Neural pruning is the process by which axons and dendrites of neuronal synapses are destroyed, in order to eliminate extra neurons and their connections, thereby increasing the efficiency of neuronal transmissions.
- 1 When does neuronal pruning occur?
- 2 The influence of neuronal pruning in adolescence
- 3 Why is synaptic pruning important for the developing brain?
When does neuronal pruning occur?
From our embryonic stage and up to 2 years of age, new neurons and synapses (synaptogenesis) are formed in our brain continuously and at a surprising rate, reaching up to 40,000 new new synapses per second. At the end of this process, babies have many more neurons and synapses than are functionally necessary.
It is then that the stage of destruction of the synapses that are not used and the strengthening or myelination of those that are used appear. This strengthening will make the remaining connections faster and more effective.
The Neural pruning (or destruction of neuronal synapses) is the process by which additional synapses are eliminated, which serves to increase the efficiency of the neural network. The whole process continues until sexual maturation, at which time almost 50% of the synapses present at 2 years of age have been eliminated. The pattern and pruning in the timeline varies according to the region of the brain.
The infant brain increases 5 times its size until reaching adulthood, reaching approximately a whopping up to 86 (± 8) billion neurons inside. Two factors contribute to this brain growth: the proliferation of synaptic connections between neurons and myelination of nerve fibers, The total number of neurons, however, remains the same.
Neural pruning is strongly influenced by environmental factors and is believed to represent learning. From adolescence (at approximately 14 years old), the volume of synaptic connections decreases again because an important synaptic pruning occurs at this time of life.
The influence of neuronal pruning in adolescence
Numerous studies indicate that while this is true that a large neuronal pruning occurs in many regions of the brain, the same does not occur in all. For example, in the prefrontal cortex, neuronal synapses continue to be created in preadolescence (11-12 years) and then decrease and strengthen those that remain, a task that does not end until after 20 years.
The prefrontal cortex is primarily responsible for the executive function (design of future plans, goal setting, start of activities, etc.) and self-regulation of behavior. In addition, thanks to the development of the prefrontal lobe during adolescence,connections with some other structures already developed are improved during the first years of life, such as the amygdala, which will make many of its automatic reactions become better controlled, progressively decreasing the impulsiveness of the first years of puberty.
As the different brain areas are integrated with each other, the regulation of impulses and emotions that are immature at the beginning of adolescence, at the end of this stage and during adulthood, will become much more effective.
Why is synaptic pruning important for the developing brain?
As we have just seen, one of the great strategies that nature uses to build nervous systems is overproduce neuronal elements, such as neurons, axons and synapses, and then prune excess. In fact, this overproduction is so important that only half of the neurons generated by mammalian embryos will survive after birth.
But why do some neural connections persist, while others do not?
Apparently newborn neurons migrate through chemically defined pathways and when they reach their destination (the one they have genetically assigned), they compete with their "sister" neurons to connect with their predetermined targets.
Victorious neurons receive trophic or nutritional factors that allow them to survive, while losing neurons fade into a process called apoptosis or cell death. The time of cell death is genetically programmed and occurs at different stages of embryonic development of each species.
For decades, neuroscientists believed that neural pruning ended shortly after birth. But in 1979, Peter Huttenlocher, a neurologist at the University of Chicago, showed that this strategy of overproduction and pruning actually continues long after birth.
Neural synapses proliferate after birth, reaching twice its neonatal levels in mid and late childhood, and then precipitously decrease during adolescence.
These changes at the synapse level cause a neuronal restructuring that most likely has important consequences for both normal and abnormal brain function. The rationalization of neural circuits could explain the increase in cognitive skills that occurs in adolescence and early adulthood. On the other hand, the loss of many other neuronal pathways could be the cause of difficulties in recovering from a traumatic brain injury, since eliminating synaptic redundancies decreases our ability to develop alternative pathways to avoid the damaged region.
Further, many important mental illnesses begin to appear in adolescence, a fact that some scholars believe may be related or even cause the great synaptic pruning that occurs. In the 1980s Irwin Feinberg, professor of psychiatry and behavioral sciences at the University of California, began to hypothesize that disorderly synaptic pruning could explain the age of onset of schizophrenia, and in 2016 the researchers published genetic and experimental evidence which supports this neuronal association.
Although the reasons for synaptic pruning in the human brain are beginning to be unraveled, this process seems to have significant consequences in normal brain function and can provide key information about the causes of some neuropsychiatric diseases.
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Iglesias, J .; Eriksson, J .; Grize, F .; Tomassini, M .; Villa, A. (2005). "Pruning dynamics in simulated large-scale stimulatory neural networks". BioSystems
François Ansermet & Pierre Magistretti: “To each his brain. Neural and unconscious plasticity ”. Discussions