac coupling of central origin. Based on the above findings as well as on our previous ones, we proposed a new model concerning the hierarchical structure of biological oscillators. Less