How does the nervous system coordinate the control of different behaviors? For example, different rhythmic behaviors that normally function independently may need to work together under certain circumstances. One example of this phenomenon is the way that our respiratory rate goes up when we start running. Vertebrate breathing frequency increases immediately with locomotor activity, so it's not just the lack of oxygen that makes us breathe faster. Respiratory rates and limb movements can also be coupled in a one-to-one fashion during fast gaits, presumably both to maintain sufficient oxygen levels and to avoid mechanical interference.

Several mechanisms have been proposed to underlie this coordination, including mechanical coupling through whole body movements and common drive from higher brain centers to the separate neuronal networks controlling breathing and locomotion. In the article by Didier Morin and Denise Viala, an elegantly simple experimental approach was used to investigate possible mechanisms of coupling between these networks.

The pattern-generating neurons for breathing are found in the brain stem and the neurons that control hindleg movements are found in the lumbar spinal cord. By removing the spinal cord and brain stem from newborn rats and putting it into a dish, the authors could test how the pattern generating system for respiration can be influenced by the locomotor system in the absence of other inputs. They bathed the lower part of the spinal cord in the neurochemical NMDA to induce rhythmic activity in the networks that control the leg movements, and then increased the dose while monitoring the resulting activity in both the respiratory and leg motoneurons to look for coupling between the pattern generators. As they increased the level of NMDA, the frequency of the locomotor rhythm in the lumbar spinal cord rose, and once above a certain threshold, the respiratory rhythm sped up too. However, the authors saw no apparent phase coupling between “walking” and “ breathing” rhythms, suggesting that no specific timing information is contained in signals from the local locomotor centers to the respiratory networks.

In contrast, mimicking sensory feedback from leg movements by electrically stimulating nerves that contain the axons of leg proprioceptors had a dramatic effect. A brief stimulation elicited a burst in the respiratory motoneurons, and reset the respiratory rhythm. Rhythmic stimulation of the leg proprioceptors could also force the respiratory rhythm to follow a wide range of stimulation frequencies in a one-to-one fashion.

The authors go on to show that these effects are directly mediated by sensory pathways from the leg to the respiratory-rhythm-generating networks and even found reflex-like connections to the phrenic motoneurons innervating the diaphragm. Therefore, sensory feedback from the legs during walking appears to play a key role in providing timing information for the respiratory system to couple the breathing frequency to the locomotor rhythm.