Neuroplasticity (The MIT Press Essential Knowledge series)

1. Neuroplasticity: The brain's lifelong capacity for change

Far from being fixed, the brain is a highly dynamic structure, which undergoes significant change not only as it develops but also throughout the entire lifespan.

Definition and scope. Neuroplasticity refers to the brain's ability to modify its structure and function in response to experiences, learning, and environmental changes. This property is fundamental to the nervous system's ability to adapt and respond to various stimuli throughout life.

Mechanisms and timescales. Neuroplasticity occurs at multiple levels:

• Molecular: Changes in neurotransmitter release and receptor sensitivity
• Cellular: Formation and pruning of synaptic connections
• Structural: Changes in gray and white matter volume
• Functional: Reorganization of neural networks

These changes can occur over timescales ranging from milliseconds (rapid synaptic changes) to years (long-term structural modifications). The brain's plasticity allows for:

• Learning and memory formation
• Skill acquisition
• Adaptation to sensory loss or brain injury
• Cognitive development and aging processes

2. Historical perspective: From fixed to flexible brain concepts

About one hundred years ago, Santiago Ramón y Cajal, the father of modern neuroscience, stated that the adult brain is "fixed" and "immutable," and this quickly became a central dogma of the field.

Early theories. The concept of brain plasticity has evolved significantly over time:

• 1780s: Early speculation about mental exercise and brain growth
• Late 19th century: William James introduces the term "plasticity" in psychology
• Early 20th century: Cajal's influential "fixed brain" dogma

Paradigm shift. The transition from a fixed to a flexible brain concept occurred gradually:

• 1960s: David Hubel and Torsten Wiesel's work on sensory experience and brain development
• 1970s: Discovery of long-term potentiation (LTP) by Bliss and Lømo
• 1990s: Evidence of adult neurogenesis challenges the "no new neurons" dogma

This shift in understanding has revolutionized neuroscience and opened new avenues for research into brain function, learning, and potential therapeutic interventions.

3. Sensory substitution: Brain's adaptive response to sensory loss

Blind people can also learn to navigate by echolocation, by making clicking sounds with their tongue or tapping sounds with their feet, and using information in the returning echoes to perceive physical aspects of their surroundings.

Cross-modal plasticity. The brain can reorganize itself to process sensory information through alternative pathways when deprived of a specific sensory input. This remarkable adaptability allows individuals to compensate for sensory loss.

Examples of sensory substitution:

• Blind individuals using touch to "see" (e.g., reading Braille)
• Deaf people enhancing visual processing in auditory brain regions
• Echolocation in blind individuals for spatial navigation

Neuroimaging evidence:

• Activation of visual cortex during Braille reading in blind individuals
• Enhanced peripheral vision in deaf people
• Recruitment of visual areas for auditory processing in blind echolocators

These findings demonstrate the brain's capacity to repurpose neural circuits for new functions, highlighting its inherent flexibility and adaptability.

4. Developmental plasticity: Critical periods and experience-dependent changes

Sensory experience refines the microscopic structure of the visual cortex by driving maturation of the large basket cells, which puts the brakes on plasticity by consolidating the emerging circuitry at a time when its representation of the world is most accurate.

Critical periods. During development, the brain undergoes heightened periods of plasticity, particularly sensitive to environmental stimuli:

• Visual system: Ocular dominance columns form based on early visual experiences
• Language acquisition: Enhanced ability to learn languages in childhood
• Emotional regulation: Early life experiences shape stress responses

Mechanisms of developmental plasticity:

• Synaptic pruning: Elimination of unnecessary connections
• Myelination: Enhancing signal transmission efficiency
• Neurogenesis: Formation of new neurons (peaks prenatally)

Role of experience. Environmental inputs during critical periods are crucial for proper brain development:

• Sensory deprivation can lead to permanent deficits
• Enriched environments can enhance cognitive development
• Early interventions can potentially mitigate developmental disorders

Understanding these processes has important implications for education, child-rearing practices, and early intervention strategies for developmental disorders.

5. Synaptic plasticity: The cellular basis of learning and memory

LTP involves changes in both the pre- and postsynaptic components of the connection that is being strengthened.

Long-term potentiation (LTP). This process strengthens synaptic connections and is widely considered the cellular mechanism underlying learning and memory:

• Induced by high-frequency stimulation of neurons
• Involves NMDA receptor activation and calcium influx
• Results in increased synaptic strength and efficiency

Structural changes. Synaptic plasticity can lead to physical changes in neuronal structure:

• Formation of new dendritic spines
• Enlargement of existing spines
• Creation of new synaptic connections

Functional implications:

• Enhanced communication between neurons
• Formation and consolidation of memories
• Skill acquisition and habit formation

Understanding synaptic plasticity has profound implications for learning theories, educational strategies, and potential treatments for cognitive disorders.

