Concept of conditioned and unconditioned reflexes
Reflexes are traditionally divided into unconditioned and conditioned. An unconditioned reflex is a constant, species-specific, genetically determined reaction of the organism to an adequate stimulus. A conditioned reflex is an individual reaction of the organism developed during ontogenesis to a conditioned stimulus that was previously indifferent (biologically neutral) to this reaction.
The fundamental differences between the two types of reflexes concern their origin, constancy, the nature of the stimulus, and the level at which the reflex arc is closed.
Comparative characteristics of unconditioned and conditioned reflexes
- Origin: unconditioned reflexes are innate and inherited; conditioned reflexes are acquired during lifetime.
- Prevalence: unconditioned reflexes are species-specific and present in every individual; conditioned reflexes are individual and formed in a specific organism.
- Constancy: unconditioned reflexes persist throughout life; conditioned reflexes are inconstant and extinguish without reinforcement.
- Stimulus: an unconditioned reflex occurs in response to an adequate stimulus of a specific receptive zone; a conditioned reflex occurs in response to various stimuli applied to different receptive zones.
- Level of closure: unconditioned reflexes are closed at the level of the brainstem or spinal cord; conditioned reflexes are closed at the level of the cerebral cortex (according to some theories, with the participation of subcortical structures).
- Reflex arc: in an unconditioned reflex, it is phylogenetically fixed; in a conditioned reflex, a temporary connection is formed between brain structures.
Mechanism of formation of the temporary connection
A classic example is the generation of a salivary conditioned reflex to sound. The unconditioned stimulus is food entering the oral cavity and exciting taste receptors; information goes to the food center of the cortex. Sound is perceived by the organ of Corti, and the impulse travels to the auditory cortex.
With repeated pairing of sound and feeding, both cortical centers are excited simultaneously. The food center, being biologically more significant, becomes dominant and attracts excitation from the auditory center. Reciprocal irradiation of excitation occurs—it circulates between the centers, forming a "pathway facilitation" (a stable neural connection).
As a result, when only the sound is presented, excitation is transmitted via the temporary connection to the cortical representation of the unconditioned salivary reflex, then to the salivary center in the medulla oblongata, and subsequently to the salivary glands.
Cellular basis of the temporary connection
- Neuronal level: two groups of excited neurons are formed in the cortical centers, between which excitation circulates. Glial cells enhance fiber myelination, accelerating impulse conduction.
- Synaptic level: the number of active synapses increases, and previously "silent" synapses transition to an active state, which increases the efficiency of synaptic transmission.
The cortical representation of an unconditioned reflex is a group of cortical neurons connected to a subcortical center (e.g., the salivary center in the medulla oblongata) and capable of modulating its activity. It is with this representation that the temporary connection is formed.
Conditions for generating a conditioned reflex
- Presence of an unconditioned basis. A conditioned reflex is generated only on the basis of a pre-existing unconditioned reflex.
- Principle of reinforcement. Repeated pairing of the conditioned and unconditioned stimuli is necessary; without reinforcement, the connection extinguishes.
- Time condition. The conditioned signal must slightly precede the unconditioned one—this ensures maximum generation efficiency.
- Sensory deprivation. Extraneous stimuli hinder reflex formation; isolation from distracting stimuli is necessary.
- Normal CNS activity. Depression or pathological states of the central nervous system make the formation of a conditioned connection difficult.
Stages of conditioned reflex formation
Generalization stage. The reflex response occurs not only to the main reinforced stimulus but also to similar stimuli. For example, when developing a reflex to a high-frequency sound, salivation is also observed in response to low-frequency sounds.
Specialization stage. A narrower differentiation of stimuli occurs: the reflex reaction occurs only in response to the specific reinforced stimulus, while the reaction to similar stimuli extinguishes.
Key points
- A conditioned reflex is an individual acquired reaction formed in ontogenesis to a previously indifferent stimulus.
- Unconditioned reflexes are closed in the brainstem or spinal cord, while conditioned reflexes are closed at the level of the cerebral cortex.
- The basis of a conditioned reflex is a temporary connection between the cortical centers of the conditioned and unconditioned stimuli.
- The mechanisms of connection formation are dominance and reciprocal irradiation of excitation.
- At the cellular level, generation is accompanied by enhanced myelination and activation of previously silent synapses.
- Conditions for generation: unconditioned basis, reinforcement, precedence of the conditioned signal, sensory deprivation, normal CNS activity.
- A conditioned reflex goes through the stages of generalization and specialization.
- Without reinforcement, the temporary connection weakens, and the conditioned reflex extinguishes.