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Brain Struct Funct. 2018 Jan;223(1):77-90. doi: 10.1007/s00429-017-1470-7. Epub 2017 Jul 18.

Volume electron microscopy of the distribution of synapses in the neuropil of the juvenile rat somatosensory cortex.

Author information

1
Laboratorio Cajal de Circuitos Corticales, Centro de Tecnología Biomédica, Universidad Politécnica de Madrid, Pozuelo de Alarcón, 28223, Madrid, Spain.
2
CIBERNED, Centro de Investigación Biomédica en Red de Enfermedades Neurodegenerativas, Madrid, Spain.
3
Instituto Cajal, Consejo Superior de Investigaciones Científicas, 28002, Madrid, Spain.
4
Laboratorio Cajal de Circuitos Corticales, Centro de Tecnología Biomédica, Universidad Politécnica de Madrid, Pozuelo de Alarcón, 28223, Madrid, Spain. amerchan@fi.upm.es.
5
CIBERNED, Centro de Investigación Biomédica en Red de Enfermedades Neurodegenerativas, Madrid, Spain. amerchan@fi.upm.es.
6
Departamento de Arquitectura y Tecnología de sistemas Informáticos, Universidad Politécnica de Madrid, Boadilla del Monte, 28660, Madrid, Spain. amerchan@fi.upm.es.

Abstract

Knowing the proportions of asymmetric (excitatory) and symmetric (inhibitory) synapses in the neuropil is critical for understanding the design of cortical circuits. We used focused ion beam milling and scanning electron microscopy (FIB/SEM) to obtain stacks of serial sections from the six layers of the juvenile rat (postnatal day 14) somatosensory cortex (hindlimb representation). We segmented in three-dimensions 6184 synaptic junctions and determined whether they were established on dendritic spines or dendritic shafts. Of all these synapses, 87-94% were asymmetric and 6-13% were symmetric. Asymmetric synapses were preferentially located on dendritic spines in all layers (80-91%) while symmetric synapses were mainly located on dendritic shafts (62-86%). Furthermore, we found that less than 6% of the dendritic spines establish more than one synapse. The vast majority of axospinous synapses were established on the spine head. Synapses on the spine neck were scarce, although they were more common when the dendritic spine established multiple synapses. This study provides a new large quantitative dataset that may contribute not only to the knowledge of the ultrastructure of the cortex, but also towards defining the connectivity patterns through all cortical layers.

KEYWORDS:

Dendritic shafts; Dendritic spines; Dual-beam electron microscopy; Excitatory synapses; FIB/SEM; Inhibitory synapses; Serial section three-dimensional reconstruction; Somatosensory cortex

PMID:
28721455
PMCID:
PMC5772167
DOI:
10.1007/s00429-017-1470-7
[Indexed for MEDLINE]
Free PMC Article

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