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Brain Behav Evol. 1988;32(3):169-80.

The sensory trigeminal tract of Pacific hagfish. Primary afferent projections and neurons of the tract nucleus.

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Department of Biology, Wesleyan University, Middletown, Conn.


The central projections of primary trigeminal afferents in Pacific hagfish (Eptatretus stouti) were examined by transganglionic transport of horseradish peroxidase (HRP) placed in the principal nerves of the trigeminal complex. Central to the trigeminal ganglia, labeled afferents in the ophthalmic, external and dental nerves coursed caudally in the descending trigeminal tract of the ipsilateral medulla. The trigeminal tract occupies an extensive portion of the dorsolateral medulla and is subdivided into a consistent pattern of longitudinal fascicles surrounded by narrow cellular bands. Primary trigeminal afferents were arranged somatotopically in the fascicles. Large-diameter afferents of the ophthalmic, external and dental nerves segregated into separate medial fascicles extending from the dorsomedial to the ventrolateral descending trigeminal tract. Fine-fibered afferents of the ophthalmic and external nerves formed separate dorsolateral fascicles. Additional dental afferents spread through these lateral fascicles. Labeled trigeminal fibers reached the dorsolateral funiculus of the rostral spinal cord. In addition, HRP injections of the descending trigeminal tract revealed a largely crossed projection to the mesencephalic tectum. Tectal injections of HRP retrogradely labeled neurons in the contralateral sensory trigeminal tract, confirming the existence of a tract nucleus comprised in part of neurons located in the bands of cells bordering the primary afferent fascicles.

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