Properties of Layer 6 Pyramidal Neuron Apical Dendrites

Abstract

Layer 6 (L6) pyramidal neurons are the only neocortical pyramidal cell type whose apical dendrite terminates in layer 4 rather than layer 1. Like layer 5 pyramidal neurons, they participate in a feedback loop with the thalamus and project to other cortical areas. Despite their unique location in the cortical microcircuit, synaptic integration in dendrites of L6 neurons has never been investigated. Given that all other neocortical pyramidal neurons perform active integration of synaptic inputs via local dendritic spike generation, we were interested to establish the apical dendritic properties of L6 pyramidal neurons. We measured active and passive properties of the apical dendrites of L6 pyramidal neurons in the somatosensory region of rat cortical slices using dual patch-clamp recordings from somata and dendrites and calcium imaging. We found that L6 pyramidal neurons share many fundamental dendritic properties with other neocortical pyramidal neurons, including the generation of local dendritic spikes under the control of dendritic inhibition, voltage-dependent support of backpropagating action potentials, timing-dependent dendritic integration, distally located I h channels, frequency-dependent Ca 2ϩ spike activation, and NMDA spike electrogenesis in the distal apical dendrite. The results suggest that L6 pyramidal neurons integrate synaptic inputs in layer 4 similar to the way other neocortical pyramidal neurons integrate input to layer 1. Thus, L6 pyramidal neurons can perform a similar associational task operating on inputs arriving at the granular and subgranular layers.

Isocortex

“…Packing density and the size of these layer Vb pyramids increase from Par1 through FL and reach highest values in HL (Zilles and Wree, 1995). Furthermore, layer V is broader in the rostral portions of HL and FL than in Fr1, the primary motor cortex, which is associated with the most prominent layer V (Chapin and Lin, 1984;Ledergerber and Larkum, 2010).…”

“…Layer IV is not only prominent throughout the anterior parietal region, but also shows remarkable structural differences, thus providing the most conspicuous features for its subdivision into areas. Par2 can be delineated from Par1 since layer IV is thinner in the former area, less cell dense, and does not contain the socalled "barrels," or granule cell aggregates surrounded by granule cell-sparse regions, and visible as vertically oriented cell-dense patches, are surrounded by narrow dysgranular strips in frontal sections through the hemisphere (Chapin and Lin, 1984;Ledergerber and Larkum, 2010). Layer IV is broader in Par1 than in HL or FL.…”

“…They are the only neocortical pyramidal cell type whose apical dendrite, which makes frequent turns, terminates in layer IV rather than in layer I and thus integrate synaptic inputs in layer IV, similar to the way other neocortical pyramidal neurons integrate input to layer I (Ledergerber and Larkum, 2010).…”

Cognitive functions of intracellular mechanisms for contextual amplification

“…At least four differences may have important functional implications. First, their apical dendrites are substantially shorter and thinner than those of layer 5 cells (Ledergerber & Larkum, 2010). Second, in cells of layers 2 and 3 NMDA spikes in tuft dendrites can influence action potential generation in the absence of a back-propagated action potential (Palmer et al, 2014).…”

Local Connections of Excitatory Neurons to Corticothalamic Neurons in the Rat Barrel Cortex

“…Since apical and basal dendrites may separately integrate their inputs and distinctly affect the output of a cortical pyramidal neuron (Mel, 2008;Sjöström et al, 2008;Ledergerber and Larkum, 2010), we further tried to examine the difference between the horizontal distributions of boutons targeting the apical dendrites of CTNs and those targeting the basal dendrites. We first examined the histograms of frequency and relative frequency against the vertical location of apposed boutons (Fig.…”