Spinal Sensorimotor Transformation: Relation between Cutaneous Somatotopy and a Reflex Network

Abstract

The projection of primary afferents onto spinal interneurons constitutes the first step in sensorimotor transformations performed by spinal reflex systems. Despite extensive studies on spinal somatotopy, uncertainties remain concerning the extent and significance of representational overlap and relation to spinal reflex circuits. To address these issues, the cutaneous projection from the hindpaw and its relation to the topography of lamina V neurons encoding withdrawal reflex strength ("reflex encoders") was studied in rats. Thin and coarse primary afferent terminations in laminas II and III-IV, respectively, were mapped by wheat germ agglutinin-horseradish peroxidase and choleragenoid tracing. The functional weights of these projections were characterized by mapping nociceptive and tactile field potentials and compared with the topography of reflex encoders. Both anatomical and physiological data indicate that thin and coarse skin afferent input is spatially congruent in the horizontal plane. The representation of the hindpaw in the spinal cord was found to be intricate, with a high degree of convergence between the projections from different skin sites. "Somatotopic disruptions" such as the representation of central pads medial to that of the digits were common. The weight distribution of the cutaneous convergence patterns in laminas III-IV was similar to that of lamina V reflex encoders. This suggests that the cutaneous convergence and features such as somatotopic disruptions have specific relations to the sensorimotor transformations performed by reflex interneurons in the deep dorsal horn. Hence, the spinal somatotopic map may be better understood in light of the topography of such reflex systems.

Action-Based Body Maps in the Spinal Cord Emerge from a Transitory Floating Organization

“…For the horizontal sections, dorsal-view maps were constructed (using Photoshop software; Adobe Systems) by superimposing all individual labeled slices from one spinal cord. The scattered terminations generally observed in deeper laminas were not analyzed (Levinsson et al, 2002). To display the relations between labeling and the medial border of the dorsal horn, the latter was approximated to a vertical plane along which horizontal sections were aligned.…”

“…However, recent data indicate that this view may not be correct. Instead, termination patterns of different cutaneous fibers in the adult rat lumbar spinal cord appear to be closely related to the locations and receptive fields of the reflex sensorimotor circuits (Levinsson et al, 2002). In the withdrawal reflex circuits, pools of reflex encoding interneurons located in the deep part of the dorsal horn, in turn, are related to the sequence of motor neurons in the ventral horn (Schouenborg et al, 1995).…”

Pathway-Specific Bidirectional Regulation of Ca2+/Calmodulin-Dependent Protein Kinase II at Spinal Nociceptive Synapses after Acute Noxious Stimulation

“…The transganglionic labeling formed a patch centered in lamina IIi in the rostral one-half of medial sections through L5 (cf. Levinsson et al, 2002). Peroxidase-labeled sections of dorsal spinal cord were cryoprotected in glycerol overnight, plunged into liquid propane, and embedded by freeze substitution in Lowicryl HM20 (Electron Microscopy Sciences, Hatfield, PA), as described (Larsson et al, 2001).…”

Modularity of Endpoint Force Patterns Evoked Using Intraspinal Microstimulation in Treadmill Trained and/or Neurotrophin-Treated Chronic Spinal Cats

“…Flexion responses mostly occurred in response to stimulation in laminae II-IV, known regions of termination of cutaneous afferents (Levinsson et al 2002). Stimulation in this area may activate the incoming afferents or first-order inter/projection-neuron populations, as the thresholds for pre-and postsynaptic elements are similar (Baldissera et al 1972;Gustafsson and Jankowska 1976;Jankowska et al 1975;McIntyre and Grill 2002).…”