Essence and Mechanism of the Acceptor
The multilevel organization of motor control is impossible without the action result acceptor apparatus. The unique feature of this mechanism lies in its timing of formation: it arises in the central nervous system earlier than the physical action itself is performed and any actual result appears.
The main function of the ARA is continuous comparison. The central nervous system evaluates and compares the actual result obtained by the organism with the originally required (programmed) result. This mechanism is implemented because afferent impulses from various receptors directly involved in the executed behavioral act continuously flow into the brain. It is precisely this sensory feedback that allows motor commands to be corrected in a timely manner.
Significance of Feedback Afferentation
For a motor act to be successful, feedback afferentation—the stream of signals from 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 most. 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 is subject to the principle of systemic quantization—meaning 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
Precision shooting serves as a classic example of systemic quantization. The dynamics of this process are frequently studied using stabilography—a method for assessing the fluctuations of the projection of the human body's center of mass in the sagittal and frontal planes.
The goal-directed act of shooting represents a behavioral quantum divided into three successive stages:
- Stabilization of body and leg position. Takes up about 2/3 of the entire aiming time. Biomechanics are built such that one leg serves as a rigid support while the other acts as the "steering" leg. The result of this stage is adopting a posture convenient for subsequent actions.
- Stabilization of weapon position. Carried out through the work of muscles in both arms. In experienced shooters, this phase lasts 2–4 seconds. The outcome is a comfortable and reliable hold on the weapon.
- Trigger pull. Masters perform this action smoothly and rapidly. Simultaneously, the physiological background changes: the shooter holds their breath, and heart rate decreases slightly. The result is the ideal finger placement ensuring an accurate shot.