We propose to test two hypotheses: (a) contralateral projections of primary afferents mediate crossed components of dorsal horn cell excitatory RFs; and (b) crossed dendrites of dorsal horn cells also mediate contralateral components of their RFs. In each experiment we shall inject horseradish peroxidase (HRP) into one low-threshold cutaneous axon on each side of the spinal cord, at least three segments apart. Then we shall inject HRP into cells in the dorsal horn, contralateral to the injected axons. We shall inject mostly cells whose RFs overlap those of the contralateral injected axons, and some cells whose RFs do not overlap those of the contralateral injected axons. The grey matter contralateral to each injected axon will also be mapped using extracellular recording to determine the somatotopy of the area. Data obtained in this manner will be used to determine whether (1) the contralateral components of dorsal horn cell receptive fields can be accounted for by crossed afferent axons and crossed dorsal horn cells dendrites; (2) crossed axons and dendrites can be accounted for as mediating the assembly of the contralateral RF components of dorsal horn cells; (3) all axons with RFs near the dorsal midline have crossed and uncrossed lateral dorsal horn projections; (4) all axons with RFs near the ventral midline have crossed and uncrossed medial dorsal horn projections; (5) all cells with RFs spanning the dorsal midline have dendrites reaching to the contralateral lateral dorsal horn; (6) all cells with RFs spanning the ventral midline have dendritic trees projecting to the contralateral medial dorsal horn; (7) no axons with RFs far from the dorsal and ventral midlines have crossed projections; (8) no cells with only ipsilateral components of their RFs have crossed dendrites; and (9) whether these generalizations apply to (a) the full rostrocaudal length of the spinal cord, (b) all receptor types (afferents), and (c) all convergence types (cells).
