Cellular physiology of the turtle visual cortex: distinctive properties of pyramidal and stellate neurons.

The electrophysiological properties of neurons in the three-layered dorsal cortex of the turtle, Pseudemys scripta elegans, have been studied in vitro. Intracellular recordings suggested two distinct classes of neuronal behavior. Cell labeling with either Lucifer Yellow or horseradish peroxidase revealed that these behaviors correlated with the two morphological classes of cortical neurons: pyramidal cells and stellate cells. Examination of Golgi-stained neurons of dorsal cortex did not uncover any other obvious classes. Pyramidal cells had their somata in the cell layer, and extended several densely spined apical dendrites through the molecular layer to the pia. They also had spiny basilar dendrites directed through the subcellular layer toward the ependymal border. Physiologically, pyramidal cells had relatively prolonged action potentials that showed marked frequency adaptation during a sustained suprathreshold current pulse. Their most striking characteristic was a tendency to fire two discrete sizes of action potential, one small (mean = 34 mV) and of relatively low threshold, the other large (mean = 76 mV) and of higher threshold. We hypothesize that at least some small spikes arise from distal dendritic sites, whereas large spikes are somatically generated. Both spikes were tetrodotoxin-sensitive, although calcium-dependent electrogenesis occurred when potassium channels were blocked. In contrast to pyramidal cells, the somata of stellate cells were found in the molecular and subcellular zones. Their dendrites tended to be horizontally oriented and spine-free. Stellate cells had relatively brief action potentials, each of which was followed by a large but short-lasting undershoot of membrane potential. Stellate cells showed little or no spike frequency adaptation. Spike amplitudes were always relatively uniform and large (mean = 73 mV). Thus, in the dorsal cortex of turtles, the pyramidal cells, which are projection neurons, and stellate cells, which are local GABAergic inhibitory neurons, have distinctly different membrane characteristics. The physiological properties of the two types of turtle cortical neurons are very similar to their counterparts in cortical structures of the mammalian telencephalon.