Journal of Neuroscience, Vol 6, 164-177, Copyright © 1986 by Society for Neuroscience
Cellular physiology of the turtle visual cortex: distinctive properties of pyramidal and stellate neurons
BW Connors and AR Kriegstein
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.
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