Mechanisms of temporary connection formation

Classical theory of I.P. Pavlov and the dominance principle

According to I.P. Pavlov, the formation of a conditioned reflex was primarily associated with the activity of the cerebral cortex. The essence of the process was viewed as the interaction of two excitation arcs: the conditioned reflex arc and the unconditioned reflex arc.

The foundation of the closure mechanism is formed by dominant relations in the cortex:

1. Unconditioned reinforcement creates a strong focus of excitation in the cortex, which becomes dominant.
2. The indifferent (conditioned) stimulus creates a weaker focus of excitation.
3. According to its properties, the dominant center "attracts" the excitation caused by the conditioned signal.
4. A functional pathway—a temporary connection—arises between the representations of the conditioned signal and the unconditioned reinforcement.

The connection is called "temporary" because it lacks rigid genetic determination and quickly breaks down (extinguishes) if the conditioned signal is no longer accompanied by reinforcement.

Dynamics of connection formation (using the food reflex as an example)

The process can be divided into three successive stages illustrating the interaction of cortical centers:

• Stage A (Action of the indifferent stimulus): For example, a flash of light irritates the retina. The signal travels via subcortical centers to the visual cortex, forming a focus of excitation there. There is as yet no connection with the feeding center, so only an orienting reflex ("What is it?") occurs.
• Stage B (Action of the unconditioned stimulus): Food intake stimulates taste receptors. The signal travels through the brainstem (medulla oblongata) to the salivary glands and also irradiates into the cortex, creating a powerful cortical focus of feeding excitation. The unconditioned reflex is executed.
• Stage C (Closure): With repeated pairing (light preceding food), two foci simultaneously exist in the cortex. The strong feeding focus "attracts" impulses from the visual one. A functional bridge is formed. Now excitation from the visual cortex directly passes along a beaten path to the feeding center, causing salivation in response to light.

Conditioned reflex in light of modern neurophysiology

Modern data have significantly expanded classical views, proving that the entire brain participates in the closure of a temporary connection, rather than just isolated cortical points.

The most important addition was the discovery of the properties of the brainstem reticular formation. It exerts generalized ascending activating influences on the cerebral cortex. Cortic-subcortical interactions ensure the reverberation (circulation) of excitations. Moreover, it has been experimentally proven that primitive conditioned reflexes can be elicited even in decerebrate animals. This finally disproved the theory about the exclusive role of the cortex in closing absolutely all temporary connections.

P.K. Anokhin's convergence theory

These modern concepts formed the basis of convergence theory, which explains the closure mechanism at the cellular level. The formation of the connection occurs when various excitations converge on the same neurons.

The process includes the following stages:

1. Motivation (initial background): A focus of motivation (hunger) arises in subcortical structures (e.g., the hypothalamus). It exerts an ascending influence on the cortex, increasing the excitability of certain neuronal populations (creating a pre-setting).
2. Arrival of signals: Excitations from the conditioned signal (light) and unconditioned reinforcement (food) successively arrive at the cortex.
3. Convergence (merging): Three streams of excitation meet on the membranes of pre-set neurons: motivational, sensory (conditioned), and reinforcing.

Molecular mechanisms of memory consolidation

The convergence of all types of excitation on a cortical neuron is only the first step. For the temporary connection to become strong, changes at the cellular and molecular level are necessary.

The convergence of stimuli triggers a cascade of intracellular biochemical reactions involving secondary messengers. The signal is transmitted to the cell nucleus, targeting its genetic apparatus:

• Genome activation (early gene expression) occurs.
• Transcription (RNA synthesis on a DNA template) and translation processes are initiated.
• As a result, specific memory proteins are synthesized de novo.

The synthesized proteins are embedded into synaptic membranes or alter the structure of the neuron. This exact process ensures long-term plasticity and the consolidation of the memory trace (engram), serving as the intimate structural mechanism for the formation of a temporary connection.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Role and functions of the brainstem reticular formation
• Cortico-subcortical interactions and reverberation mechanisms
• The orienting reflex and its differences from the conditioned reflex
• Mechanisms of dominant motivation formation in the hypothalamus
• Molecular cascades and secondary messengers in neurons

Other topics in the section "General Principles of Behavioral Organization"

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