Short latency activation of local circuit neurons in the cat somatosensory cortex.

Intracellular horseradish peroxidase (HRP) study was performed in the cat primary somatosensory cortex (SI) under Nembutal anesthesia. Response properties of neurons were analysed with stimulation of the peripheral nerve (superficial radial nerve; SR) and thalamic ventrobasal nucleus (VB). A total number of 23 cells (15 in layers III and IV, 6 in layer V and 2 in layer VI) were identified morphologically as local circuit neurons with intracellular HRP staining. The latencies of SR-induced (7.7-8.5 ms) as well as VB-induced excitatory postsynaptic potentials (EPSPs) (1.3-1.5 ms) were significantly shorter than those of pyramidal neurons (9.1-10.6 ms for SR and 1.6-2.8 ms VB EPSPs). Morphological features of identified local circuit neurons are all, except one in layer VI, aspiny and presumed to be inhibitory in nature. The present study indicates that presumed inhibitory interneurons in the cat SI could be activated first by thalamic inputs among cortical neurons and set to inhibit the output cells for the sharp contrast in the sensory processing.