6. Adult neurogenesis: New neurons in the mature brain

We now know, however, that the adult human brain also forms new cells throughout life.

Discovery and implications. The finding that new neurons are born in the adult brain challenged long-held beliefs and opened new avenues for research:

• Initially discovered in rodents and birds in the 1960s
• Confirmed in humans in the late 1990s
• Challenges the idea of a static adult brain

Neurogenic regions in humans:

• Hippocampus: Involved in memory and spatial navigation
• Striatum: Linked to motor control and motivation

Factors influencing adult neurogenesis:

• Physical exercise: Enhances neurogenesis
• Stress: Can inhibit new neuron formation
• Learning and enriched environments: Promote neurogenesis

The discovery of adult neurogenesis has implications for:

• Understanding brain plasticity and cognitive flexibility
• Developing potential treatments for neurodegenerative diseases
• Exploring approaches to enhance cognitive function in aging

7. Brain training: Harnessing plasticity for cognitive enhancement

Musical and athletic training enhance the execution of the complex sequences of movements needed, and trainees acquiring "the Knowledge" learn how to organize huge amounts of spatial information and then use it effectively.

Types of brain training. Various activities can induce structural and functional changes in the brain:

• Learning a new language: Increases gray matter in language-related areas
• Musical training: Enhances auditory and motor cortex connectivity
• Spatial navigation (e.g., London taxi drivers): Enlarges hippocampal regions

Evidence of brain changes:

• Increased gray matter volume in specific brain regions
• Enhanced white matter connectivity
• Functional reorganization of neural networks

Limitations and controversies:

• Commercial "brain training" games show limited transfer effects
• The extent and duration of training-induced changes vary
• Individual differences in plasticity may affect outcomes

While the potential for cognitive enhancement through targeted training is promising, more research is needed to develop effective, evidence-based interventions.

8. Injury-induced plasticity: Recovery after nerve damage and stroke

Unlike nerve injury–induced plasticity, which is rarely helpful, the cortical reorganization that occurs after a stroke is believed to contribute significantly to the recovery of motor function.

Nerve injury. Following peripheral nerve damage, the brain undergoes reorganization:

• Neighboring areas expand into deprived cortical regions
• Can lead to phantom limb sensations in amputees

Stroke recovery. The brain's plasticity plays a crucial role in functional recovery after stroke:

• Recruitment of alternative motor pathways
• Reorganization of sensory and motor cortices
• Formation of new synaptic connections

Therapeutic implications:

• Early intervention is crucial for optimal recovery
• Constraint-induced movement therapy can enhance motor recovery
• Non-invasive brain stimulation techniques (e.g., TMS, tDCS) show promise in facilitating recovery

Understanding injury-induced plasticity has led to the development of novel rehabilitation strategies and highlights the brain's remarkable capacity for self-repair and adaptation.

9. Maladaptive plasticity: The dark side of brain changes in addiction and pain

Addictive drugs activate and hijack the brain's reward system, and these changes remain in place long after the substance has been cleared from the brain, leading to cravings and to compulsive, drug-seeking behavior.

Addiction. Substance abuse induces long-lasting changes in the brain's reward system:

• Enhanced dopamine signaling in the mesolimbic pathway
• Alterations in synaptic strength and connectivity
• Persistent changes that contribute to craving and relapse

Chronic pain. Prolonged pain can lead to maladaptive plasticity in pain processing circuits:

• Sensitization of pain receptors
• Altered synaptic transmission in the spinal cord
• Reorganization of somatosensory cortex

Implications:

• Challenges in addiction treatment due to persistent brain changes
• Need for early intervention in pain management to prevent chronic pain development
• Potential targets for therapeutic interventions based on reversing maladaptive plasticity

Understanding these maladaptive changes is crucial for developing more effective treatments for addiction and chronic pain conditions.

10. Lifelong brain changes: From prenatal development to aging

Neuroplasticity is a lifelong process.

Prenatal and early life. The brain undergoes rapid and extensive changes:

• Massive neurogenesis and synapse formation
• Critical periods for sensory and cognitive development
• Early experiences shape lifelong brain function and behavior

Adolescence. Significant remodeling occurs in the teenage brain:

• Synaptic pruning and myelination in the prefrontal cortex
• Changes underlying improved cognitive control and decision-making

Adulthood and aging. The brain continues to change throughout life:

• Ongoing synaptic plasticity supports learning and memory
• Age-related declines in certain cognitive functions
• Potential for cognitive reserve and compensatory mechanisms

Implications across the lifespan:

• Importance of prenatal and early childhood environments
• Opportunities for intervention and cognitive enhancement throughout life
• Potential for maintaining cognitive health in aging through lifestyle factors

Understanding the dynamic nature of the brain throughout life has profound implications for education, healthcare, and strategies for maintaining cognitive health across the lifespan.

Last updated: September 19, 2024