(Not So) Open Access

I've been going through my posts, checking out some of the references there for subjects of future posts, and I discovered a footnote link that I couldn't get into, although I'd read it before.  Just now, I discovered another, and fixed it (Note 6 in "Modularity Part I: Metabolic Pathways").  What a nuisance.

I'm going to go through my back posts, as I get time, and fix any others, most of which (IIRC) are in Science (magazine).  Meanwhile, here's a trick you can use for most (hopefully all) of the Science links like this in my posts, and likely a great number of other older articles there.

1. Go to HighWire Press, which will bring up a search screen (if it doesn't, you'll have to navigate around the site till you find one).


2. Paste the title of the article you want to read into the field labeled Anywhere in Text:, and click on the phrase selector.


3. Click the Search button.


4. Scan down the resulting list of articles until you find one with a "this article is FREE" panel under it.  To be fair, you should pick a free article related to the reason you want to read one you're after.  (Even better if you want to read it too.)


5. Click on the Full text link at the right (the PDF won't work, if that's all there is, you need another choice).


6. Drop down to the footnotes, and find the one for the title you're after.  (I normally use a text search in the browser.)


7. If there's a link for "Free full text", click it.  If there isn't, you'll have to try another.  (I've always seen one so far.)

So far, this has worked for me, although most of the papers were those I followed free text links to before, and only with Science.  And only with older papers.

Of course, this may not work forever.  And there may be other tricks needed with other publishers, although it may work for articles in other magazines published by HighWire.  I guess it depends how eager you are to read an article, how much effort you'll put into finding a way to it without paying.  It's different for me, since I need to make them available to readers. Read more!

The Origin of the Nervous System(s)

Most of us know what a nerve cell is, at least generally.  It usually consists of a soma (body) with dendrites that collect information, and an axon that carries information about when the cell fires an action potential to other cells, often over long distances.  But how and when did the neuron evolve?  How did the various systems that use neurons evolve?  How much and what kind of communication did the animals before nerves use, and for what? 

I'm going to address these questions in this post, starting with the model I proposed earlier: 

Thus, we have a small ball of cells, with a bunch of collared flagella, probably in front, ciliated cells along the equator, and some sort of sensory abilities. 

[...]

It's completely plausible, IMO, that a small animal like a floating sponge might have had a system of internal water-canals, while still having a regular shape and the ability to swim.  Thus, each lineage would have branched off from the main line (with swimming adults), then developed a sessile lifestyle. 

[...]

It may well be that there was a steady progression of advance in developmental mechanisms, with one lineage of sponges branching off after each advance.  Thus, the last branch, the Homoscleromorpha, would have had almost the full suite of mechanisms, probably including biradial symmetry. 

[...]

Finally, let me mention communication. It seem unlikely that this animal had nerve cells (although it's been suggested^[1]), however much of the cellular machinery needed for chemical communication is present in at least some lineages of sponges.^[1] [I've updated the footnotes.]

So the question is, sort of internal communication did this animal have, and what was the path followed by the lineages leading to the eumetazoan nerve cell?  ... Before we address that question, however, we need to take a closer look at the animal involved, and it's evolution.

The Nature of the Urmetazoan

I've mentioned the way we can look at the paraphyly of the sponges and what it can tell us regarding the progressive evolution of the various improvements found in the eumetazoans relative to what we normally see in (adult) sponges.^[2] Quoting from the referenced paper:

[...] The demonstration that Porifera is paraphyletic and therefore represents an evolutionary grade has important implications for the polarization of character states and understanding the sequence of character acquisition at the base of Metazoa.

Metazoa (Fig. 2, node 1) is characterized, in part, by the acquisition of multicellularity and the presence of the extracellular matrix, a complex of collagen, proteoglycan, adhesive glycoprotein and integrin, which mediates cell motility and transitions between epithelial and motile cell types ([ref]). Because of sponge paraphyly, the WCS [water channel system] with choanocytes (itself a likely plesiomorphic cell type), had also evolved by this point as well. The unnamed clade Calcarea + Epitheliozoa (epitheliozoans are the homoscleromorph + eumetazoans, see below) (Fig. 2, node 2) is potentially characterized by the presence of cross-striated rootlets. Most metazoan ciliated cells have a system of cross-striated rootlets that originates in the ciliary basal body and extends into the cytoplasm. Calcisponge larvae, as well as those of homoscleromorphs, have long, crossstriated cell rootlets ([refs]) that were perhaps incorporated into adult eumetazoans through neotenous evolution ([ref]). However, the choanoflagellate [arguably the closest relative to the metazoans, see here] Monosiga, the placozoan Trichoplax, as well as several other protistan taxa, also have striated rootlets ([refs]) implying that either the rootlets of Calcarea + Epitheliozoa are not homologous with those of choanoflagellates or this trait is plesiomorphic for Metazoa and has been secondarily lost in Hexactinellida and Demospongiae.

The clade Homoscleromorpha + Eumetazoa is herein recognized as Epitheliozoa (Fig. 2, node 3). Ax ([ref]) defined the clade Epitheliozoa for the clade of epithelial animals, and it is usually considered to include Ctenophora, Cnidaria and triploblasts (e.g. [ref]). The position of the homoscleromorphs as the sister taxa to Eumetazoa, as well as the presence of basal laminae ([refs]), suggests that the Epitheliozoa should include the Homoscleromorpha. A second potential apomorphy of the Epitheliozoa is the presence of an acrosome ([refs]). Thus, of the four primary eumetazoan characters—tissues, an acrosome, a nervous system and a gut—the acquisition of tissues and the acrosome antedated the last common ancestor of homoscleromorphs and eumetazoans. The expression of features such as epithelia in the adult, along with the acquisition of these new characters (nervous system and gut), and the loss of the WCS, could be due to a coordinated character change ([ref]) accompanying the neotenous evolution of a non-feeding sponge larva to a predatory eumetazoan.^[2]

Let's take a look at what that means, given our model of a small, free-swimming spheroidal animal with internal water canals:

Figure 1: Summary of discussion points from quoted paper. (Original)

1. Node 1:  Metazoa:
   
   
   1. multicellularity
   
   
   2. extracellular matrix
   
   
   3. WCS (Water Canal System) with choanocytes
   
   
   4. post-synaptic protein homologs^[1] (see below)


2. Node 2:  Calcarea + Epitheliozoa:
   
   
   1. cross-striated rootlets
   
   
3. Node 3:  Epitheliozoa:
   
   
   1. basal laminae
   
   
   2. acrosome
   
   
   3. developmental cell signaling and adhesion genes^[4] (see below)
   
   
   4. Runx genes encoding proteins that bear at their C-terminus a WRPY sequence, which functions to recruit the Groucho/TLE corepressor^[3] (see below)

As you can see, according to Sperling et al., the metazoa already possessed the water canal system and the extra-cellular matrix. The primary development leading to the next mode was cross-striated rootlets, although there is considerable doubt about this development. Finally, the epithelium characterizes the final node that includes modern sponges. Each of these nodes, then, presumably underwent an adaptive radiation as free-swimming animals after refining its development(s), but only a few sessile, benthic, lineages survived when the next radiation came. Only the Eumetazoans actually had free-swimming lineages that survive today.

Figure 2: Sequences of adaptive radiations associated with proposed nodes:  1:  Metazoa, 2:  Calcarea + Epitheliozoa, 3:  Epitheliozoa, 4:  Eumetazoa. (Original, replaced earlier version 7/01/09.)

