Evolution of approaches: from the reflex arc to the functional system
For a long time, physiology was dominated by the principle of the classical reflex arc. However, the cybernetic approach required a revision of established dogmas. The theory of functional systems describes step-by-step how open-loop mechanisms evolved into feedback-controlled tracking systems, ultimately forming the full architecture of a behavioral act.
Open-loop regulation system (linear principle)
This is a baseline mode of nervous system operation, representing a classical reflex arc without feedback elements.
Mechanism of action:
- A stimulus acts on a receptor located in the working structure (e.g., in a muscle).
- The resulting afferent signal is directed to a specialized nerve center.
- The nerve center generates an efferent command, which is transmitted to the effector mechanism, eliciting a response.
Main disadvantage: Such a system operates completely "blindly". The control center receives no information about whether a useful result has been achieved. If an unforeseen external disturbance interferes with the process, the system will be incapable of correcting its action.
System with feedback (mismatch principle)
To compensate for the shortcomings of linear regulation, a control loop is introduced, turning the structure into a so-called "servo system." This loop operates on two critically important parameters: Set Point (target parameter, goal) and Actual Value (current state).
Algorithm of the tracking system:
- The receptor apparatus registers the actual state of the target organ.
- The gathered information is transmitted to the Comparison Element.
- A comparison is made between the specified Required parameter and the Actual one.
- In case of a discrepancy (when Required does not equal Actual), an "error signal" (mismatch signal) is generated.
- This signal is sent to the control unit for immediate correction of activity.
This principle is vital for maintaining homeostatic constants and ensuring high precision of motor acts, as it smooths out any external and internal disturbances.
Complete architecture of a behavioral act
The most detailed model according to P.K. Anokhin unites all regulatory mechanisms into a single functional system.
Main stages of the deployment of a behavioral act:
- Triggering afferentation: the action of an external stimulus acting as a trigger to launch the behavioral program.
- Formation of the action result acceptor (ARA): even before the physical execution of the task, an ideal neural model of the future result is created in the CNS. This is a manifestation of anticipatory reflection of reality, containing the parameters of that very "Required" state.
- Action: the controlling nerve center sends a command to the effector organ (e.g., causes muscle contraction).
- Feedback afferentation: receptors collect data on the actually achieved result and send this stream (feedback information about the result) back to the CNS.
- Stage of comparison: in the ARA unit, the initial prediction is compared with the incoming reality.
Possible outcomes of the comparison stage:
- Parameters match: the useful result has been achieved. The action is immediately terminated, which at the level of higher nervous activity is accompanied by the emergence of a positive emotion.
- Parameters do not match (mismatch): an orienting-exploratory reaction is triggered. The central nervous system makes adjustments to the initial action program, and the entire cycle is repeated anew until the set goal is successfully achieved.
Briefly: answers to topical questions
What components form the stage of afferent synthesis in a functional system?
The stage of afferent synthesis is formed by four main components that constitute the basis of goal-directed behavior.
- Motivation (motivational excitation) — forms the foundation of behavior oriented toward achieving a useful adaptive result.
- Situational afferentation — the totality of environmental influences that creates the conditions for task execution and activates memory.
- Memory — retrieves information associated with the satisfaction of the dominant need (includes genetic and individually acquired memory).
- Triggering afferentation — an external stimulus that unlocks the established integration of the first three components and launches the behavioral act.
How does triggering afferentation differ from situational afferentation?
Triggering afferentation directly unfolds (launches) the behavioral act, whereas situational afferentation participates in the formation of the stage of afferent synthesis as a set of external factors of the specific situation.
| Characteristic | Triggering afferentation | Situational afferentation |
|---|---|---|
| Function | Directly unfolds (launches) the behavioral act | Participates in the formation of afferent synthesis; along with dominant motivational excitation, it activates memory apparatuses, creates pre-trigger integration, and enlivens goal-directed behavioral programs appropriate to the situation |
| Sources/stimuli | Numerous conditioned stimuli, time | The entire totality of external factors of a specific situation; in humans, social factors as well |
| Mechanism in the CNS | Unlocks the pre-trigger integration established in the CNS prior to its action and determines dominance in a particular case | Together with dominant motivational excitations, it activates memory and participates in pre-trigger integration |
What types of useful adaptive results does the theory of functional systems distinguish?
According to the cited sources, the useful adaptive result is considered by the level of the result in homeostatic systems and by the informational content of the functional system.
Hierarchy of results in homeostatic systems:
- Pre-terminal result — for example, in the respiratory system, this is the level of pO₂, pCO₂, and pH in the blood; it has a protective, precautionary significance.
- Final result — stable metabolism in tissues.
Levels of informational content of the functional system:
- Physicochemical level — if the result is characterized by physicochemical properties associated with homeostasis and metabolism.
- Sign (speech) level — characteristic of behavioral activity, especially in humans; if the result has speech or sign parameters, the informational content of the system reflects these properties. The verbal result is associated with emotional or semantic meaning, and words acquire the significance of conditioned stimuli.
Class and exam questions
What is the fundamental weakness of an open-loop reflex arc?
Its main disadvantage is operating "blindly" due to the lack of feedback. The system cannot know whether the goal has been achieved and is unable to compensate for external interference hindering task execution.
What is the function of the action result acceptor?
It serves as a leading model of the future result, formed in the CNS before the action itself begins. It contains the parameters of the ideal goal with which the actual outcome will later be compared.
What happens when there is a mismatch between prediction and reality?
An error signal is generated in the Comparison Element, which activates an orienting-exploratory reaction, forces the CNS to make adjustments to the action program, and prompts a retry.
What else to study on this topic
- Mechanisms of triggering afferentation
- The phenomenon of anticipatory reflection of reality
- The role of positive and negative emotions in the stage of comparison
- Components of the orienting-exploratory reaction during mismatch
- Cybernetic approach to maintaining homeostasis parameters
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