Doublet and burst firing patterns within the dorsal column nuclei of cat and man.

We have examined extracellularly the firing pattern of neurons in the cat external cuneate nucleus and in the human main cuneate nucleus, focusing upon both the spontaneous firing patterns and its modification by natural stimulation. Many of these neurons exhibit stereotyped doublet or burst firing patterns, e .g., the interval between the spikes might be 1 .0 + 0 .1 msec in a given cell . For most cells, this characteristic doublet interval was between 0.8 and 2.0 msec, with a few extending to 5 msec . While doublets were most common, the number of spikes per burst ranged to six or more . When the external cuneate neurons were synaptically driven by forelimb position changes, the firing rate increased but the proportion of spikes occurring within bursts (the "burst index") often fell . The doublets that occurred became broader during synaptic drive. This paradoxical behavior (the peak instantaneous firing rate falling as the average firing rate rises) is analogous to the doublet firing patterns occasionally observed in spinal motoneurons ; there, the doublet is caused by a large depolarizing afterpotential (postspike hump) which rises through the falling threshold at the end of the relative refractory period to elicit an "extra spike ."