We can imagine that the original animal, then, had refined its system of water canals, using cilia to swim and some sort of visual and olfactory (chemotaxic) system to steer, allowing it to chase down the populations of bacteria or other food they required. The development of cross-striated rootlets probably allowed the skin-mounted cilia to apply greater pressure to the water as they pushed it, permitting greater size and speed. The development of the epithelia allowed the extra-cellular area to be kept under pressure relative to the surroundings.

An important difference with Sperling et al. is that I'm assuming that the common ancestor's adult form was free-swimming, and that as each lineage evolved a sessile, benthic lifestyle, it also (independently) acquired a larval form with some vestigial characteristics of the ancestral adult.

Internal Communications

Quoting again from the model I proposed earlier: 

If this model is true, our most distant ancestor might have had a fairly sophisticated system of communication long before the invention of nerve cells. It would be directly ancestral to our modern system of chemical emotions, and may have been even more sophisticated, since there were no nerve cells to handle more "digital" types of calculation. Indeed, the chemical intelligence displayed by modern vertebrates in controlling their development may well have been used to control active behavior in this ancient ancestor.

Note here that the presence of a full epithelium permits isolating those chemical communications from the outside, as well as potentially compartmentalizing chemical communications into separate modules.

In three recent post, Nerve Cells and Glial Cells: Redefining the Foundation of Intelligence, Beyond the Synapse, and The Analog Axon, I discussed the large variety of inter-cell communications that don't depend on action potentials flowing along the axon. Not only nerve cells, but at least one, and likely two, types of glial cells can fire action potentials, although they may not do so in vivo. Even without action potentials, voltage variations in cell membranes can carry signals created by chemical signals from other cells. Small patches of cell membrane can act as semi-independent calculators, responding to local conditions with local releases of various transmitters. These conditions include voltage, but also the local concentrations of all sorts of signal molecules on both sides of the cell membrane.

It would be easy to assume that the sophisticated communications just mentioned evolved after the neuron did, but it's more plausible that they were there first, and that the action potential evolved later, as a high-speed, long-distance mechanism for carrying a small subset of time-dependent control information. Thus, we can assume that, by the time the ancestors of the eumetazoans diverged from those of the Homoscleromorphs, much of the system of calculation used in the modern brain was already present, at least in rudimentary form. In this regard, we may note that many of the genes used in brain communications appear to have already been present in even the early nodes.^[1]

This type of calculation/communication is energy intensive, so it's quite likely that the original cells to undertake this process were ectodermal, since such cells would have had the best access to external oxygen. They may well have started by extending small processes toward other cells through the extra-cellular matrix, from which they released transmitters carrying their current message.

It is with the development of the epithelium (Node 3) that things really took off. Both the descendant lineages of this node appear to possess the full complement of developmental cell signaling and adhesion genes.^[4] In addition, a very recent paper,^[3] The evolution of Runx genes II. The C-terminal Groucho recruitment motif is present in both eumetazoans and homoscleromorphs but absent in a haplosclerid demosponge (by Anthony J Robertson, Claire Larroux, Bernard M Degnan and James A Coffman), demonstrates that a specific pair of genes that already existed in ancestral animals were linked in the common ancestor of node 3 but not before.

These two genes, between them, occupy a key position in controlling cell differentiation, and the fact that they are present at this node strongly suggests that the free-swimming ancestors had a complex organization of their bodies. Since the organization of internal water channels probably doesn't require this, the best hypothesis (IMO) is that is was required to organize the communication system.

For an animal a few millimeters long, this system probably didn't require the speed of the action potential, but the complex calculations possible to an organized structure of differentiated cells would have (potentially) made this a quite smart little animal. It seems very plausible to me that the primary advantage of the sophisticated system of developmental control was its ability to support the development of this powerful system of calculation and communication, long before it could gain the advantage of more complex bodies.

The Action Potential

It isn't only neurons that have sophisticated adaptions to carry action potentials, muscle cells do as well.^[5] In fact, they may well have had them first, since they would need synchronized activity over their entire lengths. Most likely the signal speed of action potentials would not even be needed until there were muscles to respond to them, and muscle fibers are so large in comparison to axons that they could have extended a process to the appropriate calculating cell more cheaply than the cell could have done to them.

We can see the neural action potential evolving, then, as the specializations already present in muscle cells are adopted by nerve cells for high-speed communication.

Rather than control of muscles, I suspect that the original value of the neural action potential was to communicate overall chemical "emotional" state: the rudiments of the modern peripheral nervous system. Once the structure of the nerve cell was fully refined, it was then adopted to the control of muscles, a process that probably wasn't complete until the early bilaterians.

Robertson, A., Larroux, C., Degnan, B., & Coffman, J. (2009). The evolution of Runx genes II. The C-terminal Groucho recruitment motif is present in both eumetazoans and homoscleromorphs but absent in a haplosclerid demosponge BMC Research Notes, 2 (1) DOI: 10.1186/1756-0500-2-59

Links: (Some of these are duplicates of links from earlier posts. Not all have been called out in the text. Many are derived from the main article. Use the back key if you came via clicking a footnote.) (I've included only the link referenced in this leader.)

1.  A Post-Synaptic Scaffold at the Origin of the Animal Kingdom



2.  Poriferan paraphyly and its implications for Precambrian palaeobiology

3.  The evolution of Runx genes II. The C-terminal Groucho recruitment motif is present in both eumetazoans and homoscleromorphs but absent in a haplosclerid demosponge

4.  Early evolution of animal cell signaling and adhesion genes

5.  t-tubules and sarcoplasmic reticulum function in cardiac ventricular myocytes Read more!

The Analog Axon

In the traditional view, the action potential is considered a "choke-point" for information:  it's fired (usually at the axon hillock), travels down the axon, and by the time it reaches the synapse there's no information from the pre-synaptic cell left to be passed through the synapse besides the timing of the spike. 

This isn't exactly true, however.  Recent research has shown that within a few hundred microns of the soma the shape of the action potential and the "the average amplitude of the postsynaptic potential" evoked can vary depending on the "somatic membrane potential of the presynaptic neuron".^[2]  Indeed, even without an action potential, voltage changes created by synaptic inputs can propagate a similar distance, modifying the synaptic activity within that range.^[3] 

We now find out, in a recent paper,^[1] that there's another analog signal that can be carried by the axon: small delays in propagation can be introduced as part of a "learning process" on a time-scale of seconds to minutes.  In Long-Term Activity-Dependent Plasticity of Action Potential Propagation Delay and Amplitude in Cortical Networks (by Douglas J. Bakkum, Zenas C. Chao, and Steve M. Potter) we discover that various types of activity experienced by "cortical neurons cultured on extracellular multi-electrode arrays (MEAs)" can modify the timing of signals received at their synapses.  ...

The actual experiments done by Bakkum et al. involved "varying a simple low frequency stimulation pattern every 40 minutes", which "induced changes in the time elapsed for dAPs [direct electrically-evoked action potentials] to propagate from a probe electrode to a recording electrode and also in their extracellularly recorded amplitude (Fig. 3)." 

