Synaptic pruning is a natural process that occurs in the brain between early childhood and adulthood. During synaptic pruning, the brain eliminates extra synapses. Synapses are brain structures that allows the neurons to transmit an electrical or chemical signal to another neuron.
Synaptic pruning is thought to be the brain’s way of removing connections in the brain that are no longer needed. Researchers have recently learned that the brain is more “plastic” and moldable than previously thought. Synaptic pruning is our body’s way of maintaining more efficient brain function as we get older and learn new complex information.
During infancy, the brain experiences a large amount of growth. There is an explosion of synapse formation between neurons during early brain development. This is called synaptogenesis.
This rapid period of synaptogenesis plays a vital role in learning, memory formation, and adaptation early in life. At about 2 to 3 years of age, the number of synapses hits a peak level. But then shortly after this period of synaptic growth, the brain starts to remove synapses that it no longer needs.
Once the brain forms a synapse, it can either be strengthened or weakened. This depends on how often the synapse is used. In other words, the process follows the “use it or lose it” principle: Synapses that are more active are strengthened, and synapses that are less active are weakened and ultimately pruned. The process of removing the irrelevant synapses during this time is referred to as synaptic pruning.
Early synaptic pruning is mostly influenced by our genes. Later on, it’s based on our experiences. In other words, whether or not a synapse is pruned is influenced by the experiences a developing child has with the world around them. Constant stimulation causes synapses to grow and become permanent. But if a child receives little stimulation the brain will keep fewer of those connections.
The timing of synaptic pruning varies by brain region. Some synaptic pruning begins very early in development, but the most rapid pruning happens between roughly age 2 and 16.
Early embryonic stage to 2 years
Brain development in the embryo starts just a few weeks after conception. By the seventh month of a pregnancy, the fetus starts to emit its own brain waves. New neurons and synapses are formed by the brain at an extremely high rate during this time.
During the first year of life, the number of synapses in the brain of an infant grows more than tenfold. By age 2 or 3, an infant has about 15,000 synapses per neuron.
In the visual cortex of the brain (the part responsible for vision), synapse production hits its peak at about 8 months of age. In the prefrontal cortex, peak levels of synapses occur sometime during the first year of life. This part of the brain is used for a variety of complex behaviors, including planning and personality.
Ages 2 to 10 years
During the second year of life, the number of synapses drops dramatically. Synaptic pruning happens very quickly between ages 2 and 10. During this time, about 50 percent of the extra synapses are eliminated. In the visual cortex, pruning continues until about 6 years of age.
Synaptic pruning continues through adolescence, but not as fast as before. The total number of synapses begins to stabilize.
While researchers once thought the brain only pruned synapses until early adolescence, recent advancements have discovered a second pruning period during late adolescence.
According to newer research, synaptic pruning actually continues into early adulthood and stops sometime in the late 20s.
Interestingly, during this time the pruning mostly occurs in the brain’s prefontal cortex, which is the part of the brain heavily involved in the decision-making processes, personality development, and critical thinking.
Research that looks at the relationship between synaptic pruning and schizophrenia is still in the early stages. The theory is that schizophrenic brains are “over-pruned,” and this over-pruning is caused by genetic mutations that affect the synaptic pruning process.
For example, when researchers looked at images of the brains of people with mental disorders, such as schizophrenia, they found that people with mental disorders had fewer synapses in the prefrontal region compared to the brains of people without mental disorders.
Then, a large study analyzed post-mortem brain tissue and DNA from more than 100,000 people and found that people with schizophrenia have a specific gene variant that may be associated with an acceleration of the process of synaptic pruning.
More research is needed to confirm the hypothesis that abnormal synaptic pruning contributes to schizophrenia. While this is still a long way off, synaptic pruning may represent an interesting target for treatments for people with mental disorders.
Scientists still haven’t pinpointed the exact cause of autism. It’s likely that there are multiple factors at play, but recently, research has shown a link between mutations in certain genes related to synaptic function and autism spectrum disorders (ASD).
Unlike research into schizophrenia, which theorizes that the brain is “over-pruned,” researchers hypothesize that the brains of people with autism may be “under-pruned.” Theoretically, then, this under-pruning leads to an oversupply of synapses in some parts of the brain.
To test this hypothesis, researchers looked at brain tissue of 13 children and adolescents with and without autism who passed away between ages 2 and 20. The scientists found that the brains of the adolescents with autism had a lot more synapses than the brains of neurotypical adolescents. Young children in both groups had roughly the same number of synapses. This suggests that the condition may occur during the pruning process. This research only shows a difference in synapses, but not whether this difference might be a cause or an effect of autism, or just an association.
This under-pruning theory may help explain some of the common symptoms of autism, like oversensitivity to noise, lights, and social experiences, as well as epileptic seizures. If there are too many synapses firing at once, a person with autism will likely experience an overload of noise rather than a fine-tuned brain response.
Additionally, past research has linked autism with mutations in genes that act on a protein known as mTOR kinase. Large amounts of overactive mTOR have been found in the brains of autism patients. Over-activity in the mTOR pathway has also been shown to be associated with an excess production of synapses. One study found that mice with overactive mTOR had defects in their synaptic pruning and exhibited ASD-like social behaviors.
Synaptic pruning is an essential part of brain development. By getting rid of the synapses that are no longer used, the brain becomes more efficient as you age.
Today, most ideas about human brain development draw on this idea of brain plasticity. Researchers are now looking into ways to control pruning with medications or targeted therapy. They’re also looking into how to use this new understanding of synaptic pruning to improve childhood education. Researchers are also studying how the shape of the synapses might play a role in mental disabilities.
The process of synaptic pruning may be a promising target for treatments for people with conditions like schizophrenia and autism. However, research is still in the early stages.