In my former life as a speech-language pathologist working in an early intervention/prevention project focused on the mental health of preschoolers, I enjoyed giving parent/teacher talks on language development, communication difficulties, how to recognize when help is needed and how to promote healthy development. Lovely work, eh? In many of these talks I presented a brief glimpse into the complexity of development that started something like this: After fertilization, the egg begins to divide and multiply. When there are roughly 50-100 cells, one cell appears that becomes the mother of all neurons. Slowly this cell line multiplies to form a neural tube and from within that tube more cells would be born that would then travel to the outer edges and form the brain. 10,000,000,000 cells would arise and find their place in this way in just a few months, so a few traffic jams and mis-directions might be expected along with some individual variability. Wow! I would also talk about some of the maturational/developmental differences between boys and girls and then get into the specifics of language development.
Now I am finishing up Georg Striedter’s text, Principles of Brain Evolution, and understand that my earlier rendition of complexity was more a 2 page Reader’s Digest version of the Encyclopedia Britannica (anyone else remember those?) The task neuroanatomists take on is enormously complex and even knowing a little bit for sure requires diligent, rigorous, and assiduous study. Understanding how brains increase in size and connectivity and then how brain functions change and increase in power is a humbling endeavor, one that I am glad those with such talents work on and one that I find spiritual in Monod’s sense of spirit (see post on 3/25/17). So let me add some to my story above.
Those 1010 cells find their way along a variety of chemical trails and gradients and then when they arrive they send out dendrites and axons to connect with other cells and this connectivity is also developed through a variety of biochemical trails, and then synapses are formed and coordinated so that integrated intercellular communication can begin. Striedter cites estimates that each mammalian neuron connects with around 500 other neurons through 8000 synapses. Let’s see: 1010 x 500 x 8000 = a lot. Also, remember that neurogenesis, that early embryonic stage when virtually all of our neurons appear, produces many cells that disappear in the first years after birth through apoptosis, i.e., cells die because they are not in the right place or connected in viable networks. Streidter says that brain areas vary in how many cells are lost and cites evidence that different systems have 20% to 80% fewer neurons at maturity than at birth. Finally, remember that neurons communicate with over 50 neurotransmitters that form the substrates of different systems processing information in their various ways, e.g., inhibitory, excitatory, etc.
The individual brains of any one species are remarkably similar in terms of neuronal systems, etc. The genetic controls and epigenetic forces are quite rigorous in their replication of each organism. I especially like the story of C. elegans, a roundworm whose nervous system comprises 302 neurons that connect in very consistent ways. Thank you, diligent researchers for finding that out through marvelously detailed work.
So I learn again and remember what I will not forget, that understanding enough to know what we do not know is the prime intellectual task, and good scholars and mystics look at our ignorance with excitement. Travel on.