Delays ranged up to 4 milliseconds (ms), and changes in amplitude reached 80% (20µV).^[1]  This is enough to produce major differences in how the signal from one axon is treated by the receiving dendrites, as dendrites perform very tight calculations of the relative timing of signals from different axons.^[6]  The changes in delay, introduced in response to activity, took place over a period of minutes, consistent with the activity of short-term memory.^[1] 

There were also much more transient delays, from "the impedance mismatch due to the change in volume from the axon into the soma causes a delay in propagation proportional to the somatic membrane potential, which varies with synaptic input [ref]."  This actually may be more interesting from a calculative point of view, as it occurs with a time-frame measured in milliseconds,^[1] the same as that of action potentials. 

I've discussed the ways in which the brain's functioning depends on much more than action potentials, the intelligence of individual synapses, and the need for a "membrane centric" approach to the brain's functioning.  This discovery integrates into these discussions.

It's not a totally new discovery, of course, that some nerve cells use more analog types of communication than just action potentials.^{[4] [5]}  Sensory nerves especially, often make use of analog calculations.  Nevertheless, except for the sensory margins of cognition, the central nervous system is usually pictured as communicating through action potentials that carry only their relative timing as information.  Over long distances, this is probably true.

However, a great many of the connections in the brain, including the neocortex, arguably the most important part of the relative expansion of the human brain, are within a few hundred microns, the distance discussed here. 

This means that not only are the axons (and their membranes) integrated into the system of membrane-level analog computation I discussed in Nerve Cells and Glial Cells: Redefining the Foundation of Intelligence and Beyond the Synapse, but even their action potentials carry much more analog information than the notion of an invariant action potential suggests.

They can alter their propagation time to synchroninze (or not) with other neurons, and their output at synapses within a few hundred microns is partially analog depending on the current activity at the soma (or axon hillock).

This information constitutes another nail in the coffin of the more traditional picture of neurons communicating by means of only the timing of their action potentials.  Quite to the contrary, the vast majority of calculations in the brain are linked into an analog signaling network where even the axons carry much more analog information than just the timing of action potentials.

Bakkum, D., Chao, Z., & Potter, S. (2008). Long-Term Activity-Dependent Plasticity of Action Potential Propagation Delay and Amplitude in Cortical Networks PLoS ONE, 3 (5) DOI: 10.1371/journal.pone.0002088

Links: (Not all of these have been called out in the text.  Many are derived from the main article.  Use the back key if you came via clicking a footnote.) (I've included only the links referenced in this leader.)

1.  Long-Term Activity-Dependent Plasticity of Action Potential Propagation Delay and Amplitude in Cortical Networks

2.  Modulation of intracortical synaptic potentials by presynaptic somatic membrane potential

3.  Combined Analog and Action Potential Coding in Hippocampal Mossy Fibers Requires Free Registration



4.  Impulse-Coded and Analog Signaling in Single Mechanoreceptor Neurons Requires subscription.  Otherwise, click on Ref 3, go to foonote #2 in the article, and click on the /Free Full Text] link.

5.  Information processing by graded-potential transmission through tonically active synapses

6.  Enhancement of presynaptic neuronal excitability by correlated presynaptic and postsynaptic spiking

7.  Recurrent Excitation in Neocortical Circuits Requires subscription.  Otherwise, click on Ref 3, go to foonote #6 in the article, and click on the /Free Full Text] link. Read more!

Under Construction - Adding Folds: Complete!

This should be short.

Assuming everything works:

For a while there will be "read more" links at the end of full posts.

As I modify the posts, only the first small part will show.

Updated 1710 CDT: Short posts will still have a "read more" link, although there won't be more. Longer posts should be properly truncated. If you notice something that seems strange a comment telling me about it would be appreciated. Read more!

Beyond the Synapse

I recently discussed the role of cell membranes, especially those of glial cells, in performing the complex calculations of the brain, and now I want to take a more detailed look at what sort of things may be going on in these calculations. I'm going to tie this discussion to three recent papers, all of which offer important supporting details in understanding just how "smart" the membranes of neurons and glial cells can be.

First,^[1] Fast Subplasma Membrane Ca²⁺ Transients Control Exo-Endocytosis of Synaptic-Like Microvesicles in Astrocytes (by Julie Marchaland, Corrado Calì, Susan M. Voglmaier, Haiyan Li, Romano Regazzi, Robert H. Edwards, and Paola Bezzi) looks into the both the timing and the micro-structure of vesicle-based release of transmitters by astrocytes, one of the key types of glial cell. Although the potential contributions to overall brain-power of the other key type, the NG2-glia, may be greater, we can tentatively assume that anything astrocytes have been shown to do, NG2-glia and neural dendrites can probably also do, although they may not do so in every region or area (or layer, in the neocortex) of the brain.

The second paper,^[5] Mitochondria Modulate Ca²⁺-Dependent Glutamate Release from Rat Cortical Astrocytes (by Reno C. Reyes and Vladimir Parpura), demonstrates the role of the mitochondria in supporting and modulating the use of calcium in astrocytes as a signal for transmitter release. I've discussed the role of calcium as a signal before, especially the way it ties the cell's energy systems to their information processing systems: calcium is often used as a local signal for some process that's energy-intensive, because the mitochondria both act as a buffer for the calcium itself, and when they absorb that calcium it stimulates them to increase energy production, even before the processes that use that energy get well under weigh.

The third,^[6] Modulation of Voltage- and Ca²⁺-dependent Gating of Ca_V1.3 L-type Calcium Channels by Alternative Splicing of a C-terminal Regulatory Domain (by Anamika Singh, Mathias Gebhart, Reinhard Fritsch, Martina J. Sinnegger-Brauns, Chiara Poggiani, Jean-Charles Hoda, Jutta Engel, Christoph Romanin, Jörg Striessnig, and Alexandra Koschak), demonstrates the flexibility of one particular type of voltage-gated calcium channel, in terms of modulating properties such as voltage thresholds and inactivation timing through alternative splicing of the RNA that codes for it. Since the activation voltages for these channels are in the same general range as the voltages involved in the calculations that can take place in cell membranes in the absence of action potentials, the demonstration of this flexibility adds support to our ideas of what glia and neural dendrites may be doing as part of the brain's calculations. (A brief introduction to action potentials and membranes may found here.)

The Glial Membrane

What I'm going to do is look at a small section of glial membrane, building a picture based on what these three papers tell us. Then I'm going to try to expand that to a picture of what a huge network of glial and neural membranes can achieve in the way of carrying and integrating huge amounts of transient data, orders of magnitude beyond the amount of information carried by the axons as action potentials. ...

The glial membrane divides the cytoplasm of the cell from the extra-cellular area. There is a great difference in concentrations of many substances between the inside and the out, including sodium, calcium, potassium, chloride, and a wide variety of neuro-active substances, including classic neurotransmitters, ATP and other purines, and hormones and other more generally diffusing substances such as nitric oxide. It is filled with receptors that respond to various substances outside the cell, as well as ion channels subject to control by the voltage across the membrane (voltage-gated) and neurotransmitters (ligand-gated). (Ligand-gated ion channels also count as receptors). Most of these ion channels are subject to modulation in their activity by other substances, and many are quite complex in their behavior, possessing the ability to open in response to one thing (typically voltage), then close after a short period or in response to changing ion concentrations either outside the cell, or inside.

Under the Membrane

Inside the cell, there isn't just a pool of cytoplasm, randomly filled with various molecules and organelles. Rather, the cell has a cytoskeleton made from filaments of actin and microtubules, that among them help to maintain its shape, and provide a scaffolding for the various organelles and large molecules that make up much of the cell's mass. Not far beneath the astrocyte cell membrane are the various structures of the endoplasmic reticulum (ER). These are mostly in the form of fine tubes (called "tubuli") enclosing material of the lumen of the ER, which can vary dramatically from the cytoplasm in its concentrations of many substances.


