Journal of Neuroscience, Vol 16, 1203-1218, Copyright © 1996 by Society for Neuroscience
Specificity in the axonal connections of layer VI neurons in tree shrew striate cortex: evidence for distinct granular and supragranular systems
WM Usrey and D Fitzpatrick
Department of Neurobiology, Duke University Medical Center, Durham, North Carolina 27710, USA.
Pyramidal neurons in layer VI of striate cortex are the source of
descending projections to the lateral geniculate nucleus (LGN) as well as a
major source of axon terminals in the layers of striate cortex that receive
LGN projections. This study examines how the connections of layer VI
neurons are arranged with respect to the functionally distinct classes of
neurons that compose their cortical and subcortical targets. By placing
injections of biocytin into layer VI of tree shrew striate cortex, we
identified two sublayers that differ in their intracortical and thalamic
connections. Neurons in the upper part of layer VI, layer VIa, terminate in
cortical layer IV, whereas those in the lower part of layer VI, layer VIb,
terminate throughout the supragranular layers, layers I-III. The
selectivity of layer VI subdivisions for the granular and supragranular
layers is also evident in their descending projections. Neurons in layer
VIa terminate preferentially in the LGN layers that supply layer IV (LGN
layers 1, 2, 4, and 5), whereas neurons in layer VIb terminate in the LGN
layers that supply layers I-III (LGN layers 3 and 6) and in the pulvinar
nucleus. Additional subclasses of layer VIa neurons were identified based
on the restriction of their terminal fields to narrow subtiers within layer
IV. By influencing the activity of distinct populations of cortical neurons
and the thalamic neurons that supply them, layer VI neurons could exert a
powerful influence on the flow of activity in functionally distinct
cortical circuits.
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