This is a short informative essay I wrote for the final project of the Neurobiology course from the University of Michigan.
Synaptic pruning, or synaptic elimination, is an essential aspect of the development of the brain. It is when the brain destroys itself, removing unwanted synapses, neurons or neuronal configurations to increase efficiency of connections. The process and timing of pruning is thought to be significantly changed by the experiences, genes, and even the thoughts of the developing mind.
During early childhood, there is a huge proliferation of connections between neurons, usually peaking around the age of two. The adolescent brain then cuts down the amount of connections, deciding which ones should be kept and which can be let go. While there are various theories as to the molecular mechanisms by which pruning actually occurs, most agree that pruning is primarily carried out by a very motile form of glial cell, called microglia [1], which is the first part of the nervous system that is active during synaptic pruning. These microglia are thought to remove cellular debris during the healing process of an injured brain, but in the healthy, developing brain they have a possibly more important function. If a synapse receives little activity, it is weakened and eventually deleted by microglia and other glial cells. After the synapse has been removed, the space and resources that it once used are taken by other synapses, which are then
strengthened.These processes [2] and various others take place throughout development, peaking at adolescence and ending around the age of 21, and transform the brain to create more complex and efficient neuronal configurations.
Various factors can affect the methods, timing, and results of synaptic pruning. Comprehensive research [3] has shown that mutations in genes lead to abnormalities in the pruning process that lead to unwanted connections remaining active or important connections being pruned out, impairing the brain permanently. Some neuroscientists have made educated guesses as to the nature of these mutations, and there has been relative consensus that microglia, being part of the immune system, remove the synapses because they appear to be either unnecessary, damaged, or a foreign object. This abnormal synaptic pruning could be a significant source of the symptoms of schizophrenia [3]. Pruning is also highly dependent on experience. Neural plasticity reaches its peak during childhood and before synaptic pruning begins. Thus, if a child begins to learn a language or any other complex function before synaptic pruning, that function will be deemed important and the neural connections pertaining to it will be kept and strengthened [4]. This gives new meaning to the saying “use it or lose it.” Experiences during and before synaptic pruning are perhaps some of the most important in development, because they directly affect brain function for the rest of the subject’s life. It is thought that connections deleted during pruning cannot be regained. “The fact that there are more connections [in a child’s brain] allows things to be moved around,” said physiologist Ian Campbell from the University of California, Davis. “After adolescence, that alternate route is no longer available. You lose the ability to recover from a brain injury, or the ability to learn a language without an accent. But you gain adult cognitive powers.” [5]. Your actions during pruning affect which circuits remain in existence and which do not.
It is not fully known what the function of synaptic pruning is, while the general explanation of “pruning removes unnecessary connections” is the most widely accepted and well-researched. It remains unknown whether connections removed during pruning can ever be regained. Pruning is often thought to represent learning, but this is almost certainly an oversimplified explanation for a very complex cluster of processes. Synaptic elimination may also be a method utilized by the brain to adapt to the most efficient form of thinking in its specific environment [6].
There are a wide variety of very interesting questions that deal with synaptic pruning. One that has intrigued me specifically is whether children and adolescents who have not fully undergone synaptic pruning are more creative than adults. It seems that large amount of synapses leads to more overall thought, and thus we can infer that before pruning, the brain is more sidetracked and inefficient. If distraction aids creativity, as a considerable amount of research shows, then that would mean that the more distracted, pre-pruning mind is more creative than the post-pruning mind. Some researchers believe that synaptic pruning is determined somewhat by the culture and society in which the child is raised. This would mean that after pruning, an adult in one society no longer has the same neuronal connections as an adult in a different society – those connections have been trimmed out. Is this an underlying cause of culture shock and/or genocide? Is the developing mind more open to change and new connections then the “narrow,” developed mind? Until the scientific community has a greater understanding of synaptic pruning, these questions will remain unanswered.
Synaptic pruning remains only barely understood by scientists, and the research already conducted has shown that it is far more complicated than previously thought. It is a critical part of neurobiological development that has tremendous consequences on the fully developed adult.
BIBLIOGRAPHY
[1] “The Role of Microglia in the Healthy Brain,” Marie-Éve Tremblay et al; Department of Psychiatry, University of Wisconsin-Madison, 1/3/2014
Click to access 16064.full.pdf
[2] “Synaptic Pruning in Early Development,” St. Olaf University Department of Neuroscience. http://www.stolaf.edu/depts/ciswp/buff/Documents/synapticpruning.pdf
[3] “Abnormal Synaptic Pruning in Schizophrenia: Urban myth or reality?,” Patricia Boksa, PhD; March 2012, US National Library of Medicine. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3297065
[4] “Brain Development and the Role of Experience in the Early Years,” Adrienne Tierney and Charles A. Nelson et al; Harvard Medical School/Children’s Hospital of Boston, 3/6/2009. http://main.zerotothree.org/site/DocServer/30-2_Tierney.pdf?docID=10001
[5] “Teen Brains Clear Out Childhood Thoughts,” Clara Moskowitz, LiveScience Magazine, March http://www.livescience.com/3435-teen-brains-clear-childhood-thoughts.html
[6] “Pruning Synapses Improves Brain Connections,” Ed Yong; 2/2/2013, The Scientist http://www.the-scientist.com/?articles.view/articleNo/39055/title/Pruning-Synapses-Improves-Brain-Connections/
SIDENOTE
A thought-provoking interview of Jeff Lichtman and Juan Carlos Tapia, professors at Harvard University who conducted some experiments on synaptic elimination.
https://www.youtube.com/watch?v=b5y2HyD-wH0