Figure 1: Spatial relationship between ER tubuli and SLMVs ["synaptic-like microvesicles"] in the submembrane compartment. Click on image to see original caption and enlargeable image. (From Ref 1, Figure 3.)

The cytoplasm right under the cell membrane contains ER tubuli,^[1] vesicles containing neurotransmitters,^[1] and mitochondria,^{[2] [4] [5]} at the least. It also contains cytoskeletel elements to which the various others are linked, capable of movement. When calcium concentrations rise in the cytoplasm, the vesicles move to the surface and expel some or all of their contents, a process called exocytosis.^[1] As Marchaland et al. show:

We found that the exocytic burst, similar to endocytosis, displayed a bimodal distribution (Fig. 2C), strongly suggesting the existence of two modes of exo-endocytosis. We confirmed this hypothesis with a double transfection approach in that we could directly visualize the fate of SLMVs during exocytosis and retrieval (Fig. 2A,B,D). Indeed, we noticed heterogeneity both in the origin of the SLMVs that underwent exocytosis (Fig. 2D, residents vs newcomers) and in their mode of fusion (kiss-and-run vs full-collapse type of fusion). The rapid phase of the exocytic burst (0–400 ms) was sustained almost exclusively by resident vesicles undergoing kiss-and-run fusion, whereas the slow phase (500 ms–1.6 s) mainly by newcomer vesicles undergoing full-collapse fusion. This duality is reminiscent of observations in neurons in that only readily releasable SVs [synaptic vesicles] are rapidly recycled and reused ([ref]). Our experiments do not clarify whether newcomers and resident vesicles represent distinct population of SLMVs[synaptic-like microvesicles]. The newcomers may indeed represent the same population of resident vesicles that undergo a second round of exocytosis upon rapid recycling.^[1]

What Marchaland et al. refer to as "synaptic-like microvesicles" are vesicles that look and act very similar to those of the neural synapse, but are present here in astrocytes. They are also almost certainly present in NG2-glia, and may well be present in neural dendrites outside the synapse. I doubt anyone would notice them unless they looked for them, and even if they looked and didn't find them, that would only mean they weren't present in that particular population of nerve cells in that species. (While there may be enough similarity between, say, mice and rats that it could reasonably be assumed that they would be alike, in this respect rodents couldn't possibly be used as a model for humans given 60-90 million years of separate evolution, evolution during which the primates specialized in neural and cognitive enhancements.)

Calcium Signaling

The process by which transmitters are released, then, takes place on a sub-second time scale, and is capable of relatively smooth variation in quantity, at least at the levels of the "kiss-and-run" release. Although this research provides no proof, it seems highly likely that some variation is possible in how much is released, depending on the severity of the rise in calcium levels.

The rise in calcium levels is part of a very complex process, in which surface activity coordinates with release of calcium from the ER tubuli, uptake by mitochondria which helps to buffer and modulate the calcium levels,^{[4] [5]} and the response by the system which controls exocytosis to the rise in calcium levels. As Marchaland et al. have demonstrated, vesicles and tubuli appear to be closely coordinated in their location, which suggests an ability to localize the release of transmitters to within a few microns, if not much less.^[1]

As for calcium signalling, the first thing to understand is that calcium is relatively fast-diffusing, over a micron scale.^[15] However, a rise in calcium levels caused by release from tubuli would be buffered by nearby mitochondria, which absorb it, localizing any particular effects.^{[4] [5]} Why do the tubuli release calcium? As it happens, there is a messenger molecule called IP₃ that reacts to receptors on the surface of the ER:

Glutamate-induced Ca²⁺ oscillations in astrocytes result from the activation of class I metabotropic receptors that induce the phospholipase-dependent accumulation of inositol trisphosphate (IP₃) that stimulates the release of Ca²⁺ from IP₃-sensitive internal stores ([ref]). Consequently, by measuring Ca²⁺ signaling, rather than membrane potential, it was discovered that astrocytes are an excitable system.

Since these initial observations it has been realized that astrocytes express a plethora of metabotropic receptors that can couple to second messenger systems ([refs]). For example, norepinephrine ([refs]), glutamate ([refs]), GABA ([ref]), acetylcholine ([refs]), histamine ([ref]), adenosine ([ref]), and ATP ([ref]) have all been shown to induce Ca²⁺ elevations in glial cells in brain slice preparations. In culture, the list of metabotropic receptors is extensive. However, because culturing astrocytes can lead to the misexpression of proteins, it is not yet clear whether all of these receptors are normally expressed in astrocytes in vivo.^[16]

So basically, what happens is that some condition outside the cell interacts with a receptor, which in turn releases IP₃, which in turn causes the ER tubuli to release calcium. If the stimulus is general, so is the response. If the stimulus is localized, so is the response, to a scale of microns or less.^[1] And it's kept localized by the quick absorption of the calcium by mitochondria.^[5]

The Effects of Voltage

Although the voltage across the membrane did not directly stimulate entry of calcium into astrocytes,^[1] many of the metabotropic receptors listed above are subject to modulation by membrane voltage. Thus, voltage has an effect on the release of second messengers. Another important point to consider is the specific experiment involved:

Finally, with pharmacological experiments shown in Figure 4D, we confirmed that submembrane Ca²⁺ events depend exclusively on Ca²⁺ released from internal stores. Thus, the number of events remained unchanged when we removed Ca²⁺ from the incubation medium (0 Ca²⁺ with 5 mM EGTA) or added nifedipine (3 µM), a blocker of L-type voltage-gated Ca²⁺ channels. In contrast, the number of events increased during the first 15 s of incubation with CPA (approximately by 3.8-fold) (supplemental Fig. 11 SI, available at www.jneurosci.org as supplemental material) and returned to a baseline value between 1 and 7 min (0.76 ± 0.25/s/ROI) and eventually almost completely disappeared in ~10 min (0.095 ± 0.014/s/ROI). After 15 min of incubation with CPA, application of DHPG failed to induce submembrane Ca²⁺ events (http://www.jneurosci.org/cgi/content-nw/full/28/37/9122/F4D). These results constitute the first evidence of spatially confined, ER-mediated submembrane Ca²⁺ events occurring in astrocytes on a millisecond time scale.^[1]

In other words, they got the same results when they disabled the use of external calcium. This certainly doesn't demonstrate that voltage activity couldn't cause similar events, especially in vivo.

This brings up Singh et al.,^[6] the third paper, which talks about calcium channels and the way their voltage response (and other properties) can be modified by alternative splicing. This is a mechanism by which newly transcribed RNA is edited in different ways depending on a number of factors. Potentially, astrocytes could make use of this to create various populations of receptors, with different voltage responses, on different parts of their membrane. This, in turn, would allow each tiny part of the cell's membrane to respond in its own way to various voltage signals traveling across the surface.

Where does the voltage come from? I discussed this recently, but let me summarize:

[E]very time an action potential arrives at a synapse, the pre-synaptic part of the synapse releases a bunch of neurotransmitters, which cross the synaptic cleft and interact with receptors on the post-synaptic part, usually called the post-synaptic density (PSD).

