Scientific Understanding of Consciousness
Dendrite Synaptic Integration
Science 7 August 2009: Vol. 325. no. 5941, pp. 756 - 760
Synaptic Integration in Tuft Dendrites of Layer 5 Pyramidal Neurons: A New Unifying Principle
Matthew E. Larkum,1 Thomas Nevian,1 Maya Sandler,2 Alon Polsky,2 Jackie Schiller2
1 Department of Physiology, University of Berne, Bühlplatz 5, 3012 Berne, Switzerland.
Tuft dendrites are the main target for feedback inputs innervating neocortical layer 5 pyramidal neurons, but their properties remain obscure. We report the existence of N-methyl-D-aspartate (NMDA) spikes in the fine distal tuft dendrites that otherwise did not support the initiation of calcium spikes. Both direct measurements and computer simulations showed that NMDA spikes are the dominant mechanism by which distal synaptic input leads to firing of the neuron and provide the substrate for complex parallel processing of top-down input arriving at the tuft. These data lead to a new unifying view of integration in pyramidal neurons in which all fine dendrites, basal and tuft, integrate inputs locally through the recruitment of NMDA receptor channels relative to the fixed apical calcium and axosomatic sodium integration points.
The pyramidal neuron is the basic computational unit of the cortex. Its distal tuft dendrite is heavily innervated by horizontal fibers coursing through layer 1 (L1), which provide long-range corticocortical and thalamocortical associational input. In the standard view of dendritic electrogenesis of L5 pyramidal neurons, the basal and apical tuft dendrites are quite different. Whereas thin basal dendrites of neocortical pyramidal neurons initiate local N-methyl-D-aspartate (NMDA) and weak Na+ spikes, the apical dendrite is able to initiate calcium spikes. However, this view is based mostly on recordings from the thick apical dendrite, and little information is presently available with regard to the actual properties of the tuft dendrites, which are thin dendrites branching from the main bifurcation forming a tree that resembles more closely the basal dendritic tree.
There are three stages (thresholds) in the integration of top-down associative information terminating at distal tuft branches: (i) NMDA spike initiation at the distal tuft branches, (ii) Ca2+ spike initiation near the main bifurcation, and (iii) sodium spike initiation at the axon hillock.
A new unifying principle emerges representing how pyramidal neurons integrate synaptic information. The thin distal tuft and basal dendrites of pyramidal neurons, which receive the overwhelming majority of synaptic inputs, appear to constitute a class of dendrite in which NMDA spikes are the predominant regenerative events summing synaptic inputs in semi-independent compartments. The output of each subunit in this class of dendrite is passed on to the major sites of integration at the axon and apical calcium initiation zones, which can all interact via actively propagated signals, enabling the interactions between top-down and bottom-up information.
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