Abstract
During mammalian development the cerebral metabolic rate correlates qualitatively with synaptogenesis, and both often exhibit bimodal temporal profiles. Despite these non-monotonic dependencies, it is found based on empirical data for different mammals that regional metabolic rate per synapse is approximately conserved from birth to adulthood for a given species (with a slight deviation from this constancy for human visual and temporal cortices during adolescence). A typical synapse uses about glucose molecules per second in primate cerebral cortex, and about five times of that amount in cat and rat visual cortices. A theoretical model for brain metabolic expenditure is used to estimate synaptic signaling and neural spiking activity during development. It is found that synaptic efficacy is generally inversely correlated with average firing rate, and, additionally, synapses consume a bulk of metabolic energy, roughly during most of the developmental process (except human temporal cortex ). Overall, these results suggest a tight regulation of brain electrical and chemical activities during the formation and consolidation of neural connections. This presumably reflects strong energetic constraints on brain development.
Highlights
The proper functioning of neural circuits depends on their proper wiring [1,2,3,4,5,6,7]
The most extreme change is in the cat visual cortex, where rs and cerebral metabolic rate (CMR) can increase by a factor of *18 and *4, respectively (Table 1)
This study shows that despite temporal changes in cerebral metabolic rate CMR and synaptic density rs during development, often exhibiting bimodal shape, the amount of metabolic energy per synapse (CMR/rs) is almost invariant in the process for a given mammal and brain region (Fig. 2; Tables 1–3)
Summary
The proper functioning of neural circuits depends on their proper wiring [1,2,3,4,5,6,7]. Despite the widespread application of recording, imaging and molecular techniques [13,14], along with modeling studies [2,15,16], it is fair to say that our understanding of brain connectivity development is still very limited, and mostly qualitative. The formation of neural circuits is an important problem in neuroscience, as its understanding may shed some light on structural memory formation in the brain and various developmental disorders [17]. Synaptic development like every physical process requires some energy. Energy consumption in mammalian brains increases fast with brain size, far more than in the rest of the body [21]
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