Dendritic synaptic integration in central neurons

sodium and calcium ion channels (Figure 1B,C). This discovery has widespread implications for the ways neurons integrate the many thousands of synaptic inputs they receive. Theoretical studies pioneered by Rall [2] laid the foundations for the way neuroscientists think about synaptic integration. He suggested that dendrites behave like electrical filters, reducing the amplitude of synaptic potentials as they travel from their dendritic site of generation to the cell body. This electrical filtering property would mean that synaptic inputs generated at dendritic sites close to the soma and axon will powerfully influence action potential firing, whereas synapses located on small caliber dendrites remote in the dendritic tree will have a dwindling influence on neuronal output. Recently, simultaneous electrical recordings from the soma and dendrites of central neurons maintained in vitro have tested the predictions of these theoretical studies. The results show that the amplitude of excitatory postsynaptic potentials in cortical pyramidal neurons are massively attenuated as they spread from basal and apical dendritic sites of generation through the dendritic tree to the cell body and axon [4,5]. Therefore, each synaptic input provides a diminutive drive for the generation of action potential firing. So why have dendritic synapses that seemingly have such a weak impact on neuronal output? Classically, it has been assumed that, although each synaptic input has little influence on neuronal output when activated alone, the activation of groups of dendritic synaptic inputs will produce a coherent signal that drives action potential firing. But this idea seems to be wrong, at least for synaptic inputs generated at dendritic sites remote from the cell body. Direct dendritic recordings have shown that barrages of excitatory synaptic input generated distally in the dendritic tree of cortical pyramidal neurons do not summate to form a coherent signal, but rather are heavily attenuated and provide a weak direct drive for action potential firing [6]. This behaviour arises because the time-course Dendritic synaptic integration in central neurons