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Activity of myotomal motoneurons during fictive swimming in frog embryos.

1. Dye-filled microelectrodes have been used to identify and to examine the electrical activity of spinal cord motoneurons during fictive swimming in amphibian embryos. 2. Impaled neurons all had ventral cell bodies, dorsal, lateral, or ventral dendrites, and most showed either damaged or complete peripheral axons projecting out to the myotomes. It was rarely possible to identify cells by recording 1:1 motor root spikes evoked by intracellular current pulses. 3. During fictive swimming, motoneurons are tonically depolarized, fire 1 spike per swimming cycle, and are inhibited in phase with motor root activity on the opposite side. Motoneurons can also fire synchronously on the two sides, at double the normal swimming frequency. They occasionally also show a pattern of lower frequency alternating activity in which there is a prolonged burst of discharge on each cycle.