Abstract
The interplay of the neuronal discharge patterns regarding respiration and locomotion was investigated using electrophysiological techniques in a decerebrate and arterially perfused in situ mouse preparation. The phrenic, tibial, and/or peroneal nerve discharge became clearly organized into discharge episodes of increasing frequency and duration, punctuated by periods of quiescence as the perfusion flow rate increased at room temperature. The modulated sympathetic tone induced by the hyperoxic/normocapnic state was found to activate the locomotor pattern generator (LPG) via descending pathways and generate a left and right alternating discharge during discharge episodes in the motor nerves. The rhythm coupling of respiration and locomotion occurred at a 1:1 frequency ratio. Although the phrenic discharge synchronized with the tibial discharge at all flow rates tested, the time lag between peaks of the two discharges during locomotion was ≈400 ms rather than ≈200 ms, suggesting spinal feedback via ascending pathways. The incidence of the phrenic and tibial discharge episodes decreased by ≈50% after spinalization at the twelfth thoracic cord and the respiratory rhythm was more regular. These results indicate that: (i) locomotion can be generated in a hyperoxic/normocapnic state induced by specific respiratory conditions, (ii) the central mechanism regarding entrainment of respiratory and locomotor rhythms relies on spinal feedback via ascending pathways, initiated by the activated LPG generating locomotion, and (iii) the increase in respiratory rate seen during locomotion is caused not only by afferent mechanical and nociceptive inputs but also by impulses from the activated spinal cord producing a locomotor-like discharge via ascending pathways.
Highlights
Entrainment of respiratory and locomotor rhythms during locomotion is well-documented in many species, including rats, cats, rabbits, dogs, and humans (Krogh and Lindhard, 1913; Dejours, 1967; Iscoe, 1981; Bramble and Carrier, 1983; Ainsworth et al, 1996)
These results indicate that: (i) locomotion can be generated in a hyperoxic/normocapnic state induced by specific respiratory conditions, (ii) the central mechanism regarding entrainment of respiratory and locomotor rhythms relies on spinal feedback via ascending pathways, initiated by the activated locomotor pattern generator (LPG) generating locomotion, and (iii) the increase in respiratory rate seen during locomotion is caused by afferent mechanical and nociceptive inputs and by impulses from the activated spinal cord producing a locomotor-like discharge via ascending pathways
The results indicate that sympathetic tone modulated by a hyperoxic/normocapnic state activates the LPG through descending pathways and produces a locomotor-like discharge in the hindlimb (Figure 8A)
Summary
Entrainment of respiratory and locomotor rhythms during locomotion is well-documented in many species, including rats, cats, rabbits, dogs, and humans (Krogh and Lindhard, 1913; Dejours, 1967; Iscoe, 1981; Bramble and Carrier, 1983; Ainsworth et al, 1996). Such coupling can be evoked by electrical stimulation of either the mesencephalic locomotor region or the subthalamic locomotor region (Bramble and Carrier, 1983; Corio et al, 1993) within a wide range of frequency ratios between locomotion and respiration (e.g., 1:1, 2:1, and 3:2). The increase in metabolism is not always reflected in that of neuronal activity and/or the sympathetic tone, it is unclear whether the sympathetic tone resulting from the increase in flow rate can generate unknown autonomic functions
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