A study of afferent input to the inferior olivary complex in the rat by retrograde axonal transport of horseradish peroxidase

Abstract

Abstract Under direct visualization, minute amounts of horseradish peroxidase were injected by controlled air pressure through glass micropipettes into the inferior olive of the albino rat. Well localized unilateral injections were obtained, without damage to other brainstem areas or spread into the adjacent reticular formation. After 24 to 36 hours, the entire brain and spinal cord were examined by light microscopy for labelled neurons supplying afferents to the inferior olive. With few exceptions, labelled somata rostral to the inferior olive were found ipsilateral to the injection site, the greatest number occurring in the subparafascicular nucleus and central gray substance around the caudal third ventricle, in the N. of Forel's field, Nn. of Darkschewitsch and Cajal, and adjacent reticular formation, and in the Edinger‐Westphal nucleus. Labelled cells were also noted in the sensorimotor cortex, red nucleus, peri‐aqueductal gray and all subdivisions of the pretectal complex. Caudal to the olive, labelled neurons were located primarily on the contralateral side in the lateral reticular nucleus, dorsal column nuclei and nucleus proprius of the spinal cord. At the level of the olive, numerous labelled cells occurred bilaterally, but mainly contralateral to the injection site in the lateral and interpositus nuclei of the cerebellum. A smaller number were seen in the N. prepositus hypoglossi and in the medial and spinal subdivisions of the vestibular complex ipsilaterally, in the N. spinal tract of V contralaterally, and in the gigantocellular reticular nucleus bilaterally. A few neurons in the N. raphe obscurus were also labelled. No positive cells were found in the caudate nucleus, globus pallidus, locus coeruleus, or medial cerebellar nucleus. The results are compared and contrasted, region by region, with previous studies of afferent olivary pathways and, where possible, correlations are made with physiologic data.