Variability in motor control: supraspino-spinal interactions underlie fractal locomotor rhythms

The motor control system in vertebrates is highly organized and well coordinated. At the same time, the motor rhythms show temporal variability. The gait rhythm in human subjects has been shown to have long-range positive correlations, reflecting a fractal nature. The mechanisms underlying these observations are unknown. Using in-vitro brain-spinal cord preparations of the lamprey, in which fictive locomotor rhythms can be generated, we show that the locomotor rhythm has a Hurst exponent that indicates presence of long-term positive correlations. Interrupting the mixed positive and negative feedforward-feedback loops between the brain and the spinal cord eliminates these long-term correlations. Hence, we conclude that mixed feedback supraspino-spinal interactions likely underlie fractal locomotor rhythms.