Geometry and the Organizational Principle of Spine Synapses along a Dendrite

Abstract

Precise information on synapse organization in a dendrite is crucial to understanding the mechanisms underlying voltage integration and the variability in the strength of synaptic inputs across dendrites of different complex morphologies. Here, we used focused ion beam/scanning electron microscope (FIB/SEM) to image the dendritic spines of mice in the hippocampal CA1 region, CA3 region, somatosensory cortex, striatum, and cerebellum (CB). Our results show that the spine geometry and dimensions differ across neuronal cell types. Despite this difference, dendritic spines were organized in an orchestrated manner such that the postsynaptic density (PSD) area per unit length of dendrite scaled positively with the dendritic diameter in CA1 proximal stratum radiatum (PSR), cortex, and CB. The ratio of the PSD area to neck length was kept relatively uniform across dendrites of different diameters in CA1 PSR. Computer simulation suggests that a similar level of synaptic strength across different dendrites in CA1 PSR enables the effective transfer of synaptic inputs from the dendrites toward soma. Excitatory postsynaptic potentials (EPSPs), evoked at single spines by glutamate uncaging and recorded at the soma, show that the neck length is more influential than head width in regulating the EPSP magnitude at the soma. Our study describes thorough morphologic features and the organizational principles of dendritic spines in different brain regions.

Functional and multiscale 3D structural investigation of brain tissue through correlativein vivophysiology, synchrotron micro-tomography and volume electron microscopy

“…5j) , a specialised endoplasmic reticulum organelle present in large spines and involved in calcium homeostasis and synaptic plasticity 1,45–48 . As suggested previously 49 , spines were denser on apical oblique dendrites compared to the trunk (Fig. 5e) as were spines with spine apparatus (Fig.…”

Three-Dimensional Structure of Dendritic Spines Revealed by Volume Electron Microscopy Techniques

“…the somatosensory cortex, striatum, and cerebellum. We found significant differences in the dimensions of the spine head and spine neck, and in the density of spines on dendrites among the brain regions (Parajuli et al, 2020a; Figure 3). Despite large differences in spine dimensions, the ratio of the PSD area to neck length was not significantly different between spines in the CA1 and cerebellum.…”

“… Morphological diversity of dendritic spines in the brain (adapted from Parajuli et al, 2020a ). Presynaptic boutons (b), dendrites (d), and spines (s) can be identified in FIB-SEM images from the cerebral cortex (A) and cerebellum (B) .…”

“…For example, thorny excrescence spines in the CA3 region of the hippocampus are extremely large (Wilke et al, 2013 ) with profuse branching of the spine heads. The cytosol of these spines contains numerous mitochondria, which is generally excluded from typical dendritic spines in the brain (Parajuli et al, 2020a ). Similarly, a recent volume EM study in the inferior colliculus of the barn owl reported doughnut-shaped spines with a hole in the center, without any obvious head and neck like features (Sanculi et al, 2020 ).…”

“…Slices as thin as 7–10 nm can be sequentially milled from a specimen block face using the gallium ion beams in the FIB-SEM imaging method ( Figure 2 ). As such, FIB-SEM has the advantage over other techniques to visualize fine subcellular structures, such as postsynaptic density (PSD), synaptic vesicles, and intracellular organelles, such as endoplasmic reticulum and mitochondria (Parajuli et al, 2020a ). However, array tomography and SBF-SEM have a wider field of view and are more appropriate for studying synaptic connectivity in local neuronal networks.…”