The sensorimotor system, part I: the physiologic basis of functional joint stability

Abstract

Objective: To define the nomenclature and physiologic mechanisms responsible for functional joint stability.

Data sources: Information was drawn from an extensive MEDLINE search of the scientific literature conducted in the areas of proprioception, neuromuscular control, and mechanisms of functional joint stability for the years 1970 through 1999. An emphasis was placed on defining pertinent nomenclature based on the original references.

Data synthesis: Afferent proprioceptive input is conveyed to all levels of the central nervous system. They serve fundamental roles in optimal motor control and sensorimotor control over functional joint stability.

Conclusions/applications: Sensorimotor control over the dynamic restraints is a complex process that involves components traditionally associated with motor control. Recognizing and understanding the complexities involved will facilitate the continued development and institution of management strategies based on scientific rationales.

Figure 1

The sensorimotor system incorporates all the afferent, efferent, and central integration and processing components involved in maintaining functional joint stability. Although visual and vestibular input contributes, the peripheral mechanoreceptors are the most important from a clinical orthopaedic perspective. The peripheral mechanoreceptors (pictured on the lower left) reside in the cutaneous, muscular, joint, and ligamentous tissues. Afferent pathways (dotted lines) convey input to the 3 levels of motor control and associated areas such as the cerebellum. Activation of motor neurons may occur in direct response to peripheral sensory input (reflexes) or from descending motor commands, both of which may be modulated or regulated by the associate areas (gray lines). Efferent pathways from each of the motor control levels (solid lines) converge upon the alpha and gamma motor neurons located in the ventral aspects of the spinal cord. The contractions by the extrafusal and intrafusal muscle fibers cause new stimuli to be presented to the peripheral mechanoreceptors.

Figure 2

Sensations arising from somatosensory sources.

Figure 3

The final common input hypothesis. Peripheral receptors from cutaneous, muscle (Golgi tendon organs and muscle spindle afferents), and articular tissues, as well as descending commands from supraspinal areas, converge onto the static and dynamic γ motor neurons. Collectively, all of these influences alter the sensitivity of muscle spindles; thus, the final afferent signals arising from the muscle spindles can be considered a function of both the preceding influential activity and muscle length.

Figure 4

The role of the articular mechanoreceptors in sensorimotor control over dynamic joint stability and conscious appreciation of proprioception. Dotted lines represent roles that are still controversial.