Developmental plasticity
Encyclopedia
Developmental plasticity is a general term referring to changes in neural connections during development as a result of environmental interactions as well as neural changes induced by learning. Much like neuroplasticity
Neuroplasticity
Neuroplasticity is a non-specific neuroscience term referring to the ability of the brain and nervous system in all species to change structurally and functionally as a result of input from the environment. Plasticity occurs on a variety of levels, ranging from cellular changes involved in...

 or brain plasticity, developmental plasticity is specific to the change in neurons and synaptic connections as a consequence of developmental processes.

Mechanisms

During development
Child development
Child development stages describe theoretical milestones of child development. Many stage models of development have been proposed, used as working concepts and in some cases asserted as nativist theories....

, the central nervous system acquires information via endogenous or exogenous factors
Neuromodulation
In Neuromodulation several classes of neurotransmitters regulate diverse populations of central nervous system neurons...

 as well as learning experiences. In acquiring and storing such information, the plastic nature of the central nervous system allows for the adaptation of existing neural connections in order to accommodate new information and experiences, resulting in developmental plasticity. This form of plasticity that occurs during development is the result of three predominant mechanisms: synaptic and homeostatic plasticity, and learning.

Synaptic plasticity

The underlying principle of synaptic plasticity is that synapses undergo and activity-dependent and selective strengthening or weakening so new information can be stored. Synaptic plasticity depends on numerous factors including the threshold of the presynaptic stimulus in addition to the relative concentrations of neurotransmitter molecules. Synaptic plasticity has long been implicated for its role in memory storage and is thought to play a key role in learning. However, during developmental periods synaptic plasticity is of particular importance as changes in the network of synaptic connections can ultimately lead to changes in developmental milestones. For instance, the initial overproduction
Synaptogenesis
Synaptogenesis is the formation of synapses. Although it occurs throughout a healthy person's lifespan, an explosion of synapse formation occurs during early brain development...

 of synapses during development is key to plasticity that occurs in the visual and auditory cortex. In experiments conducted by Hubel and Wiesel
Ocular dominance column
Ocular dominance columns are stripes of neurons in the visual cortex of certain mammals that respond preferentially to input from one eye or the other. The columns span multiple cortical layers, and are laid out in a striped pattern across the surface of the striate cortex...

, the visual cortex of kittens exhibits synaptic plasticity in the refinement neural connections following visual inputs. Correspondingly, in the absence of such inputs during development, the visual field fails to develop properly and can lead to abnormal structures and behavior. Furthermore, research suggests that this initial overproduction of synapses during developmental periods provides the foundation by which many synaptic connections can be formed, thus resulting in more synaptic plasticity. In the same way that synapses are abundant during development, there are also refining
Synaptic pruning
In neuroscience, synaptic pruning, neuronal pruning or axon pruning refer to neurological regulatory processes, which facilitate a change in neural structure by reducing the overall number of neurons or connections, leaving more efficient synaptic configurations. Pruning is a neurological...

 mechanisms that coincidently refine the connectivity of neural circuits. This regulatory process allows the strengthening of important or frequently used synaptic connections while reducing the amount of weak connections.

Homeostatic plasticity

In order to maintain balance, homeostatic controls exist to regulate the overall activity of neural circuits specifically by regulating the destabilizing effects of developmental and learning processes that result in changes of synaptic strength. Homeostatic plasticity also helps regulate prolonged excitatory responses, which can lead to a reduction in all of a neuron’s synaptic responses. While the exact mechanisms by which homeostatic plasticity acts remains unclear, recent studies raise the idea that homeostatic plasticity is modulated according to the period of development or challenges in existing neural circuits.

Learning

While synaptic plasticity is considered to be a by-product of learning, learning requires interaction with the environment to acquire the new information or behavior, whereas synaptic plasticity merely represents the change in strength or configuration of neural circuits. Learning is of crucial importance postnatally as there is considerable interaction with the environment and the potential for acquiring new information is greatest. By depending largely upon selective experiences, neural connections are altered and strengthened in a manner that is unique to those experiences. Experimentally this can be seen when rats are raised in an environment that allows ample social interaction, resulting in increased brain weight and cortical thickness. In contrast, the adverse is seen following rearing in an environment devoid of interaction. Also, learning plays a sizeable role in the selective acquisition of information and is markedly demonstrated as children develop one language as opposed to another. Another example of such experience dependent plasticity that is critical during development is the occurrence of imprinting
Imprinting
Imprinting may refer to:* Genomic imprinting , a mechanism of regulating gene expression* Imprinting , in psychology and ethology* Molecular imprinting, in polymer chemistry...

. This occurs as a result of the young child or animal experiencing a novel stimuli and rapidly learning the behavior in response.

