No description available.

The development of swimming was investigated in zebrafish aged 1.5 to 5 days postfertilization by examining both the swimming behavior and its generation by the nervous system. Upon hatching (at day 2), swimming is undirected and occurs in sustained bursts of high frequency (mean = 67 Hz) tail undulations. By 4 days, the swimming pattern matures to a more directed, less erratic, beat-and-glide pattern where slower (mean = 35 Hz) tail undulations, lasting ∼200 ms, alternate with longer gliding rest periods. Swimming is powered by two classes of embryonic muscles (embryonic red, ER and white, EW) that are electrically coupled within (but not between) classes and have physiological properties similar to vertebrate tonic and twitch muscle, respectively. ER fibers have a lower chloride ion permeability than EW fibers and do not have sodium dependent action potentials. In paralyzed preparations, motoneurons and muscle fibers received coordinated excitatory synaptic activity (with left to right alternation and head to tail propagation) corresponding to either burst or beat-and-glide swimming. ER muscle was de-recruited at the fastest swimming rates and EW fibers dropped out at the slowest swimming rates. Rhythmic motoneuron output was generated by a phasic glutamatergic and a largely tonic glycinergic synaptic drive. Glutamatergic synapses had either or both AMPA/kainate and NMDA receptors and the kinetics of these synaptic currents were fixed throughout the developmental period examined. When depolarized, motoneurons fired high frequency (up to 800 Hz) bursts of action potentials that rapidly accommodated (within ∼20 ms) due to voltage and calcium dependent outwardly rectifying conductances. These intrinsic motoneuron properties are hypothesized to interact with the rhythmic synaptic drive to pattern motor output (at ∼25--75 Hz) to locomotor muscles. The neural generation of swimming in developing zebrafish is thus fundamentally similar to locomotion in adu

No description available.

The development of spinal circuits underlying motor behaviors was examined in zebrafish. Zebrafish embryos showed three sequential, stereotyped behaviors: a transient period of spontaneous coiling contractions, followed by touch-evoked rapid coils, and swimming. Lesioning the hindbrain eliminated swimming and touch responses, but not the spontaneous contractions. / The first (spontaneous) behavior was chosen for further analysis in order to characterize the underlying circuit. In vivo patch clamp recordings were obtained from identified spinal neurons. These neurons showed periodic depolarizations that triggered rhythmic bursts of action potentials with a frequency and duration that were consistent with those of the spontaneous contractions. As with the behavior, transecting the spinal cord at the hindbrain border did not affect the rhythmic activity patterns of the neurons. Surprisingly the contractions and the periodic depolarizations were insensitive to both general and specific blockade of synaptic transmission. The periodic depolarizations were suppressed by heptanol and by intracellular acidification treatments that are known to uncouple gap junctions, indicating that electrotonic synapses could underlie network synchronization during the earliest motor behavior. / Paired recordings were obtained from identified spinal neurons. These showed that active ipsilateral neurons were electrically coupled in a simple network consisting initially of motoneurons and only three types of interneurons. Therefore, this early spinal circuit consists of rhythmically active and electrically coupled neurons. Furthermore, this circuit is also initially independent of the main neurotransmitter systems, sensory inputs, and descending hindbrain projections. The descending projections are required later in development for the onset of touch responses and swimming.