Some of these receptors are "ligand gated ion channels", which can allow currents of sodium, chloride, potassium, and calcium in response to neurotransmitters. These currents, in turn, modify the voltage at the synapse, creating a voltage wave that spreads along the cell membrane, that is the membrane of the dendritic arbor. The voltage difference created by any action potential at any synapse decays in the manner described above, even if it never reaches the threshold for an action potential. Similarly, all the other synapses on that post-synaptic cell from that axon (if any) create their local currents, at other points in the arbor. These voltage differences decay in both space and time, the current fading out as the neurotransmitters are taken up or diffuse out of the synapse. While this is happening, other action potentials, in other axons, are adding their own contribution, causing the distribution of voltage across the membrane of the entire arbor to vary in an extremely complex manner. Note that depending on the type of ions involved in the current, it can be depolarizing (tending to lower the voltage towards the threshold) or hyperpolarizing (tending to raise the voltage even farther away from the threshold).

Now, something similar to this will happen to the astrocyte, which has its own synapses from incoming axons.^[16] In addition, the various transmitters floating around in the extra-cellular medium will act on various ligand-gated ion channels, creating currents that can, in turn, modify the voltage across the membrane.

If no action potentials were arriving, the entire membrane might stabilize to constant voltage over time, but even then the voltage might vary from one place to another. With incoming action potentials, you have voltage waves sloshing around, and various ion channels reacting in a non-linear fashion, and various receptors, including ion channels, reacting to the voltage by releasing IP₃ and other second messengers, all of which drive the calcium signaling system, producing a pattern of transmitter release that varies over microns in space, and fractions of a second in time. This transmitter release, in turn, drives differences in concentration of various transmitters over similar distances and times in the extra-cellular medium, creating an interactive feedback with the local arbors of axons and dendrites of various nerve cells, as well as NG2-glia.

What's important about this is that the dendrites, and quite possibly the NG2-glia, will respond with their own changes in voltage response, depending on the local concentrations of a variety of transmitters. All of this is happening on a sub-second time scale, over distances measured in microns or less, a small fraction of the size of the cells involved, even astrocytes.

Creating Intelligence

What does this mean for the brain's computing ability? It means that the entire volume of the grey matter is a sort of massive computing machine. Of course, large amounts are given over to supporting material: blood vessels, myelinated axons, cell bodies, etc. The part where computing is concentrated is called neuropil. This is a densely packed mass of cell parts, their cell membranes typically a fraction of a micron apart.


Figure 2: Complete three-dimensional reconstruction of neuropil (2x2x2 µm). Click on image to see original caption. (From Atlas of Ultrastructural Neurocytology by Josef Spacek, MUDr., DrSc.)

The structure of the neuropil is different in different parts of the brain:^[18] cortical neuropil, neuropil of the molecular layer of the cerebellar cortex, neuropil characteristic of neocortex between pyramidal cells, neuropil between large dendrites of the Purkinje cells in the molecular layer of the cerebellar cortex, neuropil typical of thalamic nuclei, neuropil from the olfactory bulb (note the dendro-dendritic synaptic contacts), neuropil from the neurohypophysis, Neuropil from anterior horn of spinal cord, Neuropil from external plexiform lamina of the retina, and so on. In most, however, there is a tremendous amount of surface area potentially capable of the detailed electrochemical calculation. An exploration of the Atlas of Ultrastructural Neurocytology by Josef Spacek, MUDr., DrSc. from which these links come (or go to) will discover a number of features whose function is not understood.

In view of recent research, including the three papers discussed here, we have every reason to assume that the brain encompasses far more computing power than is contained in the action potentials, or even the dendritic responses to them. The highest-speed processes, involving action potentials and associated electrical waves in the dendrites, work on a time-frame of milliseconds. The processes I've been describing here work on a slightly longer scale, but still sub-second, and thus appropriate for things like short-term memory, integration of conscious awareness, and so on. Over periods of minutes or longer, cell structures can be modified, allowing the brain to change its behavior, probably part of how our longer-term memory works.

All in all, the brain is potentially a lot more intelligent than we may have thought a decade ago.

Marchaland, J., Cali, C., Voglmaier, S., Li, H., Regazzi, R., Edwards, R., & Bezzi, P. (2008). Fast Subplasma Membrane Ca2+ Transients Control Exo-Endocytosis of Synaptic-Like Microvesicles in Astrocytes Journal of Neuroscience, 28 (37), 9122-9132 DOI: 10.1523/JNEUROSCI.0040-08.2008

Reyes, R., & Parpura, V. (2008). Mitochondria Modulate Ca2+-Dependent Glutamate Release from Rat Cortical Astrocytes Journal of Neuroscience, 28 (39), 9682-9691 DOI: 10.1523/JNEUROSCI.3484-08.2008

Singh, A., Gebhart, M., Fritsch, R., Sinnegger-Brauns, M., Poggiani, C., Hoda, J., Engel, J., Romanin, C., Striessnig, J., & Koschak, A. (2008). Modulation of Voltage- and Ca2+-dependent Gating of CaV1.3 L-type Calcium Channels by Alternative Splicing of a C-terminal Regulatory Domain Journal of Biological Chemistry, 283 (30), 20733-20744 DOI: 10.1074/jbc.M802254200

Links: (I've included only the links referenced in this leader.)

1. Fast Subplasma Membrane Ca²⁺ Transients Control Exo-Endocytosis of Synaptic-Like Microvesicles in Astrocytes

2. Glutamate-induced Exocytosis of Glutamate from Astrocytes

3. Vesicular release of glutamate mediates bidirectional signaling between astrocytes and neurons

4. Mitochondria Exert a Negative Feedback on the Propagation of Intracellular Ca2+ Waves in Rat Cortical Astrocytes

5. Mitochondria Modulate Ca²⁺-Dependent Glutamate Release from Rat Cortical Astrocytes

6. Modulation of Voltage- and Ca²⁺-dependent Gating of Ca_V1.3 L-type Calcium Channels by Alternative Splicing of a C-terminal Regulatory Domain

7. Changes in dialysate concentrations of glutamate and GABA in the brain: an index of volume transmission mediated actions (Use Figure 1)

8. Nonsynaptic Chemical Transmission Through Nicotinic Acetylcholine Receptors

9. Extrasynaptic and postsynaptic receptors in glycinergic and GABAergic neurotransmission: a division of labor?

10. Glutamate Transporters Regulate Extrasynaptic NMDA Receptor Modulation of Kv2.1 Potassium Channels

11. Neuronal Synchrony Mediated by Astrocytic Glutamate through Activation of Extrasynaptic NMDA Receptors

12. Synapse-Specific Expression of Functional Presynaptic NMDA Receptors in Rat Somatosensory Cortex

13. Tonic activation of NMDA receptors by ambient glutamate of non-synaptic origin in the rat hippocampus

14. The locus coeruleus–noradrenergic system: modulation of behavioral state and state-dependent cognitive processes

15. Mechanisms of Calcium Decay Kinetics in Hippocampal Spines: Role of Spine Calcium Pumps and Calcium Diffusion through the Spine Neck in Biochemical Compartmentalization

16. Astrocyte Control of Synaptic Transmission and Neurovascular Coupling

17. Synaptic transmission onto hippocampal glial cells with hGFAP promoter activity

18. The synaptic organization of the brain by Gordon M. Shepherd

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Tinkering With Special Search

I've added a special search to my blog: it claims to search linked pages (including footnote links). It's under construction for the moment, so how it works may do a little changing. Any feedback would be welcome: ideas, experiences, etc. Read more!

