Morphological Theories of Memory

Structural Reorganizations of Neurons

According to the theories under consideration, long-term memory is inextricably linked to changes in the molecular and cellular structures of nerve tissue. For information to be reliably fixed, the brain must physically change. This process affects all key parts of the neuron.

Changes at the Synaptic Level The synaptic apparatus responds to learning in two main ways:

• Absolutely new synaptic contacts are formed directly on the bodies of nerve cells.
• The physical size of existing synapses increases, making signal transmission more reliable.

Dendritic and Axonal Changes The receptive and transmitting parts of the neuron also undergo remodeling:

• Dendrites: they actively proliferate. In addition, the number of spines—special protrusions that serve as sites for the formation of new synapses—increases significantly on the dendritic tree of neurons.
• Axons: the number of axon 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 formations increases. New terminals form on neurons. As a result of all these reorganizations, a branched network is created that facilitates the spread of a much larger volume of information through various structures of the brain.

Experimental Evidence

Morphological changes during the formation of memory traces have been convincingly proven in a series of classic experiments on various animals.

Experiments on Opisthobranch Mollusks (Aplysia) Researchers evaluated how the number of sensory terminals in the active zones of Aplysia neurons changes under different environmental conditions:

• Norm: in the normal state, active zones contain about 40% sensory terminals.
• Habituation: if the mollusk is exposed to monotonous stimuli for a long time, it stops responding to them. Morphologically, this is manifested by a reduction in the number of terminals to 10%.
• Sensitization: when sensitivity to stimuli increases, the reverse process is observed—the number of terminals sharply increases to 65%.

Experiments on Rodents Experiments on rats and mice proved the influence of the external environment on brain anatomy:

1. Enriched Environment: rats that were trained in complex visual tasks and lived in cages with a large variety of stimuli (toys, mazes) had a significantly thicker cerebral cortex compared to animals raised in an impoverished (empty) environment.
2. Light Exposure: mice that were kept in complete darkness for a long time and then sharply exposed to light demonstrated a structural response—they were found to have much more branched dendrites in the pyramidal cells of the cortex.

Hebb's Hypothesis

The foundation of modern concepts regarding the molecular and structural mechanisms of memory was the concept proposed by Canadian scientist Donald Hebb.

The essence of his hypothesis is that the long-term fixation of memory traces is ensured not only by the growth and formation of new interneuronal contacts. Persistent changes in synaptic conductivity within already existing pools of synapses play a critically important role. In other words, when neurons are activated simultaneously, the connection between them is physically strengthened, facilitating impulse transmission in the future.

Class and exam questions

What else to study on this topic

• Molecular mechanisms of synaptic conductivity changes (role of receptors)
• Differences in structural changes between short-term and long-term memory
• Processes of memory trace consolidation at the cellular level
• The role of glial cells in morphological reorganizations of the CNS

Other topics in the "Memory" section

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