Synaptic pruning

Synaptic pruning or axon pruning is the process of synapse elimination that occurs between early childhood and the onset of puberty in many mammals, including humans. Pruning starts near the time of birth and is completed by the time of sexual maturation in humans. At birth, the human brain consists of approximately 86 (± 8) billion neurons. The infant brain will increase in size by a factor of up to 5 by adulthood. Two factors contribute to this growth: the growth of synaptic connections between neurons, and the myelination of nerve fibers; the total number of neurons, however, remains the same. Pruning is influenced by environmental factors and is widely thought to represent learning. After adolescence, the volume of the synaptic connections decreases again due to synaptic pruning.

At birth, the neurons in the visual and motor cortices have connections to the superior colliculus, spinal cord, and pons. The neurons in each cortex are selectively pruned, leaving connections that are made with the functionally appropriate processing centers. Therefore, the neurons in the visual cortex prune the synapses with neurons in the spinal cord, and the motor cortex severs connections with the superior colliculus. This variation of pruning is known as large-scaled stereotyped axon pruning. Neurons send long axon branches to appropriate and inappropriate target areas, and the inappropriate connections are eventually pruned away.

Regressive events refine the abundance of connections, seen in neurogenesis, to create a specific and mature circuitry. Apoptosis and pruning are the two main methods of severing the undesired connections. In apoptosis, the neuron is killed and all connections associated with the neuron are also eliminated. In contrast, the neuron does not die in pruning, but requires the retraction of axons from synaptic connections that are not functionally appropriate.

It is believed that the purpose of synaptic pruning is to remove unnecessary neuronal structures from the brain; as the human brain develops, the need to understand more complex structures becomes much more pertinent, and simpler associations formed at childhood are thought to be replaced by complex structures.

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