Neuro Brain Thermodynamics

The brain is a metabolically expensive organ that uses quite a bit of energy. It's no surprise that it also generates a decent amount of heat during this energy usage process. A new paper has come out that poses and answers the question as to whether there is a thermodynamic limit to brain size (evolutionary wise). The author is basically asking how big can a brain get before it becomes too hot to function properly? What sort of constraints does evolution have in constructing a bigger brain, given the laws of our universe? The answer he gives, in short, is that there is plenty of room thermodynamically to evolve a larger brain.

The paper first discusses the main cause of the generation of heat in the brain. The Na+/K+ pump helps to maintain the cell membrane potential of every neuron. The pump allows a neuron to have a high concentration of K+ ions and a low concentration of Na+ ions inside of it. The protein molecule pump hydrolyzes an ATP molecule, using that energy to move 3 Na+ ions out of the neuron and 2 K+ ions into the neuron. These ions are integral to the electrochemical signaling of a brain cell and their concentrations change in response to the propagation of an action potential down a neural axon. So the pump is necessary to restore ion balance to a neuron after it fires.

The whole paper is a bit mathematically intense. The author's intent is to figure out how much energy an aggregate number of neurons use, mostly focusing on the Na+K+ pump and neglecting brain glucose utilization. The author goes on to discuss how the brain regulates the heat generated from all of this work that occurs and the specific constraints on neural processes. Some heat radiates from the scalp. Cerebral blood flow (cbf) is another method that the brain uses to cool itself. Up the evolutionary mammalian line, cbf essentially scales with brain volume. So I think that means the amount of blood vessels are basically proportional in creatures that have varying brain sizes. Due to this scaling up, there is apparently a small decrease in the rate of blood flow as you go from simpler to more complex organisms (mouse to human for example), because of more surface area coverage.

Increasing cbf in a specific deep brain region can cause a resultant decrease in brain temperature there. According to the author mammal's brains can sustain a temperature upwards of 42 Celsius without becoming damaged. Though, the optimum temperature may be much lower than that. Certain drugs have the ability to increase the brain's temperature in part by vasoconstriction. Cerebral blood flow only acts as a coolant inside deeper brain regions where the blood is cooler than the surrounding brain tissue. So it's really not a coolant in the same manner as that in a heat engine. The cbf actually heats up more superficial brain regions that are closer to the skull so the mechanism is not uniform. The main purpose of cerebral blood flow is to bring glucose to neurons for their basic energy need. So the cooling ability is sort of a secondary aspect of blood flow and is probably not ideally suited for that purpose. Evolution has basically co-opted one process for a different purpose entirely. The blood flow's ability to cool is more important for larger brained mammals and less relevant for smaller brained ones where heat can dissipate from the head easier.

The author of the paper notes some of the constraints of the brain taking into consideration excess heat production. He suggests that thinner axons/dendrites result in excess heat. However, he estimates that the axon's diameters are at a magnitude higher (averaging 12-1500 times) than the lower bound diameter that would be problematic. He also talks about the heat bounds relating to the propagation of neural signals and density of axonal packing. The author concludes that deep brain temperature is only weakly correlated with brain volume. So the brain could easily be scaled above the 5 kg limit of current land mammals. However, that is assuming that no other methods are utilized by evolution to "overclock" specific brain regions.

I think that evolution finds whatever way it can to increase the brain's computational capacity. I've mentioned previously about some of the possible ways. Evolution exploits any pathway easily available. The firing speed of neurons is an aspect of overall computational capacity. However there are limits to this facet of brain functioning. Neurons can only fire continuously so many times before the sodium concentration in the cell becomes too high. This is especially true if neuronal axons were to become too thin. Those Na+/K+ pumps can only pump sodium ions out of the neuron so fast in certain cases. Evolution can possibly add more pumps, but then that requires more energy which potentially generates more heat. The pumps work relatively slow, so even a maximum amount of them might not overcome a certain limit. Eventually you might run up into an insurmountable wall with this attribute of brain function. So evolution may have to do something else, like increase the overall amount of neurons.

Sometimes the path that evolution follows is unexpected. Average neuronal firing rates in larger brains are actually less frequent than that of neurons in smaller brains. So there may be some limitation to increasing neuronal firing rate over the course of evolution as you scale upwards in size. You would also think that evolution would first do something like maximizing the amount of proteins in the synapse before it went on to increase overall neuron count. However evolution tends to fill in some of these finer details later on, instead of in a logical linear fashion. Scientists will probably increasingly figure out why occurs as time goes on. The author of the paper does his part to elucidate a few of these possible constraints on how the brain is arranged.

With increasing brain complexity there is the potential for more pathways to open up that can be exploited to further increase overall computational capacity. With more complexity, however, there is also the possibility for evolution to have a harder time navigating a proper way forward. There may be too many entangled systems whereby changing the variable of one thing could have a negative effect on something else. When cerebral blood flow is too high, for instance, it can damage the brain. So evolution may not be able to just take obvious routes (like increasing brain blood flow speed) to decrease brain heat. Not to mention how blood flow is mainly involved with delivering energy to cells. So any change in cbf could potentially negatively affect that process as well. Short of developing a whole secondary cooling system, evolution is stuck co-opting cbf as a coolant system. Also more blood flow in the brain may mean less room for computational purposes. Of course, this paper indicates that increased cooling might not be necessary. It may tend to get difficult as evolution moves forward to undo things or reach a path that is radically different from what previously evolved. Evolution has to essentially make due with whatever it has.

In the past, scientists from Japan have constructed a "heat pipe" that can be implanted directly into discrete brain regions. This device can essentially be used to cool brain areas by diverting heat to an outside heat sink. The researchers developed this implant specifically for the purposes of reducing epileptic seizures. Over-excited neurons in an epileptic seizure are more active, use more energy and thus generate more heat. This excess heat that is generated can cause a feedback loop exciting more neurons to fire thus potentially prolonging the seizure. So cooling the brain could be a way of reducing problems associated with this condition. It is possible that this could be used to better regulate the temperature of future engineered brains.

I think the fact that our universe allows brains to evolve to the size they do may be an example of the anthropic principle. First, there is the rare earth perspective of life. Our planet is situated in a near perfect distance from the sun and is neither too hot nor too cold to sustain life. The gravitational pull of our planet may be at nearly the right level that allows a larger brain to evolve. Also our solar system is located far enough from the center of the galaxy so as to avoid excess radiation. These are only a few examples of our special situation in our own galaxy/universe. String theory predicts that our universe is merely one region out of a larger multiverse. In the multiverse there are different vacua that may have varying constants. A majority universes in the multiverse may not be able to sustain any life at all. Some universes may contain selfish replicators, but they never be able to evolve a nervous system or the capacity for sentience. Perhaps in an even smaller subset of vacua in the multiverse, a brain/nervous system can evolve, but maybe it cannot attain a complexity greater than that of a mouse's brain or an insect's or even less. There might be too many design constraints inherent to that specific universe for it to get any bigger. The physics of our own universe is perfectly suited to developing a human level intelligence. The heat capacity of water may allow the brain to maintain a fairly stable temperature, for instance. Definitely quite amazing when you think about. Perhaps we will eventually figure out everything related to this more speculative science in the future.

Karbowski, J. (2009). Thermodynamic constraints on neural dimensions, firing rates, brain temperature and size Journal of Computational Neuroscience DOI: 10.1007/s10827-009-0153-7