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Abstract

Dendritic spines receive most excitatory inputs in the neocortex and are morphologically very diverse. Recent evidence has demonstrated linear relationships between the size and length of dendritic spines and important features of its synaptic junction and time constants for calcium compartmentalisation. Therefore, the morphologies of dendritic spines can be directly interpreted functionally. We sought to explore whether there were potential differences in spine morphologies between areas and species that could reflect potential functional differences. For this purpose, we reconstructed and measured thousands of dendritic spines from basal dendrites of layer III pyramidal neurons from mouse temporal and occipital cortex and from human temporal cortex. We find systematic differences in spine densities, spine head size and spine neck length among areas and species. Human spines are systematically larger and longer and exist at higher densities than those in mouse cortex. Also, mouse temporal spines are larger than mouse occipital spines. We do not encounter any correlations between the size of the spine head and its neck length. Our data suggests that the average synaptic input is modulated according to cortical area and differs among species. We discuss the implications of these findings for common algorithms of cortical processing.

The spine neck filters membrane potentials

“…Spine necks ranged from 0.2 to Ϸ2 m, and head diameters ranged from 0.5 to 1.2 m (average ϭ 0.77 Ϯ 0.06 m in length and 0.77 Ϯ 0.02 m in diameter, n ϭ 58 spines). These values are comparable to those measured in our light-level morphological studies of this same population of neurons (19) and our recent ultrastructural reconstruction of spines from mouse primary visual cortex (J. I. Arellano, A. Espinosa, A. Fairen, R.Y., and J. DeFelipe, unpublished work).…”

“…1 A Inset), mostly at distances of Ϸ20-80 m from the soma. This is a population of spines that we have previously characterized morphologically and that constitutes a large proportion of the spines in this class of neurons (19,20).…”

“…1 A Inset), mostly at distances of Ϸ20-80 m from the soma. This is a population of spines that we have previously characterized morphologically and that constitutes a large proportion of the spines in this class of neurons (19,20).For these experiments, we recorded the somatic depolarization after uncaging glutamate at the head of the spine, a signal that we termed ''uncaging potential.'' The glutamate uncaging protocol chosen for this study was the same for all spines, positioning the uncaging laser at the edge of the spine, within Ϸ0.2 m of its membrane, and using a 4-ms laser uncaging pulse with 2.5 mM extracellular concentration of 4-methoxy-7-nitroindoline-caged L-gluamate (21).…”

Dendritic spines linearize the summation of excitatory potentials

“…To compare the integration of synaptic inputs observed upon stimulation of dendritic spines vs. stimulation of dendritic shafts, we used two-photon uncaging of MNI-glutamate (29,30) to activate spines and dendritic shafts located 15-125 m away from the soma on basal dendrites from layer 5 pyramidal neurons of mouse V1 slices (17,34). Neurons were patch clamped and filled with Alexa Fluor 488 for imaging (Fig.…”

“…Although we have only explored basal spines at particular distances (most of them at 20-80 m) from the soma, the locations of the examined spines cover the regions of highest spine density in pyramidal neurons (34,41), and basal dendrites can host the majority of the spines of pyramidal cells (42). With respect to other cell types besides mouse layer 5 neurons, linear or near-linear summation of EPSPs has been also documented in rat hippocampal pyramidal neurons (22,23), rat neocortical layer 2/3 and layer 5 cells in vitro (24,27) and in vivo (25), and even rat Purkinje cells in vitro (43).…”