Synaptic Pruning

Gopnick et al. (1999) describe neurons as growing telephone wires that communicate with one another. Following birth, the brain of a newborn is flooded with information from the baby’s sense organs. This sensory information must somehow make it back to the brain where it can be processed. To do so, nerve cells must make connections with one another, transmitting the impulses to the brain. Continuing with the telephone wire analogy, like the basic telephone trunk lines strung between cities, the newborn’s genes instruct the "pathway" to the correct area of the brain from a particular nerve cell. For example, nerve cells in the retina of the eye send impulses to the primary visual area in the occipital lobe of the brain and not to the area of language production (Wernicke’s area) in the left posterior temporal lobe. The basic trunk lines have been established, but the specific connections from one house to another require additional signals.

Over the first few years of life, the brain grows rapidly. As each neuron matures, it sends out multiple branches (axons, which send information out, and dendrites, which take in information), increasing the number of synaptic contacts and laying the specific connections from house to house, or in the case of the brain, from neuron to neuron. At birth, each neuron in the cerebral cortex has approximately 2,500 synapses/ synaptic areas. By the time an infant is two or three years old, the number of synapses is approximately 15,000 synapses per neuron. This amount is about twice that of the average adult brain. As we age, old connections are deleted through a process called synaptic pruning.

Synaptic pruning eliminates weaker synaptic contacts while stronger connections are kept and strengthened. Experience determines which connections will be strengthened and which will be pruned. Connections that have been activated most frequently are preserved.

Neurons must have a purpose to survive. Without a purpose, neurons die through a process called apoptosis in which neurons that do not receive or transmit information become damaged and die. Ineffective or weak connections are "pruned" in much the same way a gardener would prune a tree or bush, giving the plant the desired shape. It is plasticity that enables the process of developing and pruning* connections, allowing the brain to adapt itself to its environment.

pruning: trimming of the unwanted neurons or degenerated neurons