Synaptic pruning and selective elimination during adolescence

Question

How does Synaptic pruning occur during pre-adolescence, adolescence and post-adolescence, after there is blooming overproduction of synaptic connections until the years of late childhood, and how does during this stage of brain development, "selective elimination" of synaptic connections occur, which is depended on the activity of synapses which are involved in primary functions?

Also, why, how and to what extent does the brain, after approximately age 6, receive the urge to cut back certain synapses?

Update 1: what exactly is triggered before and during adolescence, so that the brain receives the urge to perpetrate synaptic pruning? I know how synaptic pruning is carried out, I am however very intrigued in the specific processes that occur, from the signal that the brain receives to do it, to why exactly it occurs in such an early stage of life

Answer 1

An answer to this question could potentially be found in "Understanding Adolescent Brain Development and Its Implications for the Clinician"

Thanks to research by Jay Giedd and colleagues at the National Institute of Mental Health, it has become clear that, during the adolescent years, the organization and functioning of the brain go through complex changes. Importantly, these changes seem to be unique to the adolescent years and not simply the trailing remnants of childhood brain development.

Changes in frontal lobes during adolescence

Frontal lobe gray-matter volumes, which represent dense concentrations of neurons and their parts, increase throughout childhood and do not reach their peak until approximately the age of 11 (girls) or 12 (boys), at which point they decline throughout the second decade of life and into young adulthood.

So why might frontal lobe gray-matter volumes go up during childhood and down during adolescence?

Recent data suggested that, during childhood, neurons in the frontal lobes are allowed to overgrow and form far too many points of communication, or synapses, with other neurons. As a result, gray-matter volumes increase. As childhood draws to a close and adolescence begins, the brain switches from overproduction mode to selection mode. Early in the second decade of life, the brain stops overproducing synapses in the frontal lobes and puts the synapses that exist on the chopping block. Hundreds of billions of points of communication will be sacrificed through the teenage years. Only those that form meaningful, useful points of contact will be kept. Guided by a teenager’s experiences, the frontal lobes are shaped and molded into a configuration that will carry the individual, for better or worse, through the adult years. As this pruning process unfolds, gray-matter volumes decrease.

Changes in other parts of the cortex (specifically temporal lobes)

The temporal lobes, which are critical for memory formation as well as processing auditory information and seeing detailed patterns and shapes, do not reach their maximum levels of gray matter until the age of 16 to 17, at which point they plateau. The temporal lobes contain the hippocampus, a structure that is central to creating an autobiographical record of what one does and what one learns.

Sources:

1. White, Aaron M. "Understanding Adolescent Brain Development and Its Implications for the Clinician." Division of Medical Psychology, Department of Psychiatry, Duke University Medical Center, Box 3374, Durham, NC 27710, USA, 2009. Web. 7 Feb. 2016.
2. Steinberg, Laurence. "A Social Neuroscience Perspective on Adolescent Risk-Taking." Developmental Review : DR. U.S. National Library of Medicine, 28 Mar. 2008. Web. 07 Feb. 2016.