The brain of a newborn baby grows and develops very quickly. Within the first three years of life, the intense formation of new nerve cells (neurons), neural networks, and connections (axons, synapses, and dendrites) occur. At this phase, connections are made to other neighboring or distant neurons, muscles, or cells through the extension of new branches from the neurons. During these activities, hormone-like chemical substances play important roles in charge of nerve cell reproduction and their networking. For instance, neural growth factors and brain-based neurotrophic factors take a role in the structuring of neurons.
Nerve cells that fail to establish connections with other cells (nerve, muscle, or secretion gland) within the initial 6-12 month period following birth are destined for a programmed cell death (apoptosis). For example, if the eyes of newborn animals are kept shut for a couple of weeks, vision-related neurons in the brain cortex die and the animals become blind. If the initially present neurons cannot perform their duties or are not used properly, they are terminated. This situation is expressed as the “use it or lose it” principle.
The notion that there is no recuperation for injured or dead nerve cells or tissues, in addition to the recovery of bodily functions related to the damaged neural tissues, was dominant in the medical field for years. Even though this is the case for adult individuals, at least most of the time, it has been proven that this is not true and applicable all the time, especially for children. It has been discovered that lost brain functions can be recovered, even if only partially, and this potential for recuperation is present at birth. This is called the neuronal plasticity or regeneration (renewal) capacity.
Neuronal regeneration is the auto-healing process of nerve tissues that have lost their integrity, functions, or been damaged due to trauma, oxygen deprivation (ischemia), infection, or many other reasons. For example, in patients with polio, certain arm-leg nerve fibers become dead and the muscle stimulated by that nerve suffers a stroke. However, neighboring nerves manage to recover the paralyzed muscles by extending new arms. This situation may resemble the installation of a parallel phone line from your apartment to your neighbor when the old line is cut, which benefits both units with the same line. Certain short projections can grow from the root of a damaged or broken nerve fiber (axon), and in some cases healing may be achieved by this growth. According to the neuronal plasticity theory, the brain is a dynamic, flexible organ, and is open to changes. It is not static, and it can adapt to new conditions. Based on this theory, a brain that has lost an ability, such as vision or hearing, can regain these functions; an individual can hear or see again. However, there is still more research to be done on this matter.
Regeneration is a mechanism that supports the plasticity theory. The proliferation of stem cells is triggered by plasticity. The reassignment of the tasks of the dead nerve cells to other cells occurs; the surviving nerve cells are induced to develop new branches (dendrites or axons). With the discovery of the plasticity and regeneration processes, some diseases that have been considered untreatable have shown signs of being treatable. For instance, during cerebral palsy in children, imperfections take place regarding motor movements, perception, and intelligence, depending on the oxygen deprivation experienced in the brain tissue before, during, or after birth. The brain goes through an adaptive process to execute the missing functions. Especially during the early stages of life, it is understood that the brain cortex is created with extraordinary reorganization abilities after an injury.
Plasticity not only manifests itself via an increased neuron count, but also with an increase in the number of axons and dendrites, in addition to an abundance of synapses among nerve cells. Nonetheless, the onset of all the positive changes on the neurons requires a stimulator or a trigger. In recent years, certain drugs were developed for this purpose. The laboratory works showing that neuronal plasticity and regeneration were successful for polio also give hope that adult brain disorders might be treatable. Brain recuperation via neuronal plasticity is not only dependent on drugs, but also can be achieved by alternative methods like acupuncture, touching, and massage therapy. Apart from these, sports like swimming and horseback riding are known to stimulate and trigger regeneration in damaged neurons.
The studies briefly explained here have proven wrong the belief that the central nervous system cannot heal and recover to its normal mode of functioning. Mechanisms that have been established in the system (the genetic structure) are being newly discovered by mankind. New studies are needed to find out what other aspects of human biology can be discovered as the science advances. Such discoveries remind us of what a remarkable creation human beings are.