Morphological, Electrophysiological, and Synaptic Properties of Corticocallosal Pyramidal Cells in the Neonatal Rat Neocortex

Abstract

Neocortical pyramidal cells (PCs) project to various cortical and subcortical targets. In layer V, the population of thick tufted PCs (TTCs) projects to subcortical targets such as the tectum, brainstem, and spinal cord. Another population of layer V PCs projects via the corpus callosum to the contralateral neocortical hemisphere mediating information transfer between the hemispheres. This subpopulation (corticocallosally projecting cells [CCPs]) has been previously described in terms of their morphological properties, but less is known about their electrophysiological properties, and their synaptic connectivity is unknown. We studied the morphological, electrophysiological, and synaptic properties of CCPs by retrograde labeling with fluorescent microbeads in P13-P16 Wistar rats. CCPs were characterized by shorter, untufted apical dendrites, which reached only up to layers II/III, confirming previous reports. Synaptic connections between CCPs were different from those observed between TTCs, both in probability of occurrence and dynamic properties. We found that the CCP network is about 4 times less interconnected than the TTC network and the probability of release is 24% smaller, resulting in a more linear synaptic transmission. The study shows that layer V pyramidal neurons projecting to different targets form subnetworks with specialized connectivity profiles, in addition to the specialized morphological and electrophysiological intrinsic properties.

Synaptic Microcircuits in the Barrel Cortex

“…layer 3); the axon density in layers 5 and 6 is considerably lower [66]. At least a subset of the short, untufted L5 pyramidal cells projects via the corpus callosum to the contralateral S1 cortex [87,88]. This is in accodance with findings on short L5 pyramidal cells in visual and auditory cortex (e.g.…”

“…L5B pyramidal cell pairs have a connectivity ratio of ~ 5-10 % with some of them showing reciprocal connectivity [15,24,76,88,[100][101][102]. The connectivity probability was lower for thick-tufted L5B-L5B connections than for slender-tufted L5A-L5A connections; however, the number of L5B-L5B synaptic contacts (established both on basal dendrites and apical dendritic tufts at a mean distance of ~ 150 µm) was higher than reported for L5A-L5A pyramidal cell pairs.…”

“…Synaptic connections of pairs of 'untufted' pyramidal cells in both layers 5A and 5B of the S1 cortex that project via the corpus callosum to the contralateral brain hemisphere are very different from those between other L5A or L5B pyramidal cell types [88]. The connection probability of pairs of corticocallosal L5 pyramidal cells has been reported to be only ~ 3 %; this is considerably lower than that of L5A-L5A and L5B-L5B connections (e.g.…”

“…The connectivity probability was lower for thick-tufted L5B-L5B connections than for slender-tufted L5A-L5A connections; however, the number of L5B-L5B synaptic contacts (established both on basal dendrites and apical dendritic tufts at a mean distance of ~ 150 µm) was higher than reported for L5A-L5A pyramidal cell pairs. L5B-L5B connections have a high synaptic efficacy with mean uEPSP amplitudes ranging from 0.7 to 1.3 mV [24,88,100] and a release probability of 0.45 under near-physiological conditions [103], i.e. lower than that of L4-L4 and L4-L2/3 connections.…”

A high-capacity model for one shot association learning in the brain

“…We assume that the two neuronal ensembles are in different areas of the brain. A neuron in the brain is connected to 10–20% of its closest neighbors, and this number drops sharply with distance exceeding 200–300μ (see Song et al, 2005; Le Bé et al, 2006; Perin et al, 2011; Levy and Reyes, 2012). The size of the input layer of a cortical column contains roughly N ≈ 5000 neurons (Meyer et al, 2010).…”