Phasic variations of extracellular potassium during fictive swimming in the lamprey spinal cord in vitro.

The lamprey spinal cord in vitro can generate the motor pattern underlying locomotion, which can be recorded with suction electrodes in the ventral roots. To test if the extracellular level of potassium changed during rhythmic activity, potassium-sensitive microelectrodes were used to systematically (every 25 micrometers) explore the level of extracellular potassium [K+]e in different loci in the transverse plane of the spinal cord. During fictive locomotion the baseline level of [K+] increased with 0.08-0.4 mM in the grey matter. As a rule phasic variations of up to 0.2 mM, correlated to each ventral root burst, were superimposed on the tonic deviation of [K+]e. Changes in this range may cause a moderate depolarization of the spinal neurones and might also affect other neuronal functions including the rhythm-generating circuits.