Spinal Mechanisms in the Control of Lamprey Swimming

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Abstract
SYNOPSIS. The lamprey, an anguilliform fish, swims using lateral undulatory movement; a transverse wave passes backward, from head to tail, the amplitude of the wave increasing as it moves tailward. The wave of muscle activity producing this movement travels down the body faster than the mechanical wave. The way in which certain features of anguilliform movement contribute to its efficiency have been described. The neural activity underlying swimming is characterized by: 1) rhythmical alternation between the two sides of a single segment; 2) a burst duration that remains a constant proportion of the cycle time and is independent of the cycle frequency; 3) rostrocaudal phase lag that is constant and also independent of the cycle frequency. Local circuits in the lamprey spinal cord can generate this locomotory pattern in the absence of sensory feedback following activation of excitatory amino acid receptors; the pattern is centrally generated. It has been hypothesized that the spinal central pattern generator for locomotion consists of a series of segmental burst generators coupled together by an intersegmental coordinating system. The intersegmental coordinating system functions to keep the frequencies of the oscillators along the cord constant and to provide the appropriate rostrocaudal phase lag. Mechanosensitive units within the spinal cord are sensitive to movement of the spinal cord otochord and movement of the spinal cord/notochord can entrain the burst pattern. Entrainment occurs through movement-related feedback onto neurons at the local level. The possible roles this movement-related feedback plays during locomotion are discussed.

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