MULTI-SEGMENTAL INNERVATION OF SINGLE PONTINE RETICULOSPINAL AXONS INTHE CERVICOTHORACIC REGION OF THE CAT - ANTEROGRADE PHA-L TRACING STUDY

To characterize the fine morphology of individual reticulospinal axons at multiple spinal segments, localized injections of the anterograde neural tracer, Phaseolus vulgaris ris leucoagglutinin (PHA-L), were ma de into the nucleus reticularis pontis oralis (NRPo) of the cat. Follo wing survival periods of 6-8 weeks, labelled axons, between 1 and 8 mu m in diameter, were found throughout the cervical and upper thoracic segments. Thick axons (diameter greater than or equal to 3 mu m) were found to descend beyond the upper thoracic spinal cord, while most thi n axons (diameter < 3 mu m) ended in the upper cervical cord. From ser ial transverse sections (50 mu m) of segments C3 to T2, in four cats, the trajectories of 23 single, thick reticulospinal axons were traced in continuity over distances of between 21.8 and 59.4 mm, correspondin g to 3 and 8 segments, respectively. Most axons gave off at least one, and as many as four collaterals per segment, some preferentially in t he cervical enlargement. The remainder gave off collaterals almost but not all segments. Detailed reconstruction of the collateralization an d arborization in the spinal gray matter showed two major termination types, one where terminals remained ipsilateral to the stem axon, the other where additional collaterals extended across the midline from th e ipsilateral gray matter to terminate in the contralateral gray matte r. Axons tended to have collaterals of one type or the other, irrespec tive of the rostrocaudal level. Both ipsilateral and bilateral project ions terminated mainly in laminae VII and VIII although the branching patterns varied from axon to axon. Individual stem axons, in general, showed similar termination patterns at each level. (C)) 1997 Wiley-Lis s, Inc.