Nice intro describing the relation between turtle 3-layer and 6-layer cortex and evolution. Some old-school papers sounds pretty interesting that I should look into. (Connors and Kriegstein (1986), Mulligan and Ulinkski (1990), Reiner (2000)).
Well just look at the difference:
FIG. 1. Dendritic structure of turtle pyramidal neurons. A: schematic diagram of a typical coronal cortical slice positioned over a photograph of the dissected turtle brain. B: diagram of a cortical slice in more detail. The picture of the pyramidal neuron shows the region from which most recordings were made. The thick blue line is the region containing most cell bodies. The main regions of the dorsal cortex are labeled from medial (M) to lateral (L) including the dorsal–medial region (DM) and two dorsal subregions (D1 and D2) from which most of the recordings were made. C and D: outline of turtle and rat L5 pyramidal neurons determined from biocytin fills. Note that that most of the dendrites from the turtle cell are single branches that extend from the somatic region. The L5 neuron has a major apical branch with oblique branches, an apical tuft, and basal dendrites.
Many ephys properties are the same. Active back-propagation of action-potentials is likely. Dendritic APs can grow in size with successive AP initiation. APs initiated closer to the soma, not in the dendrites (Soma electrode AP come before dendrite, even if AP is triggered via dendrite electrode).
Dendritic sodium-spikes can also be initiated similar to hippocampus. Protocol to evoke Ca spikes in rat neocortical pyramids did not evoke Ca spikes in turtle pyramids. Known that Ca spikes can be generated by blocking K channels (Connors & Kriegstein).
"Prepotentials" are small spikelets that Connors & Kriegstein observed. CK thought that these were dendritic in origins, but Larkum says that they are axonal - "prepotentials are axonal spikes that fail to invade the soma and are not dendritic spikes."
Calcium spikes were present. 100 ms time constant. Some cells had calcium waves, where the calcium slowly propagates along the dendrites, likely coming from intracellular stores. Waves not as big as calcium transients in neocortical cells.
"The most obvious consequence of the lack of dendritic Ca2 spikes in reptilian pyramidal neurons
is that dendritic input does not switch the somatic firing pattern from regularly spiking to bursting as in the neocortex (Larkum and Zhu 2002). It has been suggested that the Ca2 spike in neocortical pyramidal neurons serves to associate feedback inputs arriving at the tuft with feedforward inputs in the basal regions (Larkum et al. 1999b, 2007), which clearly is not possible in the turtle cortex."
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