Respiratory and stepping frequencies in conscious exercising cats

The incidence of entrainment between respiratory and stepping frequencies has been investigated in exercising cats. Stainless steel wire electrodes were surgically implanted in the right hemidiaphragm, quadriceps, and hamstring muscles of six cats. Electromyograms were recorded while the cats were at rest or walking on a treadmill at speeds between 0.31 and 1.67 m/s (6 km/h). Autocorrelation of diaphragmatic activity showed cats to have two respiratory patterns: regular (maintained periodicity in the autocorrelogram) or irregular (flat autocorrelogram). Autocorrelation of quadriceps or hamstring activity revealed stepping frequency. Cross correlation of diaphragmatic vs. quadriceps or hamstring activities revealed the presence of entrainment between respiration and stepping. In four cats, entrainment was either very weak or absent, but the other two cats clearly showed entrainment of the two activities, even in the absence of a regular respiratory rhythm. In conclusion, at speeds up to 1.67 m/s, respiratory frequency is not tightly locked to stepping frequency in cats; however, the occurrence of entrainment in some instances clearly indicates the existence of neural circuitry linking these two pattern generators.