Brain complexity: multiple neuron types (Introduction)

by David Turell , Monday, November 06, 2017, 20:35 (2574 days ago) @ David Turell

Advanced research has found that the brain contains many different types of neurons. No two may be the same increasing the possible complexity many, many times:

http://www.the-scientist.com/?articles.view/articleNo/50700/title/Advancing-Techniques-...

"For years, neurons in the brain were assumed to all carry the same genome, with differences in cell type stemming from epigenetic, transcriptional, and posttranscriptional differences in how that genome was expressed. But in the past decade, researchers have recognized an incredible amount of genomic diversity, in addition to other types of cellular variation that can affect function. Indeed, the human brain contains approximately 100 billion neurons, and we now know that there may be almost as many unique cell types.

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"Recent technological advances have enabled a highly resolved characterization of the extent of cellular diversity in the brain, showing that there is far more heterogeneity within a given cell type than previously appreciated.

"Research has also begun to examine how somatic mosaicism might drive functional differences in individual neurons. Such neuronal diversity may help explain the origin of personality in humans and interindividual behavioral variations in other animals. Anecdotally, siblings, even monozygotic twins, often have remarkably different personalities even at young ages, despite sharing genes and environments. Diversification of neurons arising from somatic gene mutations or subtle molecular and environmental differences may help explain the origin of cognitive and behavioral individuality. The findings thus far highlight the importance of moving away from a blanket definition of “cell types” that are assumed to behave in a stereotyped manner toward a more nuanced view of neurons that includes the multidimensional combination of transcriptome, epigenome, and genome when attempting to understand the impact of a given cell state.

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"Of the 100 billion or so neurons in the human brain, there may be no two that are alike. Recent advances in single-cell omics and other techniques are revealing variation at genomic, epigenomic, transcriptomic, and posttranscriptomic levels. Such diversity can arise at all stages of development and into adulthood. In the case of genetic changes that are passed on to daughter cells, the stage at which mutations occur will dictate their frequency in the brain. Researchers are now working hard to catalog every cell type within the human brain, and understand how differences among them may underlie variation in neuronal function. There are early hints that this mosaicism may contribute to personality and behavioral differences among individuals, as well as to various neurological or psychiatric disorders.

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"While it was traditionally believed that every cell in the body contained identical genetic material, recent evidence has revealed that individual neurons actually differ significantly due to somatic DNA mutations and rearrangements, including those caused by the movement of L1 and other retrotransposons. Somatic mutations can occur both during development and in adulthood. Early progenitor cells that accumulate somatic mutations may give rise to many progeny, which also carry the same mutation, whereas a later progenitor, like an NPC in a neurogenic niche of the adult brain, may only give rise to a few progeny, limiting the spread of that particular mutation. This process could represent a lifelong flexibility of the brain, potentially making it more adaptable to changing demands.

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"The brain is capable of remarkable remodeling in response to experience. Signals originating from the environment can cause both widespread and localized adaptations. At the level of individual cells, structure and function are continually changing with the environment in a dance of lifelong brain plasticity, and some experiences, such as stress or physical exercise, affect the growth, survival, and fate of newborn neurons in neurogenic regions of the brain.15  Considering that each neuron in the human brain makes 5,000 to 200,000 connections with other neurons, changes at the synapse level could have effects on multiple brain circuits and downstream behavioral or cognitive phenotypes.

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"Once we begin to consider all of the subtle cell-to-cell variations, it becomes clear that the number of cell types is much greater than ever imagined. In fact, it may be more appropriate to place some cells along a continuum rather than into categories at all.

"Brain cells in particular may be as unique as the people to which they belong. This genetic, molecular, and morphological diversity of the brain leads to functional variation that is likely necessary for the higher-order cognitive processes that are unique to humans."

Comment: This helps explain why we can think the way we do and how the brain can adapt to individual usage creating each unique person.