Anatomy and Physiology of Proprioception and ~eu~muscular Control

M, LEBs%AR"%P9PbD, AmC. BUZ SWRltBq PhDT aqand TA#BMT BOOMRIOMG, %IlsD Neuromuscular Research Laboratory University of Pittsburgh

Developing or re- establishing pro- prioception and 9- Pro priocep tion giv neur~mu~cularcon- Peripheral receptors for proprio- us the sensl e of join ception, called mechanoreceptors, trol is a critical component in -A. r r-m I\-+ I are located in articular structures, motor performance and rehabili- to spec yial nerv tation. Proprioception can be tendons, muscle, and skin. Mech- ings cal led anoreceptors are special nerve defined as a special variation of manoreceptors.t sensory modality that encom- endings that depolarize in re- passes the sensation of joint sponse to tissue deformation. movement (kinesthesia) and joint Therefore, mechanical deforma- position. ?ptors e' tion of tissue is transduced into Proprioception involves inte- inform; neural signals (Grigg, 1994): grating peripheral sensations -- - As tissue deformation in- from afferent pathways while mu;scles an d joints creases, so does the frequency of neuromuscular control helps pro- Fee d-forwe ~rdand feed- discharge and number of mecha- cess these signals into coordinated bac k muscular con noreceptors stimulated. These sig- motor responses through efferent can be enhi nals provide sensory information pathways (Figure 1). pra~ctice. on intrinsic and extrinsic joint Both the afferent and efferent ------loads. Mechanoreceptors vary in pathways comprise the sensory- shape, location, and function and motor system. Basic science re- can be classified according to their search has provided insight on the ral pathways that integrate pe- responses to mechanical stimuli. sensory and motor characteristics ripheral receptors and motor re- They are either slow-adapting of structures that regulate pro- sponses, and uses theoretical (SA) or quick-adapting (QA), and prioception and neuromuscular models to describe the proces- either low-threshold or high- control. sing of sensory information for threshold. This paper gives an overview neuromuscular control. The other QA mechanoreceptors de- of the sensory receptors that pro- theme articles will discuss practi- crease their discharge rate to ex- vide joint motion and position cal ways to improve neuromuscu- tinction within milliseconds of the awareness. It addresses the neu- lar control in injured athletes. onset of a continuous stimulus,

O 1998Human Kinetics September 1998 ticular structures. During normal Descending pathways conditions they are inactive, but Joint receptors they become active when articu- (stimuli: noxious lar tissues are subjected to dam- aging mechanical deformation. Free nerve endings are also sensi- Muscle (stimuli: tive to certain chemical by-prod- ucts of the inflammatory process. Tenomscalar-Mechanorecepl.ovs Muscle spindles, SA mechanore- ceptors located in skeletal muscle, are sensitive to length and rate of Muscle length changes. They have the dis- tinction of being innervated by gamma motor nerves. Increased signals from the gamma motor nerves do not initiate muscle eon- traction but they do heighten the sensitivity of muscle spindles to Muscle tone \ / stretch. When stimulated, muscle Static sensitivi Dynamic sensi ivity spindles convey information SECONDARY MUSCLE about joint motion and position caused by or due to changes in muscle length. They can also elicit AFIGURE 1 Afferent and efferent pathways linking peripheral receptors with a reflex contraction of the agonist muscle spindles and motor responses. muscles. This is the mechanism, Reprinted from MedicalHypotheses, Vol. 35, H. Johansson & P. Sojka, "Pathophysiological mechanisms involved in genesis and spread on muscular tension in occupational muscle known as the stretch reflex, pain and in chronic musculoskeletal pain syndromes: A hypothesis," pp 196-203, 0 1991, whereby muscle spindles have by permission of the publisher, churchill ~ivin~stone. the capacity to mediate muscle activity. Golgi tendon organs are SA while SA mechanoreceptors con- meniscofemoral, and collateral mechanoreceptors found near the tinue to discharge. QA's are very ligaments. They are thought to musculotendinous junction; they sensitive to changes in stimulation mediate the sensation of joint function by monitoring muscle and are therefore thought to me- motion. tension. When stimulated by high diate the sensation of joint motion, Ruffini endings are low- muscle tension, they cause reflex- while SA's are maximally stimu- threshold, SA found in the ive inhibition (relaxation) of the lated at specific joint angles and superficial layer of the cruciate, involved muscle. are thought to mediate the sensa- meniscofemoral, and collateral tion of joint position. ligaments. Ruffini endings medi- Cutaneous Mechanoreceptors ate the amplitude and velocity of The primary role of skin afferents Articular Mechanoreceptors joint rotation and position. is to enhance the effects of other Four types of mechanoreceptors Golgi tendon organ-like end- proprioceptive inputs. We know, are found in the knee joint: (a) ings are high-threshold, SA found for example, that receptors lo- Pacinian corpuscles; (b) Ruffini in cruciate, collateral ligaments, cated in the dorsal skin of the endings; (c) Golgi tendon organs; and menisci. These receptors re- wrist and fingers can provide in- and (d) free nerve endings. main silent when the joint is im- formation on wrist and finger Pacinian corpuscles are mobile but are stimulated at the movements. The contribution of low-threshold, QA located in extremes of joint motion. cutaneous mechanoreceptors to the medial meniscus, extra- and Free nerve endings are widely joint motion and position sense intra-articular fat pad, cruciate, . distributed throughout most ar- continues to be explored.

September 1998 The ProBesslonal Jollranall for ASBhIeSic Tmliners amd Tlhemgplas 7