Synaptic pruning is a brain function that helps maintain normal neuronal connectivity. When synapses are lost or damaged, it may lead to neuropsychiatric disorders such as schizophrenia and autism spectrum disorder (ASD). The term “prune” refers to the process of removing dead or dying neurons from the network. The loss of connections between neurons causes problems with communication within the brain. For instance, if one neuron loses its connection to another, then the two neurons cannot communicate with each other effectively. If too many connections are lost, then the brain’s ability to learn and remember is impaired.
The term “pruning” comes from the Latin word prōprare meaning “to cut.” The process of synaptic pruning occurs during adolescence because it is thought that this stage of development allows for greater plasticity than later stages of development. During this time, the brain undergoes a period of rapid growth and maturation. As a result, there is often a loss of synapses due to the increase in neural activity.
In order to keep these new connections intact, they must be removed. The removal of dead or dying cells is done through the use of chemicals called neurotransmitters which are released into the surrounding area where they bind with receptors located throughout the body. The purpose of the pruning is to remove weak or haphazard connections in the brain.
The synaptic pruning process begins shortly after birth and continues into young adulthood. In fact, studies show that newborn infants have up to one hundred billion neurons. During infancy and early childhood, many of these neurons are lost as a result of the pruning process. After the age of two, humans have around 86 billion neurons which is enough for them to retain most of their abilities.
The human nervous system contains two types of connections: grey matter and white matter. The grey matter contains groups of cells called nuclei. These nuclei contain the cell bodies of brain cells (neurons). The white matter contains bundles of long nerve fibers that connect different groups of cells in different regions of the brain.
The pruning process involves the reduction of grey matter, the loss of unneeded cells and neural connections. During this time, white matter continues to develop and grow.
The brain uses a system called synaptic plasticity and competitive learning to determine which synapses (connections) should be preserved. The specific molecules involved in synaptic plasticity are thought to be identical to those involved in learning and memory. When two neurons are active at the same time, a chemical exchange occurs between the two cells. This exchange strengthens the connection between the two cells causing them to respond more readily to one another.
If a neuron is not active, the connection eventually fades and dies. Another method of removal involves the formation of a structure called a dendritic spine which acts as a holding area for synapses within the brain. When an excitatory synapse attaches to a dendritic spine, the spine grows. When an excitatory synapse loses its connection, the spine shrinks.
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