Prolonged and patterned direct extracellular stimulation of single neurons

¹ Laboratory of Neurophysiology, National Institute of Mental Health; and National Institute of Neurological Diseases and Blindness, National Institutes of Health, Bethesda, Maryland

Current parameters for direct extracellular excitation and inhibition of single neuron activity in the frog brain (by application of current through the recording micropipette) are described. Rheobase for normally inactive cells is between 1 and 3 x 10^–9 A; threshold for modulation of activity in spontaneous cells (25% effects) is about 2.5– 3.5 times lower than for initiation of activity in inactive cells. Rebound from inhibition, accumulation of post-inhibitory rebound, escape from inhibition, postexcitatory rebound depression and off-responses and after-discharges to direct stimulation are described at the unit level. The comparative aspects of current sensitivity of neurons is discussed and with the exception of the specialized electrical receptor in some fish, it is concluded that neurons so far investigated have no more sensitivity (slope, impulses/sec/10^–9 A) to current than a nonadapting 20–30 µ diameter invertebrate unmyelinated nerve. The responses of cells after sustained depolarizations and the behavior of cells to patterned intermittent depolarizations are described; long after effects are revealed. Evidence is presented that some of the effects appear to be due to positive and negative feedback presumably from a small population of interconnected neurons whose activity can be modified by the one cell being excited. It is suggested that the classical excitabilities so far studied in single neurons are perhaps only rarely the limiting factors in natural patterned neuronal activity.