Ultrastructure and synaptic contacts in barrels of mouse SI cortex

Barrels in the posteromedial barrel subfield (PMBSF) in layer IV of mouse somatosensory cortex are consistently identifiable and somatotopically related to the large whiskers on the animal's snout^32,38. This is a study of the ultrastructure - in particular the synaptic connections - of mouse PMBSF barrels. A technique was developed which enabled the precise selection for electron microscopy of specific regions of barrels identified in the light microscope. Barrel 'sides', containing a high density of neuronal cell bodies and myelinated axons, contrasted with barrel 'hollows' in which these elements were relatively sparse. The types and frequencies of synapses were examined in series of thin sections through sides and/or hollows of barrels B₂, C₃, C₇, D₄, E₄, E₅ and E₈ from 6 mice. In all, 3042 synapses from 20 fields, each at least 10 μm × 10 μm × 4.8 μm in size, were classified. Four distinct kinds of presynaptic terminal were identified: (1) darkly, and (2) lightly stained, asymmetrically synapsing terminals (AD, AL respectively), and (3) darkly, and (4) lightly stained, symmetrically synapsing terminals (SD, SL respectively). Synapses of these terminals were distributed in roughly similar proportions throughout the neuropil of barrel sides and hollows. Thus in all barrel regions approximately 92% of the synapses in neuropil were made by AD terminals, 1.0% by AL, 2.4% by SD and 4.0% by SL terminals. Clusters of vertically oriented dendrites, many of which are probably apical dendrites of layer V pyramidal neurons, were preferentially located in sides and in areas ('septa') between barrels. The synaptic input to 54 segments (each ≈ 5 μm long) of 42 clustered dendrites in the sides and/or hollows of barrels B₂, C₃, C₇, D₄, E₅ and E₈ in 6 mice was analyzed. Almost all synapses onto clustered dendrite spines were from AD terminals; however, AD terminals formed only about two-thirds of the synapses onto clustered dendrite shafts. The remainder of the shaft synapses were from SD and SL terminals.