Drugs, Receptors, and the Brain

My last couple of posts were pretty heavy, certainly for me, so I'm going to drop back to a slightly lighter level, and comment on something I've noticed, both in the literature and in blogs, about neurotransmitters and their pharmacology. Let me start with a few posts from Scicurious at Neurotopia, whose posts on this subject I try to always catch:

• General Stuff I Blog About: Dopamine!
• The Serotonin System and All that Goes With it
• Depression Post 4: The Serotonin Theory (and why it's probably wrong)
• Things I like to Blog About: Amphetamine
• The elegant logic of dopamine
• Dopamine Neurons: Reward, Aversion, or Both?

All of these posts are discussing the action of neurotransmitters such as dopamine and serotonin, which tend to modify the action of other neurotransmitters rather than directly cause voltage changes in the post-synaptic cell.

A tendency I notice is to assume that a neurotransmitter such as dopamine has a particular "meaning", such as that more dopamine in the brain means you're experiencing something you like, or that dopamine is part of the "reward" system in the brain. We can see this in the last link, which discusses "Two types of dopamine neuron distinctly convey positive and negative motivational signals" by Masayuki Matsumoto and Okihide Hikosaka. Here, it's shown that only a subset of neurons that deliver dopamine to their synapses are entirely reward-motivated, while others seem to respond more to unexpected stimuli, whether desirable or otherwise.

IMO this is a somewhat simplistic view. The first thing we have to remember is that there are many receptors for each of these neurotransmitters, with different actions. The second is that, as Matsumoto and Hikosaka have demonstrated, different populations of neurons that deliver the same transmitter "mean" different things by their message. Let's start with that. ...

Suppose you have a population of cells that deliver dopamine when some significant class of event has occurred. They deliver their message to each region of the brain that needs this message, and the cells in each region modify their behavior in response. However, there are many types of cells in each region, and each may have to modify its behavior in a different way. Moreover, even when the same modification is being made, cells may use different receptors to trigger their changes.

Now, many regions in the brain will have to receive many different messages from different populations of dopaminergic neurons. Different cells within those regions will have synapses receiving dopamine from different populations of neurons, and many different synapses, receiving different messages, will probably use the same receptors.

But what do drugs do? They affect the activity of one or more specific type of receptor, either making them more or less sensitive, or "turning them on" without reference to whether the neurotransmitter is present at all. Their actions are, at best, specific to one type of receptor, and some, such as reuptake inhibitors, are independent of which receptors are used, although they may be specific to one type of reuptake transporter. Still, it's hard to imagine that there's any specific correlation between which receptor(s) are present, and which transporter is used. But as we saw above, there will be many different cells, with different functions, responding to different messages, all using the same receptors. And they'll all be affected.

What this means is that any drug used to affect the activity of these neurotransmitters is going to be like a shotgun, or maybe a claymore mine in its effect. It's hardly likely to only affect whatever specific issue you are trying to treat.
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Nerve Cells and Glial Cells: Redefining the Foundation of Intelligence

Our traditional idea of how the brain works is based on the neuron: it fires action potentials, which travel along the axon and, when the reach the synapses, the receiving neuron performs a calculation that results in the decision when (or whether) to fire its own action potential. Thus, the brain, from a thinking point of view, is viewed as a network of neurons each performing its own calculation.

This view, which I'm going to call the axon-centric view, is simplistic in many ways, and two recent papers add to it, pointing up the ways in which the glial cells of the brain participate in ongoing calculation as well as performing their more traditional support functions.

In the traditional view, the brain was made up neurons, which processed data, and glial cells, which performed support functions, including "surround neurons and hold them in place, to supply nutrients and oxygen to neurons, to insulate one neuron from another, and to destroy pathogens and remove dead neurons." (Wiki). In recent years, this picture has been breaking down.^{2, 24, 25, 31, 32}

Glia are generally divided into two broad classes, microglia and macroglia. Microglia are part of the immune system, specialized macrophages, and probably don't participate in information handling. Macroglia are present in both the peripheral and central nervous systems, in different types. For purposes of analyzing the brain, which is part of the central nervous system (CNS), we will ignore the types not present there. ...

Traditionally, there were four types of glia in the CNS: astrocytes, oligodendrocytes, ependymal cells, and radial glia. Of these, the one type that's most important to the developing revolution in our ideas are those cells called astrocytes.² It turns out that there are at least two types of cell (at least) subsumed under this name.^{24, 25, 31, 32} One, which retains the name of astrocyte, takes up neurotransmitters released by neurons (and glial cells), aids in osmoregulation,¹⁰ controls circulation in the brain,^{1, 31} and generally appears to provide support for the neurons and other types of glia.

The other type have several names, including NG2 glial cells,²³ Synantocytes,²⁵ oligodendrocyte precursor glia,²⁶ NG2(⁺) Progenitor Cells,^{28, 29, 33} PDGFRA/NG2 glia,³⁰ and NG2-glia,³² which last name will be used here (because it's short). While they are similar to astrocytes, there are distinct differences:

Astrocytes have asymmetrically radiating processes that consist of several primary thick processes, from which emanate multiple smaller collateral branching secondary processes, giving them a bushy appearance (Fig. 1C; [refs]). By contrast, NG2-glia have a centrally located soma, from which extend several long, slender primary processes, which bifurcate two or more times, but less extensively than astrocytes, to form a symmetrical process field (Fig. 1D). Astrocyte processes often end in bulbous swellings, or terminal end-feet, which form the vascular and pial glia limitans (Fig. 1A), whereas NG2-glial processes taper to an end and do not appear to contribute to the glia limitans (Fig. 1B; [refs]). Although both NG2-glia and astrocytes extend processes to nodes of Ranvier in white matter ([refs]) and synapses in grey matter, their geometric relationship to these neuronal elements is different. Thus, although astrocytes and NG2-glia bear a superficial resemblance, they are distinguished by their different process arborizations. This will reflect fundamental differences in the way these two glial cell populations interact with other elements in the neural network.³² [The figure number matches with here.]

Figure 1: Astrocytes and NG2-glia. A and C are astrocytes, B and D are NG2-glia. Click on image to see full image. (From Ref 32.)

Both types of glia are closely integrated with the nervous system, receiving information from action potentials via synapses²² (which, only a few years ago were thought to be limited to neurons), and returning control of neuron activity through release of neurotransmitters and other modulators. Both, then, demonstrate the potential for considerable intelligent activity, contributing to the overall intelligence of the brain.

Astrocytes probably (IMO) are limited, or mostly so, to maintaining the supplies of energy and necessary metabolites. They receive action potentials,^{3, 6} which allows them to closely and quickly monitor general activity and increase circulation in response, even before the neurons and NG2-glia have reduced their supply of ATP.²¹ They appear to be linked in a network among themselves,^{2, 5} allowing them to communicate their needs without interfering with the higher-level calculations of the brain. (They may also serve to regulate overall neural activity in response to reduced supplies of oxygen or food.^{1, 4, 8})

NG2-glia appear to have several functions, but one of the most exciting things about them is that they seem to be able to fire action potentials.³³ Their cell membranes, like those of the dendrites of neurons, have all the necessary channels and receptors to perform real-time electrical calculations in the same way as neural dendrites. They have also demonstrated the ability to learn through long term potentiation.²⁷

In one of the two new papers mentioned above,²³ Dividing glial cells maintain differentiated properties including complex morphology and functional synapses (by Woo-Ping Gea, Wei Zhoub, Qingming Luob, Lily Yeh Jana, and Yuh Nung Jan), we find that these cells are capable of maintaining their full function even while dividing. Evidently, they maintain a network of processes, which would be called dendrites if they were neurons, with the soma pretty much being a support structure. When the network grows too big, the soma divides, with each daughter soma getting part of the processes.


