current injection (Kawaguchi, 1995; Gorelova et al., 2002). Fast-spiking (FS) neurons are the most numerous and display very rapid action potentials (~0.5ms half-width) each followed by a deep after-hyperpolarization. FS neuron firing is non-accommodating and rarely do these neurons show bursting type of activity. Late spiking (LS) interneurons show a slowly developing depolarization during current injection that eventually triggers spikes. LS cells also are non-accommodating and have spike widths approximately twice that of FS neurons. Low threshold spike (LTS) neurons generate action potentials upon current injection only when cells are held at fairly hyperpolarized potentials (< –70mV). Action potentials also arise at the end of a hyperpolarizing pulse. Regular spiking nonpyramidal (RSNP) neurons show a slow adaptation to firing and have prominent hyperpolarization-induced inward currents (IH). When combined with morphological and biochemical analysis, FS neurons include some basket cells, PV positive neurons that are also positive for GABA, and some chandelier cells (Kawaguchi, 1995; Markram et al., 2004). LS neurons are neurogliaform cells while LTS neurons have axons terminating on Layer I neurons. RSNP neurons are not easily classified into any of these groups and likely are comprised of multiple and diverse members.
