Developmental plasticity refers to changes in neural connections during growth, influenced by environmental interactions and learning. Similar to brain plasticity, it specifically involves how neurons and synapses adapt during development. Most of these connections form from birth to early childhood, following three main processes, with critical periods determining lasting changes. The term can also describe how an embryo or larva adjusts its traits based on the environment. Unlike phenotypic plasticity, which can be reversible in adulthood, developmental plasticity shapes traits early in life that usually remain permanent.
Mechanisms
During development, the central nervous system acquires information via endogenous or exogenous factors 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. According to Turrigiano (2012), this form of plasticity that occurs during development is the result of three predominant mechanisms: synaptic and homeostatic plasticity, and learning. When brain areas are impaired, remaining circuits can reorganize to compensate for lost functions. Additionally, adult neuroplasticity allows for continuous learning and memory formation. Factors such as age, environment, and experience influence the extent of plasticity, with enriched environments enhancing cognitive function. These changes are driven by mechanisms like synaptic plasticity, which strengthens or weakens synapses based on activity, homeostatic plasticity, which maintains neural stability, and learning-induced plasticity, which adapts neural circuits in response to new experiences.
Synaptic plasticity Phenotypic plasticity is the ability of an organism to change its physical traits, behavior, or physiology in response to environmental conditions. This adaptability allows a single genotype to produce different phenotypes depending on the environment, helping organisms survive and reproduce in varying or changing habitats. For example, some plants can grow taller in low-light conditions to reach sunlight, while certain animals may change their coloration with the seasons for better camouflage. Phenotypic plasticity plays a crucial role in evolution and ecological interactions.
Synaptic plasticity The underlying principle of synaptic plasticity is that synapses undergo an activity-dependent and selective strengthening or weakening so that 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 of synapses during development is key to plasticity that occurs in the visual and auditory cortices. In experiments conducted by Hubel and Wiesel, the visual cortex of kittens exhibits synaptic plasticity in the refinement of 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 mechanisms that assist in the maturation of synapses in 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 lead to a reduction in all of a neuron's synaptic responses. Numerous pathways have recently been associated with homeostatic plasticity, though there is still no clear molecular mechanism. Synaptic scaling is one method that serves as a type of autoregulation, as neurons can recognize their own firing rates and notice when there are alterations; calcium-dependent signals control the levels of glutamate receptors at synaptic sites in response. Homeostatic mechanisms may be local or network-wide.
Learning While synaptic plasticity is considered to be a by-product of learning, learning involves interaction with the environment to acquire the new information or behavior; synaptic plasticity merely represents the change in strength or configuration of neural circuits. Learning is crucial, as there is considerable interaction with the environment, which is when 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 inverse is seen following rearing in an environment devoid of interaction. Also, learning plays a considerable role in the selective acquisition of information and is markedly demonstrated when children develop one language instead of another. Another example of such experience-dependent plasticity that is critical during development is the occurrence of imprinting. This occurs as a result of a young child or animal being exposed to a novel stimulus and rapidly implementing a certain behavior in response.
Neural development The formation of the nervous system is one of the most crucial events in the developing embryo. The differentiation 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, due to the impediment of the normal acetylcholine receptor activation. 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, when proper synaptic function is disrupted 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.
Neural refinement and connectivity Initial stages of neural development begin early on in the fetus with spontaneous firing of the developing neuron. These early connections are weak and often overlap at the terminal ends of the arbors. The young neurons have complete potential of changing morphology during a time span classified as the critical period to achieve strengthened and refined synaptic connections. It is during this time that damaged neuronal connections can become functionally recovered. Large alterations in length and location of these neurons can occur until synaptic circuitry is further defined. Although organization of neural connections begins at the earliest stages of development, activity-driven refinement only begins at birth when the individual neurons can be recognized as separate entities and start to enhance in specificity. The gradual pruning of the initially blurry axonal branching occurs via competitive and facilitative mechanisms, relying on electrical activity at the synapses; axons that fire independently of each other tend to compete for territory, whereas axons that synchronously fire mutually amplify connections. Until this architecture has been established, retinal focus remains diffuse. Perpetuation of these newly formed connections or the lack thereof depends on maintenance of electrical activities at the synapses. Upon refinement, the elaborate connections narrow and strengthen to fire only in response to specific stimuli to optimize visual acuity. These mechanisms can malfunction with the introduction of toxins, which bind to sodium channels and suppress action potentials and consequently electrical activity between synapses.
Quantification of synaptic networks has primarily been through retinal wave detection using Ca2+ fluorescent indicators. Prior to birth, retinal waves are seen to originate as clusters that propagate through the refractory region. These assays have been shown to provide spatiotemporal data on the random bursts of action potentials produced in a refractory period. Another assay recently developed to assess the depth of neuronal connections utilizes the trans-neuronal spread of rabies. This method of tracing employs the migration of a neurotropic virus through tightly interconnected neurons and specific site labeling of distinct connections. Patch-clamping experiments and calcium imaging are often conducted based on preliminary results from this assay in order to detect spontaneous neuronal activity. A method for in vitro synaptic quantification has been developed that uses immunofluorescence to measure synaptic density in different cell cultures.
Critical period The concept of critical periods is a widely accepted and prominent theme in development, with strong implications for 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 rely on biological mechanisms including endogenous or exogenous factors. Another notable example includes the development of sensory systems, which also undergo plastic changes during critical time periods. 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 interactions from both the environment and intrinsic factors. Experimentally exposing two-week-old kittens and rats to hyperoxic conditions completely eliminates the carotid chemoreceptor response to hypoxia, resulting in respiratory impairment. This has remarkable 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. When stimuli appear or experiences occur outside of the critical period, any potential outcome is typically not long-lasting.
Spontaneous network activity