Plasticity, or neuroplasticity, is the lifelong ability of the brain to re-organise neural pathways based on new experiences. As we learn, we acquire new knowledge and skills through instruction or experience. In order to learn or memorise a fact or skill, there must be persistent functional changes in the brain that represent the new knowledge. The ability of the brain to change with learning is what is known as ‘neuroplasticity’.
To illustrate plasticity in a way, imagine making an impression of a coin in a lump of clay. In order for the impression of the coin to appear in the clay, changes must occur in the clay — the shape of the clay changes as the coin is pressed into the clay. Similarly, the neural circuitry in the brain must reorganise in response to experience or sensory stimulation. Neuroplasticity does not consist of a single type of morphological change, but rather includes several different processes that occur throughout an individual’s lifetime. Many types of brain cells are involved in neuroplasticity, including neurons, glia, and vascular cells and endothelial cells. Neuroplasticity has a clear age dependent determinant. Although plasticity occurs over an individual’s 78 lifetime, different types of plasticity dominate during certain periods of one’s life and are less prevalent during other periods.
Neuroplasticity occurs in the brain under two primary conditions
• During normal brain development, when the immature brain first begins to process sensory information through adulthood (developmental plasticity and plasticity of learning and memory).
• As an adaptive mechanism to compensate for lost function and/or to maximise remaining functions in the event of brain injury.
The environment plays a key role in influencing plasticity. In addition to genetic factors, the brain is shaped by the characteristics of a person’s environment and by the actions of that same person.
Developmental Pasticity: Synaptic Pruning
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.
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. 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 as a gardener would prune a tree or a 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.
Plasticity Learning Memory
It was once believed that as we age, the brain’s networks become fixed. In the past two decades, however, an enormous amount of research has revealed that the brain never stops changing and adjusting. Learning, as defined is the ability to acquire new knowledge or skills through instruction or experience. Memory is the process by which that knowledge is retained over time. The capacity of the brain to change with learning is plasticity. So how does the brain change with learning? There appear to be at least two types of modifications that occur in the brain with learning:
• A change in the internal structure of the neurons, the most notable being in the area of synapses.
• An increase in the number of synapses between neurons.
During brain repair, following injury due to stroke, head injury etc, plastic changes are geared towards maximizing function in spite of the damaged brain. In studies involving rats, in which one area of the brain was damaged, brain cells surrounding the damaged area underwent changes in their function and shape that allowed them to take on the functions of the damaged cells. Although this phenomenon has not been widely studied in humans, data indicate that similar changes (though less effective) occur in human brains following injury.