Spinal Cord Processing of Sensory Information: Spatial Organization and Adaptive Mechanisms

Abstract: Principles for sensorimotor transformation and plasticity in the spinal cord and functional repair in the root-avulsed spinal cord were studied. A comparative study confirmed that the nociceptive withdrawal reflex (NWR) system is composed of reflex modules, with each performing a finely tuned transformation of skin sensory input to activity in one muscle. This suggests that modularity of the NWR system is a general organizational principle. The sensorimotor transformation in the reflex modules was adapted to species-specific specializations of the biomechanics of the motor plant, such as digitigrade or plantigrade stance. During postnatal development, the NWR modules undergo profound experience dependent changes from an unadapted to a functionally well-tuned system. This tuning was abolished after neonatal spinal transection, which indicates that input from supraspinal systems is needed for the formation of spinal memory engrams that specify input-output relations of NWRs. To further clarify principles for spinal sensorimotor transformation, a comprehensive morphological and electrophysiological mapping of the nociceptive and tactile cutaneous inputs to the spinal cord and their relation to the NWR network was made. The cutaneous somatotopy in the dorsal horn was found to be complex, exhibiting a high degree of representational overlap. The weight distribution of the monosynaptic tactile input to laminae III–IV was similar to that of ‘reflex encoder’ neurons in the deep dorsal horn. Therefore, it is suggested that the organization of input to the spinal cord reflects the demands of spinal motor systems rather than being primarily a body representation used for sensory discriminative functions. Furthermore, because the weight distribution of the cutaneous tactile input is similar to that of the tuned cutaneous input to reflex networks in the adult, it is conceivable that it is subject to a significant tuning during development. In a parallel study, transplanted human embryonic sensory neurons were shown to grow into the spinal cord and form functional connections with the recipient rat dorsal horn neurons. Although only weak activity could be evoked in the dorsal horn, the transplants had the capacity to evoke large amplitude motor reflexes, indicating compensatory mechanisms further downstream in the reflex connections. The bearing of spinal plasticity on the normal function of the spinal cord and on repair strategies after cord injuries are discussed.