Intracellular Labeling with HRP of Identified Neurons, Primary Afferents and Cortical Fibers to the Cuneate Nucleus of the Cat

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Intracellular or intra-axonal injection of horseradish peroxidase (HRP) enables the visualization of single, physiologically identified cell or axon, together with their dendritic trees or terminal arborizations, and enables the investigator to directly tackle structural- functional questions. The present study investigates thalamic-projection neurons and primary and cortical afferents within the cuneate nucleus. Cells were identified by antidromic activation from the ventrobasal nuclei of the thalamus, their receptive fields determined, and HRP was iontophoretically injected. After HRP histochemistry, sections were embedded in plastic wafers and viewed by LM prior to EM examination of HRP-labeled profiles. Soma diameters ranged from 10 -25 um. Dendrites were directed towards the center of the clusters and carried many appendages and complex axon-like profiles carrying en passant and terminal enlargements. The axons of cuneothalamic cells gave rise to collateral arborizations in the ventral parts of the cuneate nucleus. Single primary afferent axons were identified following natural stimulation. In contrast with the afferent fiber patterns in the spinal cord (Brown, 1981), cutaneous axon collaterals in the cuneate nucleus show few distinctions according to fiber type; rather the terminal arborizations of each axon are associated with one or more of the cell clusters. The large primary afferent boutons (primarily en passant) are distributed mainly in the core of each cluster. Ia fibers from muscle have few projections in the cell clusters region but have terminations at the base of the cuneate. Corticocuneate fibers were recorded ventral to the cuneate nucleus and were identified by cortical stimulation. They gave rise to arborizations which extend up to 2.5 mm rostrocaudally in the ventral parts of the cuneate nucleus. Other corticocuneate projections arise as collateral branches of corticospinal axons; their arborizations are restricted rostrocaudally (0.2 - 0.5 mm). Supported by USPHS grants NS12440 and NS16264.


Presented at the 96th Session of the American Association of Anatomists, Atlanta, GA.



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