|
The many input layers of the neocortex| old_uid | 11537 |
|---|
| title | The many input layers of the neocortex |
|---|
| start_date | 2012/06/19 |
|---|
| schedule | 12h |
|---|
| online | no |
|---|
| location_info | salle Piaget |
|---|
| summary | The thalamocortical (TC) projection to layer 4 (L4) of primary sensory cortex is thought to represent the main route by which sensory information from the periphery reaches the neocortex. TC synapses onto L4 neurons have indeed been found to be highly convergent in many sensory systems. What is the function of the supposedly “sparse, modulatory” TC axon collaterals that innervate other cortical layers? Using new semi-automatic axonal reconstruction methods, my laboratory has found that anatomically these TC synapses outside of L4 are not as sparse as previously thought. Our calculations show that the potential convergence onto non-L4 neurons can be nearly as high as measured convergence onto L4 cells. To measure the actual convergence and strength of TC synapses in these deep layers, we have recorded in vivo from pairs of neurons in barrel cortex and somatotopically aligned thalamic neurons in the ventroposterior medial (VPM) nucleus. We found that unitary “extragranular” connections are weak, like TC-L4 synapses. Though less frequent than TC-L4 connections, measured convergence in these layers is proportional to their respective sensory-evoked responses. Silencing L4 while recording whole-cell from pyramidal neurons outside of L4 does not alter their sensory-evoked responses, suggesting these responses are almost entirely of thalamocortical origin. Indeed, inactivation of the VPM thalamic nucleus, but not the secondary thalamic nucleus (POm), abolished cortical responses. We conclude that L4 is not a distribution hub of cortical activity, contrary to long-standing beliefs, and that thalamus activates multiple cortical layers in parallel. |
|---|
| responsibles | Sackur |
|---|
| |
|