3. Interactions between locomotion and respiration

The lamprey model is also well-suited for the study of respiratory neural mechanisms particularly because its entire brainstem, which contains most of the respiratory network, can be isolated in vitro with the breathing rhythm going strong for many hours. The model can also be extended to a semi-intact preparation in which the brainstem and the functioning gill muscles are still connected with the respiratory behavior intact.

How breathing works in lampreys

In the lamprey, the branchial muscles that make the gills contract and allow water to come in and out are innervated by the branchial motoneurones of the facial, glossopharyngeal and vagus nerve, homologues of the facial motor nucleus and nucleus ambiguus of mammals. These motoneurons are driven at a frequency of around 1 Hz by a group of neurons located rostrally in the hindbrain, near the rostro-lateral tip of the trigeminal motor nucleus, hence its name the paratrigeminal group (pTRG). The pTRG is considered the central pattern generator for respiration in lampreys.

Occasionally but regularly, there are stronger contractions of the gills (arrowheads on the traces in the figure) that are completely independent of the pTRG, and that are driven by a group of neurons more or less diffusely arranged in the lateral part of the caudal hindbrain, the caudal respiratory group (CRG). This caudal group is located in a region of the hindbrain much more reminiscent of the pre-Bötzinger complex of mammals and some homologies between the two structures have been proposed.

Interactions between locomotion and respiration

Our work on the supraspinal control of locomotion and, more precisely, on the mesencephalic locomotor region (MLR) has led us to the observation that the respiratory and locomotor activities are linked together by connections within the central nervous system. We have shown that MLR neurons send projections to the central pattern generator for respiration (pTRG described above) and that MLR stimulation increases respiratory activity even before locomotor activity has even started. Our findings support the presence of strong proactive interactions between the neural centres controlling movement and those controlling respiration. The adaptation of respiratory activity during movement (i.e. an increase of the respiratory rhythm) may thus result from such central interactions. It is perhaps not so surprising that two of the most fundamental and crucial motor activities of vertebrates, locomotion and respiration, and tightly interconnected within the central nervous system.

This research is supported by the Natural Sciences and Engineering Research Council of Canada.