The efficiency of birds’ brains is higher than of mammals

in hive-109160 •  2 months ago 

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(Lewis Hulbert / Wikimedia Commons https://bit.ly/3ROAnRv)

German scientists found that a gram of pigeon brain consumes, on average, 2.5 times less glucose than the brain of mammals of comparable size, and 3.5 times less per neuron.

The team from the Ruhr University Bochum hasn’t precisely determined the reason for such a high energy efficiency of avian neurons.

However, most likely, the answer lies in the histological features of the nervous tissue and the temperature of the brain.

The larger the brain and the number of neurons, the more complex behavior the animal has.

And it's not just the size of the brain, but also the energy cost of its work.

It is known that the larger the surface area of ​​a warm-blooded animal, the less energy each gram of its body spends.

With the brain, the situation is similar.

For example, if a horse needs on average about 1% and 0.5% of the main metabolism to ensure the functioning of the brain, then in a rat this value is 3.5 times higher.

Despite the large difference in the cost of working a gram of brain, in terms of one cell, the numbers are similar in all mammals, since the density of neurons in small animals is higher.

But if we compare the behavior of mammals and birds, then the latter demonstrate more complex behavior (with a comparable body size and brain).

In birds, the density of neurons is much higher than in mammals (at least in parrots and corvids, intelligent birds that often serve as model for studies of cognitive functions).

This means that birds must either have extremely high costs for maintaining the work of the brain, or this work must take little energy.



THE STUDY
Now German neurophysiologists and psychologists decided to understand the problem and measure the intensity of the aerobic metabolism of the brain in pigeons.

For this, they performed PET-MRI of the brain in four awake and six anesthetized rock pigeons after intravenous administration of 18F-fluorodeoxyglucose.

The method is based on the fact that the organs capture the intravenously administered glucose derivative, and the fluorine isotope undergoes beta decay in the cells, which is recorded by the detector.

Thus, knowing the concentration of glucose in the blood and the amount of drug administered, it is possible to calculate the consumption of glucose.

To prevent the pigeon from moving during the procedure, the scientists fixed the head’s animal inside the gantry tomograph.

When measured while awake, pigeons tended to have higher brain energy consumption than at rest, although the difference did not reach the statistical significance criteria.

This difference was primarily due to a drop in forebrain activity during anesthesia.

In mammals of comparable size (gray rat), this parameter averages 68 ± 4 micromoles per 100 grams of brain per minute.

Having obtained this result, the authors decided to calculate how much glucose one neuron of the pigeon brain consumes, using data from previous studies.

It was known that the contribution of neurons to the energy consumption of the brain is 70-80%, and the density of neurons in the brain of a pigeon is 150,000 cells per milligram of tissue.

The calculation showed that in a pigeon one neuron should consume about 1.86 ± 0.2 femtomol of glucose per minute

That is 3.6 times less than in rats, according to the data of other researchers available in the scientific literature.

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(von Eugen et al. / Current Biology, 2022 https://bit.ly/3RWQqwI)



MYSTERY
The team notes that the mechanisms underlying the discovery are still unknown.

A comparison of the density and number of neurons calculations shows that the average size of neurons in birds is smaller, although this clearly does not apply to all populations of neurons.

The smaller the neuron, the smaller the area of ​​its plasma membrane, the smaller the surface area of ​​the membranes inside the mitochondria, and the shorter the length of the processes.

Accordingly, such cells require less energy to maintain the resting potential and to transmit the action potential.
And, they simply cannot produce oxidative phosphorylation reactions with the same intensity as large neurons with a large area of ​​the inner mitochondrial membrane.

It is also possible that the parameters of activation and action potentials of neurons differ in birds (for example, the frequency of activation in response to stimuli is lower).

Another factor that can affect the energy consumption of cells is related to the temperature of the brain.

The temperature optimum of Na+/K+-ATPase (the enzyme responsible for maintaining the resting potential of neurons) lies above body temperature.

Therefore, an increase in brain temperature can lead to a decrease in the energy cost of the resting potential.

Given that the temperature of the pigeon's brain is a couple of degrees higher than that of most mammals of comparable size, this can make a significant contribution to the "cost" of activity and increase its speed.

That efficiency might explain why Australian humans are now in an arms race with cockatoos.

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