Figure 2: NG2-glia cell division while retaining differentiated morphology. Click on image to see original caption, and enlargeable image. (From Ref 23.)

Dividing NG2-glia also retain the ability to fire action potentials, as well as receiving synaptic inputs from neurons.²³ Presumably, they continue to perform their full function, including retaining any elements of long term potentiation or depression contained in their synapses.²⁷

The second paper,³⁰ PDGFRA/NG2 glia generate myelinating oligodendrocytes and piriform projection neurons in adult mice (by Leanne E Rivers, Kaylene M Young, Matteo Rizzi, Françoise Jamen, Konstantina Psachoulia, Anna Wade, Nicoletta Kessaris & William D Richardson), shows evidence that this type of cell can mature into either oligodendrocytes or neurons. Oligodendrocytes are responsible for the insulation of the axons, wrapping around approximately 1 mm of each of up to 50 axons within their reach, and forming the myelin sheath.

Although the precise type of neuron formed by maturing cells hasn't been determined, the very fact that cells of this type can change into neurons is very important. We actually don't know whether the cells that do this maturation are the same as those that perform neuron-like activities, there appear to be two separate types of NG2-glia, spiking and non-spiking.²⁶ It may very well be that the "spiking" type have actually differentiated, while the "non-spiking" type may be doing the maturing. Of course, very few differentiated cell types remain capable of division, as even the "spiking" type do.²³

Another possibility is that the "non-spiking" type may be able to mature into either oligodendrocytes or "spiking" types, while the "spiking" types may be able to grow an axon and turn into neurons. There's much we don't know yet about these cells.³³

Although the NG2-glia appear to be able to fire (or at least propagate) action potentials, they probably don't in vivo. Rather, they probably act much like dendrites, which are also capable of propagating action potentials, but are usually thought not to fire them. (Rather, they will back-propagate action potentials fired by the axon hillock.¹⁸)

What's important about both dendrites and NG2-glia isn't so much their ability to propagate action potentials, as that their entire cell membranes are capable of "intelligent" manipulation of the voltage across it. Let's start by looking at the action potential.

Action Potentials and Ion Channels

While there are many ion channels involved in controlling the voltage across the cell membrane, the only type we really need to worry about for action potentials is voltage-gated sodium channels. These are channels that sometimes allow sodium ions to pass through the cell membrane, which they will do because the concentration of sodium ions outside the cell is very much higher than inside. When and how much they open depends, among other things, on the voltage across the membrane.

A normal neuron will have a voltage of around -60 to -80mV (millivolts), in a direction that tends to push the sodium ions (which are positive) into the cell (the same direction as the concentration is pushing). When the voltage falls to around -55mV, the primary type of gate will open for a millisecond or so, after which it will close and rest for several milliseconds. It won't be able to open again until the voltage is somewhere between -55 and around -10mV. Meanwhile, the sodium current has caused the voltage to swing past zero to around +20mV. (The high concentration of sodium outside the cell allows it to drive the ions "uphill" electrically.)

When one part of the cell membrane is "depolarized" in this fashion, the voltage near it is also depressed. Thus, if the voltage is at zero at one point, it might be at -20mV 10 microns (μm) away, and -40mV 20μm away, and -60mV 30μm, and so on. Notice that somewhere between 20μm and 30μm, it has passed the threshold for the ion channels, which means that they are open, allowing a current that drives the voltage further down. This will produce a wave of voltage drop along the membrane, which is what the action potential is.

After the action potential has passed, and the gates have closed (see above), the voltage is recovered by diffusion of ions towards and away from the membrane, the opening of other gates (primarily potassium), and a set of pumps that push the ions back to their resting state. These pumps are mostly powered by the sodium gradient, except for the sodium/potassium pump that maintains it, which is powered by ATP. (See my Wiring the Cell for Power for more on the cell's power systems.)

In the laboratory, action potentials are usually fired by applying a voltage to the cell membrane at some point, but how are they fired in vivo? Although I know of no research to back me on this, IMO the cell creates "hot spots", where a voltage greater than -55mV is capable of opening the ion channels. How is this accomplished? Most ion channels come in a large variety of types: there are nine types of primary (alpha, α) subunits of the voltage-gated sodium channel, which can operate on their own but usually associate with secondary (beta, β) subunits, of which there are at least 4 types. In addition, there are other types of modulation that can be applied. If a bunch of channels that are modified to open at a voltage greater than -55mV, say -60mV, are concentrated in one place, then if the voltage at that place falls below that value the action potential fires. In almost all neurons, the "hot spot" will be on the axon hillock.

What modifies the voltage on the soma and axon hillock? Well, every time an action potential arrives at a synapse, the pre-synaptic part of the synapse releases a bunch of neurotransmitters, which cross the synaptic cleft and interact with receptors on the post-synaptic part, usually called the post-synaptic density (PSD).


Figure 3: Synapse. (From Wiki.)

Some of these receptors are "ligand gated ion channels", which can allow currents of sodium, chloride, potassium, and calcium in response to neurotransmitters. These currents, in turn, modify the voltage at the synapse, creating a voltage wave that spreads along the cell membrane, that is the membrane of the dendritic arbor. The voltage difference created by any action potential at any synapse decays in the manner described above, even if it never reaches the threshold for an action potential. Similarly, all the other synapses on that post-synaptic cell from that axon (if any) create their local currents, at other points in the arbor. These voltage differences decay in both space and time, the current fading out as the neurotransmitters are taken up or diffuse out of the synapse. While this is happening, other action potentials, in other axons, are adding their own contribution, causing the distribution of voltage across the membrane of the entire arbor to vary in an extremely complex manner.¹⁷ Note that depending on the type of ions involved in the current, it can be depolarizing (tending to lower the voltage towards the threshold) or hyperpolarizing (tending to raise the voltage even farther away from the threshold).

If the cell membrane away from the synapses were passive, with no change in current in response to voltage changes, the voltage distribution at any point could be calculated using passive cable theory.^{38, 40} However, membranes outside the synapses are well supplied with voltage-gated ion channels,^{11, 13} which create their own non-linear reactions to voltage changes,^{16, 17} of which the action potential is only one example, and IMO not the most important. In dendrites, the result is an incredibly complex, non-linear, response to the synaptic inputs, which contains the potential for enormous calculating capacity, much higher than the point calculations performed by synapses.

This is especially true because many of these ion channels are not only sensitive to voltage, but their voltage-sensitivity can be modified by a wide variety of different neuro-modulators,¹⁵ many of which are released by axon arbors in response to action potentials. In addition, both astrocytes and NG2-glia have been shown to release chemicals that function as neuro-modulators, in response to their own cellular circumstances.^{1, 8, 9, 11, 12, 13}

Neuro-Glial Calculations Beyond Action Potentials

The fact that astrocytes can release these neuro-modulators, partly in response to electrical signals received from action potentials in axons,³ demonstrates that action potentials within a specific cell aren't necessary for that cell to perform its release. Unless and until NG2-glia have been shown to have action potentials in vivo, we can assume the same to be true of them.

