Firing properties of muscle spindle afferents in the intrinsic foot muscles and tactile afferents from the sole of the foot during upright stance.

We review our approach for undertaking microelectrode recordings from the posterior tibial nerve at the ankle, which has allowed us to identify, for the first time, the firing properties of muscle spindle endings in the intrinsic muscles of the foot and of cutaneous mechanoreceptors in the sole during unsupported standing. The responsiveness of muscle spindles in the short muscles of the foot to stretch and related joint movements was similar to that of spindles located in the intrinsic muscles of the hand. Only 27% were spontaneously active in the unloaded condition, whereas 50% were active during unsupported free standing. Moreover, in the latter condition firing rates of 67% of the endings were correlated with changes of the centre of pressure (CoP), primarily (88%) along the anteroposterior axis. The firing of cutaneous afferents supplying the sole of the foot in unsupported free standing depended on the receptor type and location of the receptive field: fast-adapting type I and slowly adapting type I afferents responded transiently during contact with the substrate on standing and to spontaneous postural adjustments, whereas the tonic firing of slowly adapting type II endings encoded fluctuations in the CoP. We conclude that muscle spindle endings in the intrinsic muscles of the foot are recruited or increase their spontaneous discharge on standing and can faithfully encode changes in CoP during spontaneous or evoked postural sway, a function shared by slowly adapting type II afferents in the sole. These data emphasize the important contributions of sensory sources in the foot to maintaining and responding to perturbations in upright posture.