Go read the rest of the article.A New Integrative Theory for Cortical Pyramidal Neurons
A discovery reported in last Friday's Science Magazine offers a new unifying principle for how pyramidal cells perform their function in the neocortex, as well as other cortical areas: Synaptic Integration in Tuft Dendrites of Layer 5 Pyramidal Neurons: A New Unifying Principle (by Matthew E. Larkum, Thomas Nevian, Maya Sandler, Alon Polsky, and Jackie Schiller), unfortunately behind a paywall. What this paper does is demonstrate that a type of neural activity called an NMDA spike (see below), already known to occur in the basal branches of the dendrites of pyramidal cells, also occur in the branches in the apical tuft, the arborization that occurs at the top of the apical dendrite, the main dendrite that reaches from the soma (body) up to the top layer (Layer I) where the majority of synaptic connections are made between incoming axons (from other brain regions) and pyramidal cells (which produce outgoing axons to other brain regions).[1]
As of late 2007, "NMDA spikes have been observed in basal dendrites but not apical dendrites",[4] but now: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.[1]Before we look at what these NMDA spikes are, let's look briefly at the implications of this discovery:
Integrative Calculations in Pyramidal Neurons
Figure 1: Structures of selected pyramidal neurons from different cortical areas. Click on image to see full image and original caption. (From Reference 4, figure 1..)
Figure 2: Simplified (!) diagram of the calculating modules according to the "New Unifying Principle" in Reference 1. Each box represents an integrative calculating unit, capable of performing a large variety of calculations with its inputs. The circles represent input information coming via synapses. Note that the logical organization here doesn't map exactly to the physical organization of the neuron: the non-linear voltage responses of the synapses, the locations of the synapses on the local branches, and the non-linear voltage responses of the dendritic membrane of the local branches should all be considered part of the calculation box. The input, then, consists of the flow of neurotransmitters across the synapse. The timing of the arrival of the action potential, and any calculations that take place in the pre-synaptic neuron, aren't included in this diagram. Click on image to see larger version. (Original. You may link to, copy, and/or modify this image, as long as you give credit with a link to this post.)
Looking at Figure 2, we can see that there are a number of modules, potentially nested, which perform semi-independent calculations. They feed their results to modules progressively closer (and ultimately identical) to the soma, which (along with the axon hillock and the first 50-100 microns of the axon) performs the final calculation regarding whether, and when, to fire an action potential. Prior to this research, the modules in the apical tuft (those feeding the proximate apical dendrite, see also figure 1) were regarded as different in kind from those in the basal branches.[4] What this research has (tentatively) demonstrated is that the integrating modules called "Fine Dendrite Branches" in Figure 2 appear to act in very similar fashions, although they provide their outputs to different places.[1]
The outputs of from the tuft branches feed the integrating/calculating module represented by the proximate apical dendrite, which performs a calculation that involves a process called a "calcium spike" (see below), which had been thought to be caused in the tuft branches,[4] but with this research are (tentatively) shown not to be caused in the branches, but only in the proximate apical dendrite.[1] This permits the "New Unifying Principle":The thin distal tuft and basal dendrites of pyramidal neurons, which receive the overwhelming majority of synaptic inputs ([ref]), 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 ([ref]), enabling the interactions between top-down and bottom-up information.[1]This represents a major change to how pyramidal cells should be viewed, offering different pictures of their modular breakdown and evolution. ... We can reasonably suppose that the evolution of the neocortex involved, among other things, some improvements and refinements to the integrating calculation process of the apical calcium initiation zones in the proximate apical dendrite. Pyramidal neurons "are abundant in the cerebral cortex of virtually every mammal that has ever been studied, as well as in those of birds, fish and reptiles, but not amphibians."[4] They are:found in most mammalian forebrain structures, including the cerebral cortex, the hippocampus and the amygdala, but not the olfactory bulb, the striatum, the midbrain, the hindbrain or the spinal cord. Thus, they are found primarily in structures that are associated with advanced cognitive functions[.][4]We can reasonably suppose that the original version, developed either by early amniotes (and independently by fish), or much earlier in vertebrate evolution (and lost by ancestral amphibians), was only capable of supporting the three-layer cortical structure of reptiles and the allocortex of mammals.
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