What are the stages of a sensorimotor cycle?

Taking a simple step involves a nervous ballet that loops several hundred times per second. This sensorimotor cycle can be broken down into six main stages.

Collection of Afferents

Four channels inform the nervous system about the body's position: vision, vestibular, proprioception, and plantar touch. Each sensor converts energy (light, pressure, stretching...) into nerve impulses.

Transmission to the Central Nervous System

Afferent fibers ascend to the spinal cord and brainstem, while efferent fibers will later descend to the muscles; this afferent/efferent distinction forms the basis of every neural circuit (Purves 2019).

Integration and Prediction

At the level of the brainstem and then the cerebellum, multisensory signals are compared to the "expected command." The cerebellum acts as a comparator: if the error exceeds a threshold, it generates a correction in < 100 ms.

Postural Pre-programming

Even before the movement begins, the system triggers anticipatory postural adjustments (APA) to subtly shift the center of mass and prevent falling. Their timing (≈ 150 ms before the action) demonstrates the pre-conscious aspect of the cycle (Shumway-Cook & Woollacott 2017).

Descending Motor Command

The vestibulo-, reticulo-, and corticospinal pathways then orchestrate muscle contraction. The reticulospinal pathways, in particular, modulate extensor tone from the very first milliseconds (Richard & Orsal 2007).

Continuous Feedback

The executed movement immediately generates new afferents; these enter the next cycle, allowing for constant updates. This looping thus transforms action into a series of successive predictions and corrections, ensuring both balance and precise movements.

References

1. Purves, D. et al. (2019). Neuroscience (6th ed.). Oxford University Press.
2. Richard, P., & Orsal, D. (2007). Organization of Reticulospinal Pathways and Postural Tone.
3. Shumway-Cook, A., & Woollacott, M. (2017). Motor Control: Translating Research into Clinical Practice (5th ed.).