Fig. 7.

(A) Aspiny cells fall into two patterns of responsiveness to perforant path stimulation. (A) Simultaneous extracellular recordings from the granule cell layer (top, ‘GCL’) and intracellular recordings from an aspiny cell (bottom, ‘INT’). This fast-spiking cell was very sensitive to stimulation of the perforant path compared to granule cells, since it burst at a stimulus intensity that produced only a small population spike (left, ‘PRE STIM’). After several minutes of intermittent stimulation (right, ‘POST STIM’), the fast-spiking cell was depolarized and unable to fire action potentials. This brief period of intermittent stimulation was not sufficient to decrease paired-pulse inhibition of granule cells, as does happen after prolonged intermittent stimulation (see B). Calibration = 5 mV (GCL), 10 mV (INT), 10 ms. (B) Similar experimental arrangement as in A, except recordings were from a different slice. This fast-spiking cell was not more sensitive to perforant path stimuli than granule cells, since it fired one action potential at a stimulus strength which produced a large population spike. After prolonged intermittent stimulation blocked paired-pulse inhibition (right, ‘POST STIM’), the responses of the fast-spiking cell were similar to the ‘PRE STIM’ condition. Calibration (in A) = 12 mV, 20 ms. (C) Individual granule cells excite individual fast-spiking cells. Simultaneous intracellular recordings are shown of a granule cell (top) and a fast-spiking cell (bottom). Stimulation of the granule cell by current injection evoked a series of action potentials. Following each granule cell action potential the fast-spiking cell depolarized or fired an action potential. A spontaneous action potential is marked by the arrow. Small filled circles mark the start and end of the current pulse used to cause granule cell discharge. Calibration (in A) = 12 mV, 30 ms. Reprinted with permission from Scharfman and Schwartzkroin⁵² and Scharfman et al.⁵³.