Neural Development

The formation of the nervous system is one of the most crucial events in the developing embryo. Specifically, the differentiation
Cellular differentiation
In developmental biology, cellular differentiation is the process by which a less specialized cell becomes a more specialized cell type. Differentiation occurs numerous times during the development of a multicellular organism as the organism changes from a simple zygote to a complex system of...

 of stem cell precursors into specialized neurons gives rise to the formation of synapses and neural circuits, which is key to the principle of plasticity. During this pivotal point in development, consequent developmental processes like the differentiation and specialization of neurons are highly sensitive to exogenous and endogenous factors. For example, in utero exposure to nicotine has been linked to adverse effects such as severe physical and cognitive deficits as a result of impeding the normal activation of acetylcholine receptors. In a recent study, the connection between such nicotine exposure and prenatal development was assessed. It was determined that nicotine exposure in early development can have a lasting and encompassing effect on neuronal structures, underlying the behavioral and cognitive defects observed in exposed humans and animals. Additionally, by disrupting proper synaptic function through nicotine exposure, the overall circuit may become less sensitive and responsive to stimuli, resulting in compensatory developmental plasticity. It is for this reason that exposure to various environmental factors during developmental periods can cause profound effects on subsequent neural functioning.

Critical Period

The concept of critical periods is a widely accepted and prominent theme in development, with strong implications to developmental plasticity. Critical periods establish a time frame in which the shaping of neural networks can be carried out. During these critical periods in development, plasticity occurs as a result of changes in the structure or function of developing neural circuits. Such critical periods can also be experience-dependent, in the instance of learning via new experiences. Or can be independent of the environmental experience and be dependent on biological mechanisms including endogenous or exogenous factors. Again, one of the most pervading examples of this can be seen in the development of the visual cortex in addition to the acquisition of language as a result of developmental plasticity during the critical period. A lesser known example, however, remains the critical development of respiratory control during developmental periods. At birth, the development of respiratory control neural circuits is incomplete, requiring complex inetractions from both the environment and internal factors. Experimentally exposing two week-old kittens and rats to hyperoxic
Hyperoxia
Hyperoxia is excess oxygen or higher than normal partial pressure of oxygen.In medicine, it refers to excess oxygen in the lungs or other body tissues, which can be caused by breathing air or oxygen at pressures greater than normal atmospheric pressure...

 conditions, completely eliminates the carotid chemoreceptor response to hypoxia
Hypoxia (medical)
Hypoxia, or hypoxiation, is a pathological condition in which the body as a whole or a region of the body is deprived of adequate oxygen supply. Variations in arterial oxygen concentrations can be part of the normal physiology, for example, during strenuous physical exercise...

, and consequently resulting in respiratory impairment. This has dramatic clinical significance as newborn infants are often supplemented with considerable amounts of oxygen, which could detrimentally affect the way in which neural circuits for respiratory control develop during the critical period. Additionally, when stimuli or experiences are elicited outside of the critical period, usually the results have little to no lasting effect, which could also lead to severe developmental impairment.

Spontaneous Network Activity

Another lesser known element of developmental plasticity includes spontaneous bursts of action potentials in developing neural circuits, also referred to as spontaneous network activity. During the early development of neural connections, excitatory synapses undergo spontaneous activation, resulting in elevated intracellular calcium levels which signals the onset of innumerable signaling cascades and developmental processes. As an example, prior to birth neural circuits in the retina undergo spontaneous network activity, which has been found to elicit the formation of retinogeniculate connections. Examples of spontaneous network activity during development are also exhibited in the proper formation of neuromuscular circuits
Neuromuscular junction
A neuromuscular junction is the synapse or junction of the axon terminal of a motor neuron with the motor end plate, the highly-excitable region of muscle fiber plasma membrane responsible for initiation of action potentials across the muscle's surface, ultimately causing the muscle to contract...

. It is believed that spontaneous network activity establishes a scaffold for subsequent learning and information acquisition following the initial establishment of synaptic connections during development.

See also

  • Hebbian theory
    Hebbian theory
    Hebbian theory describes a basic mechanism for synaptic plasticity wherein an increase in synaptic efficacy arises from the presynaptic cell's repeated and persistent stimulation of the postsynaptic cell...

  • Long-term potentiation
    Long-term potentiation
    In neuroscience, long-term potentiation is a long-lasting enhancement in signal transmission between two neurons that results from stimulating them synchronously. It is one of several phenomena underlying synaptic plasticity, the ability of chemical synapses to change their strength...

  • Long-term depression
    Long-term depression
    Long-term depression , in neurophysiology, is an activity-dependent reduction in the efficacy of neuronal synapses lasting hours or longer. LTD occurs in many areas of the CNS with varying mechanisms depending upon brain region and developmental progress...

  • NMDA receptor
    NMDA receptor
    The NMDA receptor , a glutamate receptor, is the predominant molecular device for controlling synaptic plasticity and memory function....

  • GABA receptor
    GABA receptor
    The GABA receptors are a class of receptors that respond to the neurotransmitter gamma-aminobutyric acid , the chief inhibitory neurotransmitter in the vertebrate central nervous system...

  • Cultured neuronal network
    Cultured neuronal network
    A cultured neuronal network is a cell culture of neurons that is used as a model to study the central nervous system, especially the brain. Often, cultured neuronal networks are connected to an input/output device such as a multi-electrode array , thus allowing two-way communication between the...

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