The spatial compartmentalization of motor axons in cranial nerves has not been previously demonstrated. In the present study, motor axons in the medial lingual ramus of the hypoglossal nerve of rats were labeled using horseradish peroxidase and diaminobenzidine staining methods. Neuronal axons were segregated into fascicles within the main nerve trunk (average total length: 10.4 mm). Light microscopic examination of stained nerve sections revealed reaction product within individual axons and showed that the grouping of labeled nerve fibers in the medial ramus was maintained for at least the peripheral half of the nerve before the stain faded. The authors propose that the fascicular anatomy of the rat hypoglossal cranial nerve resembles that of peripheral spinal nerves in humans. This functional and structural compartmentalization may have a clinical impact on the repair and treatment of cranial nerve pathologies.
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