Modifications of locomotor pattern underlying escape behavior in the lamprey.

Two forms of undulatory locomotion in the lamprey (a lower vertebrate) have been described earlier: fast forward swimming (FFS) used for long distance migrations and slow backward swimming (SBS) used for escape from adverse tactile stimuli. In the present study, we describe another form of escape behavior: slow forward swimming (SFS). We characterize the kinematic and electromyographic patterns of SFS and compare them with SBS and FFS. The most striking feature of SFS is nonuniformity of shape and speed of the locomotor waves propagating along the body: close to the site of stimulation, the waves slow down and the body curvature increases several-fold due to enhanced muscle activity. Lesions of afferents showed that sensory information critical for elicitation of SFS is transmitted through the dorsal roots. In contrast, sensory signals that induce SBS are transmitted through the dorsal roots, lateral line nerves, and trigeminal nerves. Persistence of SFS and SBS after different lesions of the spinal cord suggests that the ascending and descending pathways, necessary for induction of SBS and SFS, are dispersed over the cross section of the spinal cord. As shown previously, during FFS (but not SBS) the lamprey maintains the dorsal-side-up body orientation due to vestibular postural reflexes. In this study we have found that the orientation control is absent during SFS. The role of the spinal cord and the brain stem in generation of different forms of undulatory locomotion is discussed.