Molecular mechanisms of memory retrieval
Memory recall represents an active biological cascade. It is based on dominant motivation, which arises when a need appears. This motivation accesses the neuronal genome in a feed-forward manner.
Specific genes are expressed, leading to the formation of information molecules (effector proteins). It is through these molecules that the previously formed engram (memory trace) is "revived". If protein synthesis blockers or specific anti-immunoglobulins are administered during an experiment, experience retrieval becomes impossible. As a result, afferent synthesis is disrupted and the architectonics of the behavioral act are completely destroyed.
The role of dominant motivation
Without motivation, memory remains inaccessible, as convincingly proven by A.S. Sosnovsky's experiments on rabbits. The animals were trained in a skill: when a light turned on in a specific setting, they had to pull a ring to receive a carrot.
When the rabbits were fully fed (food motivation was absent), they ignored all situational and triggering stimuli. However, as soon as the "hunger center" in the lateral hypothalamus was artificially stimulated via electrodes, their behavior changed drastically. The fed rabbits started pulling the ring again upon seeing the signals. Thus, it is motivation within a specific setting that pulls the entire chain of learned behavior out of memory.
Levels of the human recall process
In humans, the retrieval of experience can occur at two different levels:
- Conscious recall. The subject purposefully recognizes external impacts, relying on past experience. The process is built on current motivation and instantaneous recognition of the object.
- Subconscious memory. It engages automatically during the execution of stereotyped behavioral acts. It accompanies learned actions that have turned into dynamic stereotypes (e.g., driving a car, playing music, reciting poetry by heart, or athletic training).
Memory retrieval is associative in nature and is triggered by sensory stimuli. Past scenes provoked by olfactory stimuli are distinguished by particular vividness.
Emotional memory and structural foundations
Emotional memory stands out because the emotional state itself becomes the triggering trigger (cause) for information retrieval. It is supramodal (not tied to a single sensory system) and forms extremely rapidly, often on the first attempt.
Crucial data on the structural localization of memory were obtained by neurosurgeon W. Penfield while stimulating patients' brains during surgeries. It was revealed that the medial regions of the temporal lobes of the cortex are responsible for retrieval. Their electrical stimulation evoked detailed, "vivid" scenes of the distant past, reproduced in an adequate temporal sequence. Furthermore, stimulation of these areas can elicit the déjà vu phenomenon, where new events are evaluated by the subject as already experienced.
Briefly: answers to topical questions
Which specific neurotransmitters (e.g., acetylcholine or glutamate) ensure synaptic transmission during engram retrieval?
The provided sources do not contain a complete list of neurotransmitters that specifically "ensure synaptic transmission during engram retrieval." The cholinergic and noradrenergic systems are directly associated with the reproduction/retrieval of memory traces.
- Acetylcholine — activation of cholinergic synapses causes conformational changes in postsynaptic membranes and an increase in synaptic conductivity. Acetylcholine antagonists impair learning and recall, and cause amnesia.
- Norepinephrine — pharmacological reduction of norepinephrine levels slows down learning, causes amnesia, and impairs memory trace retrieval.
For dopamine, a general role in memory mechanisms is indicated in the sources, while for serotonin, involvement in memory consolidation is noted. The glutamatergic system is indicated as participating in the regulation of the cortex, limbic system, and subcortical ganglia, but the direct involvement of glutamate specifically in engram retrieval is not described in the sources.
Which closed neural circuits (e.g., the Papez circuit) are involved in the mechanisms of memory recall?
Cortico-subcortical ring neural circuits ("neural traps") and the great limbic Papez circuit are involved in memory mechanisms.
- Ring neural circuits — interconnected by axons, they ensure prolonged circulation (reverberation) of excitation. They serve as the neural substrate for short-term memory.
- The great limbic Papez circuit — a closed pathway of excitation circulation, the integrity of which is necessary for the full functioning of declarative memory.
The Papez circuit includes:
- Parahippocampal gyrus;
- Hippocampus;
- Fornix (fornix);
- Septum;
- Mamillary bodies (corpora mamillaria);
- Anterior thalamus;
- Entorhinal cortex.
Damage to the structures of the Papez circuit leads to impairments in transferring information into long-term memory.
To memorize the molecular memory chain, use the abbreviation MGEN: Motivation $\rightarrow$ Genome (activation) $\rightarrow$ Effector molecules $\rightarrow$ Network/Engram (trace revival).
Class and exam questions
Why does a satiated animal fail to perform a learned feeding skill?
Dominant motivation is absolutely essential for retrieving a memory trace from systemic processes. In a well-fed animal, it is absent, so even in the presence of all triggering signals, the behavioral act is not initiated.
Which sensory stimuli evoke the most vivid associations?
According to physiological observations, memories evoked by olfactory stimuli are distinguished by the greatest vividness and clarity.
What happens upon stimulation of the medial regions of the temporal lobes?
Stimulation of this cortical area leads to the reproduction of "vivid scenes" from the distant past in a strict chronological sequence, and can also induce the illusion of having seen it before—the déjà vu phenomenon.
What else to study on this topic
- Mechanisms of memory engram formation and synthesis of specific proteins
- The role of the lateral hypothalamus in the formation of food motivation
- Dynamic stereotype: mechanisms of formation and extinction
- The effect of amyloid protein on neurons in Alzheimer's disease
- Afferent synthesis as a stage of the behavioral act
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