Structural Neuronal Reorganization
According to these theories, long-term memory is inextricably linked to changes in the molecular and cellular structures of neural tissue. For information to be reliably consolidated, the brain must physically change. This process involves all key regions of the neuron.
Synaptic-Level Changes The synaptic apparatus responds to learning via two main pathways:
- Absolutely new synaptic contacts form directly on the neuronal cell bodies.
- The physical size of existing synapses increases, making signal transmission more reliable.
Dendritic and Axonal Modifications The receiving and transmitting parts of the neuron also undergo remodeling:
- Dendrites: They actively branch out. Furthermore, the number of dendritic spines—specialized protrusions that serve as sites for new synapse formation—significantly increases along the dendritic tree.
- Axons: The number of axonal collaterals (side branches) increases, allowing a single neuron to connect with a larger number of target cells.
Molecular Level Inside the cells themselves, the number of microtubules and other molecular structures increases. New terminals form on the neurons. As a result of all these rearrangements, an expanded network is created that facilitates the distribution of a much larger volume of information across various brain structures.
Experimental Evidence
Morphological changes during memory trace formation have been convincingly demonstrated in a series of classic experiments across various animals.
Experiments on Sea Slugs (Aplysia) Researchers evaluated how the number of sensory terminals in active zones of Aplysia neurons changes under different environmental conditions:
- Baseline: Under normal conditions, active zones contain about 40% of sensory terminals.
- Habituation: If the mollusk is subjected to prolonged monotonous stimuli, it stops responding. Morphologically, this manifests as a reduction in terminal count to 10%.
- Sensitization: When sensitivity to stimuli increases, the reverse process occurs—the number of terminals sharply rises to 65%.
Rodent Experiments Experiments on rats and mice have proven the influence of the external environment on brain anatomy:
- Enriched Environment: Rats that learned complex visual tasks and lived in cages with a variety of stimuli (toys, mazes) developed a significantly thicker cerebral cortex compared to animals raised in an impoverished (empty) environment.
- Visual Deprivation and Exposure: Mice kept in total darkness for an extended period and then abruptly exposed to light demonstrated a structural response, displaying significantly more branched dendrites in cortical pyramidal cells.
Hebb's Hypothesis
The foundation of modern concepts regarding the molecular and structural mechanisms of memory is the framework proposed by Canadian psychologist Donald Hebb.
The core of his hypothesis is that the long-term fixation of memory traces is ensured not only by the growth and formation of new interneuronal connections. Critically important roles are played by persistent changes in synaptic efficacy within already existing pools of synapses. In other words, when neurons are activated simultaneously, the connection between them is physically strengthened, facilitating future impulse transmission.