Fig. 2.

Morphology and electrophysiology of layer IV spiny neurons. a, b, Photomicrographs (scale bar, top panel, 25 μm) and somatodendritic reconstructions (scale bar,bottom panel, 100 μm) of biocytin-stained layer IV spiny neurons. The lighter shaded areas mark the dimensions of the respective barrels.a, Spiny stellate cell with dendrites showing a spherical organization restricted to the barrel. b, Pyramidal cell with apical dendrite reaching layer I and basal dendrites forming a skirt-like pattern. Action potential firing pattern (c, d) and correlation of first-ISI versus second-ISI (e). f, Synaptic responses of layer IV spiny neurons. Spiny stellate as well as star pyramidal and pyramidal cells revealed one of the following firing patterns on injection of a suprathreshold depolarizing current pulse at resting membrane potential. In regular spiking cells, depolarizing current evoked a low-frequency train of single action potentials (c). Intrinsically bursting neurons revealed an initial high-frequency burst consisting of an action potential followed by a depolarizing afterpotential with at least two spikes of decreasing amplitude (d). The initial burst was followed by a sequence of single action potentials. e, Correlation of first-ISI versus second-ISI revealed two clusters representing the two types of action potential firing pattern: IB firing (small cluster at bottom left) and RS firing (larger cluster attop right). Data are shown for 46 spiny neurons (28 spiny stellate cells and 18 pyramidal cells). f, Postsynaptic responses of a spiny neuron to orthodromic electrical stimulation at different membrane potentials. The stimulus elicited an action potential and an EPSP followed by a fast (asterisk) and slow (double asterisk) IPSP.