Proprioceptive population coding of limb position in humans

Abstract

The present study investigates the coding of positions reached in a two-dimensional space by populations of muscle spindle afferents. The unitary activity of 35 primary muscle spindle afferents originating from the tibialis anterior, extensor digitorum longus, extensor hallucis longus, and peroneus lateralis muscles were recorded from the common peroneal nerve by the microneurographic technique. The steady mean frequency of discharge was analyzed during 16 passively maintained positions of the tip of the foot. These positions were equally distant from and circularly arranged around the "neutral" position of the ankle. The results showed that a same position of the foot was differently coded depending on whether it was maintained for several seconds or whether it was attained after a movement. Muscle spindle activity was increased or decreased, respectively, when the previous movement lengthened or shortened the parent muscle; the magnitude of change in activity depended on the amount of lengthening or shortening in relation to movement direction. Each muscle surrounding the ankle joint was shown to encode the different spatial positions following a directional tuning curve. Data were analyzed by using the "neuronal population vector model". This model consists of calculating population vectors representing the mean contribution of each muscle population of afferents to the coding of a particular position, and by finally calculating a sum vector. The direction of the sum vector was shown to accurately describe the direction of a given maintained position compared to the initial position. We conclude that muscle spindle position coding is based on afferent information coming from the whole set of muscles crossing a given joint. A given spatial position is associated with a stable muscle afferent inflow where each muscle makes an oriented and weighted contribution to its coding.

Modulation of flexion reflex induced by hip angle changes in human spinal cord injury

“…Nonetheless, evidence from studies in both humans and lower vertebrates suggest that the contribution of hip muscle spindle afferents responding to stretch cannot be ignored. Muscle spindle afferents are considered as receptors for detecting limb position (Prochazka et al 1989), with their static discharges known to depend on the amount of the muscle shortening or lengthening (Ribot-Ciscar et al 2003). These findings are in agreement with the reports of patients, stating that they can still detect the position of their hip after a total hip joint replacement (Grigg et al 1973).…”

Effects of hip joint angle changes on intersegmental spinal coupling in human spinal cord injury

“…In a similar way, ankle muscle afferents responding to stretch modulate significantly soleus H-reflex excitability (Pinniger et al 2001). It is worth noting that muscle receptors have been reported as receptors for detecting limb position, with their static discharges to depend on the amount of the muscle shortening or lengthening (Ribot-Ciscar et al 2003). Further, in the absence of any background force, it is unlikely that group Ib afferents were activated during static hip angle changes.…”

Discharges in Human Muscle Receptor Afferents during Block Grasping

“…Accordingly, their responses to movements imposed on their passive parent muscles can easily be predicted from anatomy (Roll et al, 2000;Ribot-Ciscar et al, 2002, 2003. Natural hand and finger movements are, however, characteristically associated with extensive cocontractions (Johansson and Westling, 1988;Maier and Hepp-Reymond, 1995), that is, during grasping behaviors muscle spindles are subjected not only to length changes but also affected by the fusimotor system and extrafusal muscle contractions.…”