The Journal of Neurophysiology Vol. 87 No. 3 March 2002, pp. 1186-1195
Copyright ©2002 by the American Physiological Society

Position Sensitivity of Human Muscle Spindles: Single Afferent and Population Representations

 ¹Neurological Sciences Institute, Oregon Health & Science University, Beaverton, Oregon 97006;  ²Department of Neurology, Psychiatry, and Medical Psychology, University of São Paulo, 3900 Ribeirão Preto, Brazil;  ³Department of Kinesiology, Katholiecke Universiteit Leuven, 3000 Leuven, Belgium; and  ⁴School of Human Kinetics, Brain Research Centre, The University of British Columbia, Vancouver, British Columbia V6T 1Z1, Canada

Cordo, Paul J., Carmen Flores-Vieira, Sabine M. P. Verschueren, J. Timothy Inglis, and Victor Gurfinkel. Position Sensitivity of Human Muscle Spindles: Single Afferent and Population Representations. J. Neurophysiol. 87: 1186-1195, 2002. The representation of joint position at rest and during movement was investigated in 44 muscle spindle primary afferents originating from the extensor carpi radialis brevis (ECRb) and extensor digitorum (ED) of normal human subjects. Position sensitivity was estimated for each afferent, and 43 of 44 were position sensitive. In each trial, six sequential ramp-and-hold movements (2-6°, 2°/s, total 24°) flexed the relaxed wrist, beginning from the angle at which the afferent was just recruited. Joint position was represented by three specific features of afferent firing patterns: the steady-state firing rate during the 4-s hold period between ramps, the initial burst at the beginning of each ramp, and the ramp increase in firing rate later in the movement. The position sensitivity of the initial burst (1.27 ± 0.90 pps/°, mean ± SD) was several times higher than that of the hold period (0.40 ± 0.30 pps/°) and not different from that of the ramp increase in firing rate (1.36 ± 0.68 pps/°). The wrist position sensitivities of ECRb and ED afferents were equivalent, as were their recruitment angles and angular ranges of position sensitivity. Muscle spindle afferents, both individually and as a population, were shown to represent static joint position via the hold rate and the initial burst. Afferents were recruited over the entire 110° range of wrist positions investigated; however, the angular range over which each feature represented joint position was extremely limited (15°). The population response, based on the summed activity of the 43 afferents, was monotonically related to joint position, and it was strongly influenced by the afferent recruitment pattern, but less so by the position sensitivities of the individual afferents.