Plasticity of the Brain
The ability of the nervous system to change is referred to as neural plasticity. While it was always thought that the nervous system could adapt throughout life, there is now ample evidence that the adult brain retains the ability for reorganization or plasticity. 17–19 Previously, it was assumed that flexibility was restricted to the developing nervous system. When neurons compete for synaptic locations, they are said to be in a critical period. Changes in brain circuitry caused by activity typically occur over a limited development period or during a critical period when the organism is sensitive to the effects of experience. The ability of the nervous system to create structural changes in response to internal and external forces is included in the idea of plasticity. Throughout life, learning and behavior appear to influence neurogenesis. Plastic alterations can also involve neurons’ ability to adjust their function and the amount and type of neurotransmitter they generate. 20 Changes in the structure and function of the nervous system’s basic blocks are notable instances of nervous system plasticity.
The nervous system continues to develop after birth. Although most of the 100 billion neurons are generated at birth, neurons continue to link with other structures via dendritic branching and remodeling of existing connections. During critical periods, neuronal projections compete for synaptic sites. 7 When synaptic strength grows, there are long-term changes in spine shape. 13 Neurons that fire simultaneously links together. The strength of the synapse formed by the spine is proportional to its size. 14 Postnatal experience has a significant role in further inducing developmental alterations by enhancing the system’s pattern of synaptic connections. The interaction of development and experience brings about change. Spine density abnormalities have been connected to certain types of mental retardation and cognitive problems (Figure 9-9). 13
Experience is essential for growth. The literature describes two types of neuronal plasticity. 21 Unfortunately, the names assigned to them are perplexing. The first is experience-expectant, while the second is experience-dependent. The infant is supposed to be exposed to sufficient environmental stimuli at appropriate periods during typical prenatal and postnatal development. Indeed, if the newborn is not exposed to the appropriate quality and quantity of input, the development will be disrupted. This form of experience-expectant neuronal plasticity is demonstrated by sensory systems, which are ready to operate at birth but require exposure to light and sound to fully mature. Deprivation during vital stages can fail to develop vision and hearing as planned.
Experience-dependent brain plasticity permits the nervous system to incorporate other information from unpredictable and distinctive external events. These experiences are unique to the individual and are influenced by the setting in which they occur, such as the physical, social, and cultural surroundings. This is referred to as ecological plasticity by Lebeer22 and activity-dependent plasticity by Johnston23. Movement experiences can be influenced by climate, social expectations, and child-rearing techniques. What each child learns is determined by the physical hurdles they face. Motor learning is an example of experience-dependent neuronal plasticity as part of motor development. Infants’ experiences in different cultures may result in differences in the acquisition of motor ability. Similarly, not every child hears the exact words, but every youngster learns language. Changes in synapses or neural circuits as a result of experience or learning are driven by activity-dependent plasticity.