Plasticity can be thought of as changes to the brain and brain structure as a consequence of both natural brain development and in response to trauma in the already developed brain. The major cell of the brain is the neuron. To carry out various functions in the brain, neurons communicate with each other through synapses. When plasticity occurs, both neurons and synapses increase in number. There is a clear relationship between plasticity and aging. Synaptic connections increase dramatically between birth and two or three years of age; they are reduced by half during adolescence and remain relatively static throughout adulthood.
Plasticity and the Young Brain
The young brain displays the greatest plasticity. Neurons and synapses experience a huge increase in number even before a person can perform basic functions like talking and walking. Between birth and two or three years of age, the number of synapses in the brain increases from 2,500 to 15,000 per neuron. The average toddler has twice as many synapses as an adult.
Plasticity and the Adolescent Brain
Between youth and adulthood, a phenomenon known as pruning occurs in the brain. Pruning is the reduction in the number of neurons and synapses that formed during early age. This elimination is based on experiences the person has in life; the connections a person uses the most are kept, and weak connections are eliminated. By the time an individual reaches late adolescence, the number of the synaptic connections between neurons has been reduced by about half.
Plasticity and the Adult Brain
Although the number of neurons and synapses were long thought to be static in adulthood, there is evidence that plasticity can occur in older individuals as a result of learning or experience. Learning, which can cause the brain to increase the number of synapses, is an instance of plasticity. Changes also occur in different parts of the brain's cells. For example, dendrites, which extend from the perimeter of the neuron to receive signals from other neurons, have been found to be more extensively branched in old individuals than in those who are middle aged.
Plasticity and Brain Damage
An exception to age-related plasticity occurs when the brain undergoes trauma from conditions such as accident or stroke. While the number of neurons remains relatively constant, the strength of the connections -- or the ability for neurons to "talk to" one another -- can increase to compensate for the loss that occurs with brain damage.