Shao Z, Burkhalter A
Department of Anatomy and Neurobiology, Washington University School of Medicine, St. Louis, Missouri 63110, USA.
J Neurosci. 1996 Nov 15;16(22):7353-65. doi: 10.1523/JNEUROSCI.16-22-07353.1996.
Different cortical areas are linked reciprocally via forward and feedback connections. Forward connections are involved in the representation of retinal images, whereas feedback pathways may play a role in the selection and interpretation of visual information. To examine the synaptic mechanisms of forward and feedback connections between primary and secondary visual cortical areas directly, we have performed intracellular recordings in slices of rat visual cortex. Irrespective of stimulus intensity and membrane potential, 78% (45/58) of the cells in striate cortex activated by feedback input showed monosynaptic responses that were depolarizing only, and inhibitory inputs were evident merely as a slight acceleration in the decay of EPSPs. In contrast, in 89% (17/19) of the cells, stimulation of forward input evoked monosynaptic excitatory postsynaptic potentials (EPSPs), followed by disynaptic, hyperpolarizing inhibitory postsynaptic potentials (IPSPs). EPSPs followed by IPSPs also were recorded after stimulation of local connections within primary visual cortex (92%, 12/13) and after activation of thalamocortical input (91%, 10/11). These results suggest that the synaptic organization of feedback connections are distinct from forward, local, and thalamocortical circuits. The findings further indicate that intracortical back projections exert modulatory influences via synaptic mechanisms in which weak inhibitory input is strongly dominated by excitation.
不同的皮质区域通过前馈和反馈连接相互关联。前馈连接参与视网膜图像的表征，而反馈通路可能在视觉信息的选择和解释中发挥作用。为了直接研究初级和次级视觉皮质区域之间前馈和反馈连接的突触机制，我们在大鼠视觉皮质切片上进行了细胞内记录。无论刺激强度和膜电位如何，由反馈输入激活的纹状皮质中78%（45/58）的细胞显示出仅为去极化的单突触反应，抑制性输入仅表现为兴奋性突触后电位（EPSP）衰减的轻微加速。相比之下，在89%（17/19）的细胞中，刺激前馈输入诱发单突触兴奋性突触后电位（EPSP），随后是双突触、超极化抑制性突触后电位（IPSP）。在刺激初级视觉皮质内的局部连接后（92%，12/13）以及激活丘脑皮质输入后（91%，10/11），也记录到了先有EPSP后有IPSP的情况。这些结果表明，反馈连接的突触组织与前馈、局部和丘脑皮质回路不同。这些发现进一步表明，皮质内的反向投射通过突触机制发挥调节作用，在这种机制中，微弱的抑制性输入被兴奋性输入强烈主导。
