The Journal of Neuroscience, December 15, 1998, 18(24):10525-10540
The Connection from Cortical Area V1 to V5: A Light and Electron
Microscopic Study
John C.
Anderson1,
Tom
Binzegger1,
Kevan A. C.
Martin1, and
K. S.
Rockland2
1 Institute for Neuroinformatics, 8057 Zürich, Switzerland, and 2 University of Iowa
Hospitals and Clinics, Iowa City, Iowa 52242
Area V5 (middle temporal) in the superior temporal sulcus of
macaque receives a direct projection from the primary visual cortex
(V1). By injecting anterograde tracers (biotinylated dextran and
Phaseolus vulgaris lectin) into V1, we have examined the
synaptic boutons that they form in V5 in the electron microscope.
Nearly 80% of the target cells in V5 were spiny (excitatory). The
boutons formed asymmetric (Gray's type 1) synapses with spines (54%), dendrites (33%), and somata (13%). All somatic targets and some (26%) of the target dendritic shafts showed features characteristic of
smooth (inhibitory) cells. Each bouton formed, on average, 1.7 synapses. The larger boutons formed multiple synapses with the same
neuron and completely enveloped the entire spine head. On most
dendritic shafts and all somata the postsynaptic density en
face was disk-shaped but in about half the cases the
reconstructed postsynaptic densities of synapses on spines appeared as
complete or partial annuli. Even in the zones of densest innervation
only 3% of the asymmetric synapses were formed by the labeled boutons. Although the V1 projection forms only a small minority of synapses in
V5, its affect could be considerably amplified by local circuits in V5,
in a way analogous to the amplification of the small thalamic input to
area V1.
Key words:
visual cortex; area MT; corticocortical; synapse
morphology; postsynaptic target; 3-D reconstruction
Copyright © 1998 Society for Neuroscience 0270-6474/98/182410525-16$05.00/0
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