Essence and mechanisms of "neural traps"
The physiological and anatomical basis of short-term memory consists of specific loop chains of nerve cells that the researcher Lorente de Nó called "neural traps".
The architectonics of these chains are built upon reciprocal connections of axons. The mechanism of operation is as follows:
- Excitation originates in a single neuron and is transmitted to neighboring cells within the chain.
- Through a system of axonal collaterals (side branches), the nerve impulse returns to the original cell.
- This creates a prolonged circulation (reverberation) of the signal through a closed cyclic formation.
To maintain this continuous running of the impulse, the constant generation of excitatory postsynaptic potentials (EPSPs) in the synapses of the chain is required. The mechanism of reverberation is not limited to local areas; it is capable of engaging quite extensive regions of the brain.
How is reverberation terminated? Circulation cannot last forever. Physiological cessation of the process occurs when an inhibitory neuron comes into play. Under its influence, an inhibitory postsynaptic potential (IPSP) develops on one of the cells of the closed chain, which instantly blocks further signal transmission. Furthermore, crude external interventions, such as electroshock, can break the reverberating circuit. This explains the phenomenon of short-term memory loss following such interventions.
Cortico-subcortical connections and circulation pathways
Large-scale reverberation loops are built on the basis of a branched system of collaterals originating from pyramidal neurons of the cerebral cortex. These connections can be divided into two major blocks.
Intracortical interactions They provide local information processing. Collaterals of pyramidal cells are directed toward associative neurons in the prefrontal and sensorimotor areas of the cortex. Concurrently, recurrent collaterals connect neurons with other associative and projection areas of the cortex.
Connections with brainstem structures For more complex processes, excitation extends beyond the cortex.
- Side branches of the pyramidal tract descend to subcortical nuclei — the thalamus and the reticular formation of the brainstem.
- For the circuit to close, axons from these brainstem structures ascend back into the cerebral cortex.
- An important anatomical detail: the returning fibers terminate strictly on neurons of the 4th layer of the cortex, from where the signal is transmitted again to pyramidal cells, initiating a new cycle of reverberation.
Emotional memory and the Papez circuit
The classic and most studied example of cortico-subcortical reverberation is the system responsible for the formation of emotional memory. This system is known as the "great limbic ring" or the Papez circuit (named after the neuromorphologist who described it).
Excitation circulation in the Papez circuit follows a strict sequence and passes through key structures of the limbic system:
- The impulse originates in the hippocampus.
- Via axons, the signal is transmitted to the mamillary bodies of the hypothalamus.
- From there, the excitation proceeds to the anterior nuclei of the thalamus.
- The next station is the cingulate gyrus.
- From the cingulate gyrus, axons return back to the hippocampus, completely closing the loop.
It is the repeated passage of nerve impulses along this anatomical route that ensures the fixation of emotionally significant information and the formation of short-term emotional memory.
Briefly: answers to topical questions
Are there other loop circuits in the limbic system besides the Papez circuit?
Yes, in addition to the Papez circuit, other closed pathways of excitation circulation exist within the limbic system.
The following additional limbic circuits are distinguished:
- Septo-amygdaloid pathway — circulation follows the route: septum → diagonal band → amygdala → stria terminalis → septum.
- Septo-hippocampal pathway — excitation follows the pattern: septum → supracallosal striae → hippocampus → fornix → septum.
To memorize the sequence of structures in the Papez circuit, use the mnemonic: "Hungry Mammoths Trot Across the Meadow to the Mountains" (Hippocampus → Mamillary bodies → Thalamus (anterior nuclei) → Cingulate gyrus → return to Hippocampus).
Class and exam questions
What is the concept of "neural traps"?
These are closed loop circuits of interconnected neurons described by Lorente de Nó. Through their collaterals, excitation returns to the original cell, ensuring prolonged impulse circulation.
Why is short-term memory impaired during electroshock?
External interventions, such as electroshock, abruptly disrupt reverberating neural circuits. Since impulse circulation is the substrate of short-term memory, its interruption leads to the loss of recent memories.
At which cortical layer do subcortical reverberation loops close?
Axons of thalamic and reticular formation neurons, returning to the cerebral cortex, terminate on neurons of layer 4, after which they again act upon pyramidal cells.
What physiological phenomenon terminates reverberation?
The process stops when an inhibitory postsynaptic potential (IPSP) arises on one of the chain's neurons under the influence of an inhibitory nerve cell.
What else to study on this topic
- Mechanisms of EPSP and IPSP formation in neural networks
- Architectonics and layers of the cerebral cortex
- Functions of the brainstem reticular formation
- Physiology of the limbic system and hippocampus
- The pyramidal tract and its collaterals
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