Essence and mechanism of the acceptor
The multilevel organization of motor control is impossible without the action result acceptor apparatus. A unique feature of this mechanism is the timing of its formation: it arises in the central nervous system earlier than the physical action itself takes place and any actual result appears.
The main function of the ARA is constant comparison. The central nervous system evaluates and compares the actual result obtained by the organism with the initially required (programmed) one. This mechanism is implemented due to the continuous influx of afferent impulses into the brain from various receptors directly involved in the executed behavioral act. It is sensory feedback that allows motor commands to be corrected in a timely manner.
Significance of afferent feedback
For a motor act to be successful, the presence of afferent feedback—a stream of signals from the receptors of the musculoskeletal system—is critically important. If this stream is interrupted, movement control is grossly impaired.
Practice shows that damage to afferent pathways affects different muscle groups unevenly:
- Distal segments: Fine and maximally precise movements (e.g., hand movements) suffer the greatest impairment. With complete deafferentation of a limb, the ability to control the hand is completely lost.
- Proximal segments: Larger segments, such as the shoulder or hip, retain the general direction of movement even when sensation is lost.
Systemic quantization of complex actions
Complex movements are not an absolutely continuous stream of activity. By their very nature, they are discrete reactions (unlike basic tonic reactions or simple reflex arcs).
Any complex behavior obeys the principle of systemic quantization—that is, the continuous process is broken down into separate, logically completed stages. The transition between these stages is achieved through the sequential dispatch of new motor commands from the brain and spinal cord directly to the muscles. The discrete nature of motor activity is particularly evident when performing tasks that require high precision.
Behavioral quantum using shooting as an example
A classic example of systemic quantization is target shooting. The dynamics of this process are frequently studied using stabilography, a method for assessing the oscillations of the projection of the human body's center of gravity in the sagittal and frontal planes.
The goal-directed act of shooting represents a behavioral quantum, which is divided into three consecutive stages:
- Stabilization of body and leg position. Takes about 2/3 of the entire aiming time. Biomechanics is based on the fact that one leg serves as a rigid support, while the other acts as the "steering" leg. The result of this stage is the adoption of a posture convenient for subsequent actions.
- Stabilization of weapon position. Implemented through the work of the muscles in both arms. In experienced shooters, this phase lasts 2–4 seconds. The result is a comfortable and secure hold of the weapon.
- Trigger pull. Masters perform this action smoothly and quickly. At the same time, the physiological background changes: the shooter holds their breath, and heart rate decreases slightly. The result is an ideal finger position ensuring an accurate shot.
Briefly: answers to topical questions
Who is the author of the concept of the action result acceptor and the theory of functional systems?
The author of the concept of the action result acceptor and the theory of functional systems is P. K. Anokhin. This theory is used to describe the architecture of a behavioral act and a functional system; the diagram illustrates the cybernetic principle of self-regulation of physiological processes and behavior.
The action result acceptor is a key mechanism of goal-directed activity. Its apparatus pre-programs the properties of the final result, against which the actually obtained result is compared using afferent feedback. The action result acceptor forms in the CNS before the action is executed and the result appears.
Imagine the ARA as a quality control department in a factory: the part's "blueprint" (action program) is ready in advance, and as soon as the part (movement) is made, the department immediately checks the actual result against the blueprint using information from its "sensors" (receptors).
Class and exam questions
At what point in the nervous system does the action result acceptor form?
It forms in a feedforward manner—significantly earlier than the physical movement itself is performed and its result appears.
What happens upon the loss of afferent feedback (deafferentation) of a limb?
Movement control will be substantially impaired. Fine and precise movements of the hand will be completely lost, but the general direction of movements in large proximal joints (shoulder, hip) will be preserved.
What is systemic quantization of behavior?
This is the division of a single continuous behavioral process into separate, discrete stages (quanta). The transition between them occurs due to a change in motor commands from the central nervous system.
What does the stabilography method assess?
Stabilography studies the dynamics of oscillations of the projection of the human body's center of gravity in the frontal and sagittal planes (for example, during the posture stabilization stage).
What else to study on this topic
- Morphological substrate and levels of motor control organization
- The role of the brain and spinal cord in generating sequential motor commands
- Physiological mechanisms of stabilography
- Autonomic support of the motor act (changes in heart rate and respiration)
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