Although it hasn't been proven yet, we can certainly theorize that this release is partly voltage-dependent, probably through the use of calcium currents stimulated by voltages that don't reach the threshold for action potentials. (I'll note that calcium currents are almost always associated with the release of vesicles full of neuro-transmitters and -modulators,^{8, 9} because of the energy required. See my How Smart is the Cell: Part IV: Local Intelligence, especially The Calcium System, for further discussion.) Certainly calcium channels can be stimulated by a variety of sub-threshold voltages.²⁰ This means that both dendritic networks and NG2-glial networks are tied together in an extremely complex network of electro-chemical analog calculations,²⁸ with far greater potential calculating ability than even the collection of dendrites that mediates the high-speed action potential-type communication.

We might think of the brain, then, as an extremely large organization, with offices spread out through an entire densely populated country, but without telephones or other modern communications; sort of the way things were in the 1800's. Most communication has to take place via letters in the post, or face-to-face communication. However, there is a telegraph network to provide high-speed transmission of a very small subset of the total information involved.

Similarly, the vast majority of calculation that goes into human intelligence takes place at the level of the network of dendrites and NG2-glia, with the whole system of axons, dendrites, and action potentials only carrying a tiny subset of the total information over long distances. This is especially important considering that the human brain has a much higher proportion of glial matter than our relatives.^{34, 35, 36, 37}

The Membrane-Centric Approach

This, in turn, suggests that our overall approach to understanding the brain has been far too axon centric, there needs to be a shift to a more membrane-centric approach to understanding how the brain creates intelligence.

Another point to keep in mind is the differences between various regions of the brain. Although brain cells are divided into broad classes: pyramidal cells, interneurons, astrocytes, NG2-glia, and so on, the cells in each region of the brain may well behave differently. Each cell "knows" where it is, it had to in order to create the right connections, which vary depending on which region of the brain, even which region of the neocortex, they're in. Not only that, but in each layer of the neocortex the cells make different types of connections, so they must know that as well. If they know enough to make connections differently, they know enough to express different patterns of ion channels and vesicles at appropriate locations within the membrane.¹⁴ Thus, whenever research finds some capability in some type of cell in one part of the brain, we know that same capability may be used anywhere (or everywhere) else, but we don't know if it will be used in the same way. And when research fails to find some mechanism used in one part of the brain, that certainly doesn't mean it isn't used in other parts.

This adds great complexity to the membrane-centric approach, since it's not only the elements of the action potential and synaptic communication that may be specific to each region, but the way in which the neuro-glial network operates. For instance, astrocytes have been investigated in several regions, and while they have the ability to respond to neuro-transmitters released by axons, no sign of action potentials have been found in them. But that doesn't mean that the astrocytes in some other regions of the brain haven't evolved this capacity, presumably in response to a need for faster coordination of blood flow.

In my view, the membrane-centric approach, as it is implemented in research, will provide tremendous insight into the functionality of the brain. The opportunities for new research, and exciting findings, are enormous. It'll be interesting to see what turns up.

Ge, W., Zhou, W., Luo, Q., Jan, L., & Jan, Y. (2009). Dividing glial cells maintain differentiated properties including complex morphology and functional synapses Proceedings of the National Academy of Sciences, 106 (1), 328-333 DOI: 10.1073/pnas.0811353106

Rivers, L., Young, K., Rizzi, M., Jamen, F., Psachoulia, K., Wade, A., Kessaris, N., & Richardson, W. (2008). PDGFRA/NG2 glia generate myelinating oligodendrocytes and piriform projection neurons in adult mice Nature Neuroscience, 11 (12), 1392-1401 DOI: 10.1038/nn.2220

1. Astrocyte Control of Synaptic Transmission and Neurovascular Coupling

(I've included only the links referenced in this leader.)

2. Astrocytes, from brain glue to communication elements: the revolution continues

3. NMDA Receptors Mediate Neuron-to-Glia Signaling in Mouse Cortical Astrocytes

4. Synapse-Specific Expression of Functional Presynaptic NMDA Receptors in Rat Somatosensory Cortex

5. The astroglial syncytium summary only

6. P2X1 and P2X5 Subunits Form the Functional P2X Receptor in Mouse Cortical Astrocytes

7. Inhibition of the ATP-gated P2X7 receptor promotes axonal growth and branching in cultured hippocampal neurons

8. Glutamate-induced Exocytosis of Glutamate from Astrocytes

9. Fast Subplasma Membrane Ca2+ Transients Control Exo-Endocytosis of Synaptic-Like Microvesicles in Astrocytes

10. Substantia nigra osmoregulation: taurine and ATP involvement

11. Tonic activation of NMDA receptors by ambient glutamate of non-synaptic origin in the rat hippocampus

12. Regulation of Synaptic Transmission by Ambient Extracellular Glutamate

13. Glutamate Transporters Regulate Extrasynaptic NMDA Receptor Modulation of Kv2.1 Potassium Channels

14. Potassium Channel Phosphorylation in Excitable Cells: Providing Dynamic Functional Variability to a Diverse Family of Ion Channels

15. Dendritic Excitability and Synaptic Plasticity

16. Spine Neck Plasticity Controls Postsynaptic Calcium Signals through Electrical Compartmentalization

17. Timing and Location of Synaptic Inputs Determine Modes of Subthreshold Integration in Striatal Medium Spiny Neurons

18. Differential Excitability and Modulation of Striatal Medium Spiny Neuron Dendrites

19. G-Protein-Coupled Receptor Modulation of Striatal CaV1.3 L-Type Ca2+ Channels Is Dependent on a Shank-Binding Domain

20. Modulation of Voltage- and Ca2+-dependent Gating of CaV1.3 L-type Calcium Channels by Alternative Splicing of a C-terminal Regulatory Domain

21. Rapid Astrocyte Calcium Signals Correlate with Neuronal Activity and Onset of the Hemodynamic Response In Vivo

22. Synaptic transmission onto hippocampal glial cells with hGFAP promoter activity

23. Dividing glial cells maintain differentiated properties including complex morphology and functional synapses

24. Polydendrocytes: NG2 Cells with Many Roles in Development and Repair of the CNS abstract only

25. Synantocytes: the fifth element

26. Spiking and non-spiking classes of oligodendrocyte precursor glia in CNS white matter

27. Long-Term Potentiation of Neuron-Glia Synapses Mediated by Ca2+-Permeable AMPA Receptors requires free registration

28. Satellite NG2 Progenitor Cells Share Common Glutamatergic Inputs with Associated Interneurons in the Mouse Dentate Gyrus RB?

29. Vesicular release of glutamate from unmyelinated axons in white matter

30. PDGFRA/NG2 glia generate myelinating oligodendrocytes and piriform projection neurons in adult mice

31. Astrocyte-mediated control of cerebral blood flow

32. Astrocytes and NG2-glia: what's in a name?

33. Synapses on NG2-expressing progenitors in the brain: multiple functions?

34. The scalable mammalian brain: emergent distributions of glia and neurons

35. Evolution of increased glia–neuron ratios in the human frontal cortex

36. Cellular scaling rules for primate brains

37. Increased Cortical Expression of Two Synaptogenic Thrombospondins in Human Brain Evolution

38. Distinctive Roles for Dendrites in Neuronal Computation

39. The synaptic organization of the brain by Gordon M. Shepherd

40. Linear Electrical Circuit Theory
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