Primary afferents play a significant role in motor control (Pearson, 1993). In locomotion, primary afferents transmit sensory information on load bearing, muscle stretch, joint position, and cutaneous sensations and are involved in such matters as the transition from stance to swing or from static to dynamic balance in the initiation of stepping. Their reflex effects, ipsi- and contralaterally through Ia and cutaneous afferent activation, in the human are substantially modulated over cycles of activities such as walking (Brooke et al., 1997). In addition to spinal paths from sensory receptors to motoneurons and motor interneurons, it is clear that primary afferent activity regulates the activity of other primary afferents in complex and organized ways (Rudomin, Romo, & Mendell, 1998). Often, this involves presynaptic inhibition (Stein, 1995). Such sensorisensory conditioning is seen across much of the animal kingdom (Watson, 1992).
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