Projections from the orbital gyrus in the cat. I. To brain stem structures

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Abstract
Direct projections from the anterior part of the orbital gyrus and its immediate vicinity to the brain stem in the cat were investigated by means of the Nauta and Marchi methods. These experiments were carried out in an attempt to identify the anatomical substrates subserving orbital‐gyral inhibitory influences on somatic reflexes.The cortical layers of the anterior orbital gyrus and the immediately surrounding cortex were ablated unilaterally. Preterminal degeneration was found bilaterally in the spinal trigeminal nucleus. The most consoicuous degeneration occurred in the interpoler nuclei, caudal parts of the oral nuclei, and rostral portions of the caudal nuclei of the spinal trigeminal complex. Thus, degeneration was found in the trigeminal sensory nucleus from the level of the inferior olive rostrally to levels just caudal to the obex. Preterminal degeneration was also observed bilaterally in the mesencephalic reticular formation. Heavy preterminal degeneration was found in the reticular formation of the rostral midbrain and moderate degeneration was observed in the caudal mesencephalic reticular formation, the pontile reticular substance and medullary reticular nuclei.Based on these findings, the possible pathways involved in the mediation of orbital‐cortically induced inhibition on somatic reflexes were discussed.