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SUMMARY1. A comparison has been made of the patterns of muscle activity during swimming in the intact and spinal lamprey, and the patterns of ventral root activity in the in vitro preparation of the lamprey spinal cord.2. Electromyographic (e.m.g.) activity was recorded with intramuscular bipolar electrodes from three segmental levels in intact lampreys swimming in a swim-mill at a range of swimming speeds.3. The patterns of activity obtained were similar to those seen in elasmobranch and teleost fish.4. After high spinal transaction, lampreys could be induced to swim continuously for a period of several minutes in the swim-mill by a light initial mechanical stimulation of the tail or dorsal fin.5. The patterns of e.m.g. activity obtained from spinal animals at a range of swimming speeds were similar to those obtained in the intact state.6. Portions of spinal cord were isolated encompassing those segments from which e.m.g. recordings had been made and ventral root recordings were made in vitro of the rhythmic activity induced by bath application of D-glutamate.7. In all experiments the mean duration of the bursts of activity at any segmental level was directly proportional to the mean cycle duration, and the constant of proportionality (about 036) was similar for all three types of preparation.8. In all preparations the mean time delay for the activation of segments in the rostral-caudal direction was proportional to the cycle duration and to the number of segments between recording positions. The proportionality constant, or phase lag per segment, was approximately equal to 0-01 in all three types of preparation.

Bath application of N-methyl-aspartate induces fictive locomotor activity in the isolated spinal cord preparation of the lamprey, as well as TTX-resistant membrane potential oscillations in many individual neurons. This inherent oscillatory activity is shown to depend on a specific activation of N-methyl-D-aspartate (NMDA) receptors. This activation initiates voltage-dependent, magnesium- requiring membrane potential bistability, presumably due to a development of a region of negative slope conductance in the current- voltage relation of the neuron. When sodium ions were removed from the bathing solution, oscillations disappeared, and the membrane potential was maintained at a hyperpolarized level, suggesting that the depolarizing current during the oscillatory cycle is mainly carried by sodium ions. Replacing Ca2+ with Ba2+ also leads to a cessation of oscillatory activity, with the membrane potential remaining at the more depolarized level. This indicates an involvement of a Ca2+-dependent K+ current during the repolarization phase. These findings, together with the voltage dependence, can account for the main characteristics of the NMDA receptor-induced, TTX-resistant membrane potential oscillations. This oscillatory behavior has been demonstrated in motoneurons and in several interneurons including CC interneurons but has not been found in edge cells, dorsal cells, or lateral interneurons. The possibility that inherent oscillatory membrane properties may contribute to the activity pattern during fictive locomotion was investigated in experiments with intracellular current injection in the absence of TTX. The stimulation effects obtained required the presence of magnesium ions and were analogous to the stimulation effects seen during oscillations after TTX blockade. Together with similarities in, for instance, frequency and amplitude between the locomotor oscillatory activity and the TTX-resistant oscillations, the results are compatible with an involvement of inherent, oscillatory membrane properties during fictive locomotion in the lamprey spinal cord.