Postembryonic Maturation of a Spinal Circuit Controlling Amphibian Swimming Behaviour
In most vertebrates, including humans, the basic neural circuitry responsible for generating rhythmic locomotor movements appears to be resident in the spinal cord at very early stages in development, often at times when locomotion is not even possible. After hatching or birth, this naive circuitry then matures, often over a prolonged period, until the complex and sophisticated repertoire of adult locomotion is attained. To address the developmental mechanisms responsible for locomotor circuit maturation we are studying the ontogeny of locomotion in a simple model system — swimming activity in postembryonic amphibian (Xenopus laevis) tadpoles. The neuroanatomical simplicity of these organisms, together with their rapid development has enabled detailed description of: 1) the timecourse of developmental changes in rhythmic swimming activity of immobilized animals; 2) the axial progression of these changes; 3) the modulation of spinal neuronal properties as the swimming system develops; and 4) the central mechanisms underlying swimming rhythm maturation. We find that as the swimming circuit matures, a bursty ventral root pattern becomes established during the first 24 hours after hatching from the egg membranes. This occurs in association with a transition from single to multiple impulses per cycle in myotomal motoneurones. As we describe here this developmental change imparts flexibility on a simple stereotyped embryonic system at the precise time in ontogeny when behavioural manoeuvrability is likely to enhance survival. The acquisition of ventral root bursts follows a rostrocaudal path during larval development indicating the possible involvement of a descending neuronal process and evidence suggests that rhythm maturation is controlled by serotonergic raphespinal interneurones.
KeywordsXenopus Laevis Cycle Period Ventral Root Burst Duration Xenopus Embryo
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