TERMINATION MODE AND BRANCHING PATTERNS OF RETICULORETICULAR AND RETICULOSPINAL FIBERS OF THE NUCLEUS-RETICULARIS-PONTIS-ORALIS IN THE CAT - AN ANTEROGRADE PHA-L TRACING STUDY

By utilizing an anterograde neural tracer, Phaseolus vulgaris leucoagg lutinin (PHA-L), pontomedullary reticuloreticular connections and reti culospinal connections were studied, including their fiber trajectorie s and distribution of PHA-L labeled terminals in close apposition to t arget reticular and spinal neurons, and branching patterns of axon col laterals at the levels of the cervical and upper thoracic cord. PHA-L was focally microinjected into the medial pontine reticular formation corresponding to the nucleus reticularis pontis oralis. A great number of PHA-L labeled thin fibers descended bilaterally coursing through t he medial part of the pontine and medullary reticular formation with a n ipsilateral predominance. Labeled terminal boutons were closely appo sed to somata of various sized pontomedullary reticular neurons. Label ed thick fibers descended ipsilaterally coursing through the ventral h alf of the medial longitudinal fasciculus, and further descended throu gh the ventral funiculus of the spinal cord. At the levels of the cerv ical and upper thoracic cord, these reticulospinal fibers gave off axo n collaterals sending terminal fibers to small- to large-sized neurons in Rexed's laminae VII and VIII. Some of the axon collaterals innerva ted not only ipsilateral but also contralateral gray matter. By recons tructing branching patterns of axon collaterals, each axon collateral was found to innervate spinal neurons located in a disk-like spinal se gment with a width less than 1 mm.