Horseradish peroxidase injection of identified low threshold cutaneous mechanoreceptor (LTCM) primary afferent axons was used to assess the somatotopic organization of hindlimb projections to laminae III and IV of cat dorsal horn. Multiple injections in the same animals were used to assess bilateral symmetry and precision. Thirty-one axons were injected, with more than 1 axon injected in each of 8 animals (25 axons). Somatotopic relations between their receptive field (RF) centers and the centers of their dorsal horn projections were similar to the somatotopic relations between dorsal horn cell RF centers and cell locations. Very few reversals of mediolateral somatotopic gradients (proximodistal RF location as a function of mediolateral projection center) were observed. Two afferents with nearly identical RFs in 1 animal had nearly identical projections. These observations held for many different combinations of receptor types. A simple mathematical model was used to demonstrate that assembly of dorsal horn cell RFs via passive sampling of the presynaptic neuropil by dorsal horn cell dendrites cannot account for the sizes of dorsal horn cell LTCM RFs. Hypothesized mechanisms for assembly of dorsal horn cell RFs must take into account the functional selectivity of connections required to produce RFs smaller than those predicted by the passive assembly model.
Previous studies in this laboratory have shown 1) that 19% of L6 and L7 dorsal horn cells in normal cats respond only with excitatory postsynaptic potentials (EPSPs) to sural nerve stimulation, and 2) that the distribution of dorsal horn neurons responding with impulses to sural nerve stimulation is increased in cats with chronic lateral funiculus lesions. The present study was undertaken to determine whether strengthening of subliminal sural nerve projections could account for the changes seen after lateral funiculus lesions and to explore the nature of these changes in greater detail. Ipsilateral L6 and L7 dorsal horn cells of cats with T12 dorsolateral funiculus (DLF) lesions were studied electrophysiologically at less than 1-30 days postoperatively (DPO) and were compared with similar cells recorded in normal cats. The major results were as follows. 1. Responsiveness to peripheral stimulation was depressed for up to 3 DPO following the lesions. 2. The percentage of L6 and L7 dorsal horn cells showing spontaneous activity was elevated at 3 DPO and declined to normal levels by 28 DPO. 3. The percentage of cells that responded to sural nerve stimulation increased over 3-30 DPO and was significantly greater than normal at 28-30 DPO. The increase in the percentage of cells giving impulses to sural nerve stimulation at 28-30 DPO was similar to the percentage of cells with subliminal responses to sural nerve stimulation in normal cats. A subpopulation of identified postsynaptic dorsal column neurons also showed a significant increase in the percentage that responded to sural nerve stimulation after DLF lesions. 4. The percentage of cells that had a cutaneous receptive field (RF) overlapping the region of skin innervated by the sural nerve also increased over time and was significantly greater than normal at 28-30 DPO. 5. Median low-threshold or high-threshold RF areas were not significantly greater than normal ipsilateral to DLF lesions at any survival time. 6. The proportions of low-threshold (LT), high-threshold (HT), and multireceptive (MR) cells were not significantly different from normal at any survival time from less than 1 to 30 DPO.(ABSTRACT TRUNCATED AT 400 WORDS)
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