Effects of elevated magnesium on discharge of myenteric neurons of cat small bowel

Electrical discharge of single units in myenteric ganglia of cat intestine was recorded extracellularly, and the effects on the electrical discharge of elevating the concentration of Mg++ in the Tyrode solution from 0.2 to 10.2 mM were determined. The ongoing discharge of 83% of the burst-type units was blocked by elevation of Mg++. The remainder of the burst-type units and all of the single-spike units tested continued to discharge in elevated Mg++. Frequency distribution histograms of interburst intervals of burst units that were unaffected by elevated Mg++ showed low variance of interburst interval, and multimodes on the histograms were multiples of the first mode. The variance of interburst intervals of burst-type neurons that were blocked by elevated Mg++ was greater than variance of the burst-type neurons that were unaffected by increased Mg++. If it is assumed that the action of elevated Mg++ is blockade of synaptic transmission, then the results indicated that the ongoing discharge of the neurons that were blocked by elevated Mg++ was dependent on synaptic input to the cell. The burst-type units that were unaffected by elevated Mg++ may be endogenous oscillators that do not receive synaptic input. The burst-type units that were blocked by elevated Mg++ may be driven by synaptic input from the burst-type oscillators.