Changes of Extracellular Potassium Concentration Induced by Neuronal Activity in the Sinal Cord of the Cat

1. Changes of extracellular K(+) concentration, [K](e), arising in the spinal cord of the cat in response to an afferent stimulation were studied by means of K(+)-specific micro-electrodes.2. In the most active areas of the spinal cord a single volley in a large afferent input like the common peroneal nerve or the posterior tibial nerve produced a transient increase in [K](e) of 0.05-0.1 mM, which reached its peak in 0.2-0.3 sec and it declined in about 3 sec.3. Much higher increases in [K](e) were found during repetitive stimulation of an afferent input. The highest increase (by 3 mM) was at 100 Hz, but even at 1 Hz a significant increase of 0.25 mM was observed. Equilibration of accumulated K(+) was slow with a time constant of about 6 sec, which is much longer than could be expected for the same process in free solution.4. A characteristic distribution of increased [K](e) was found in the spinal cord in response to 100 Hz afferent stimulation. The highest increase of 3 mM was found in and around the intermediate nucleus, but at depths between 0.9-1.8 mm the [K](e) increase exceeded 1 mM.5. In the ventral horns afferent stimulation (100 Hz) increased [K](e) by 0.25 mM, while the same stimulation of the ventral root resulted in a [K](e) increase of less than 0.05 mM.6. The consequences of K(e) (+) accumulation after neuronal discharge are discussed in respect to its possible role in the depolarization of primary afferent terminals.