The afferent and efferent connections of the feline nucleus tegmenti pedunculopontinus, pars compacta

The fiber connections of the nucleus tegmenti pedunculopontinus, pars compacta (TPc) were studied by means of the anterograde autoradiographic and retrograde horseradish peroxidase tracer methods. The limits of TPc, which is not a cytoarchitecturally distinct cell group in the cat, were estimated on the basis of autoradiographic experiments in which we plotted the distribution of afferent connections from the entopeduncular nucleus, motor cortex, substantia nigra, and subthalamic nucleus. We concluded that in the cat, TPc can most readily be identified by the terminal distribution of fibers originating in the substantia nigra. Deposits of radiolabel in the motor‐pre‐motor cortex, the entopeduncular nucleus, and subthalamic nucleus weakly labeled the same tegmental region. Parallel horseradish peroxidase experiments suggested that neurons near the entopeduncular nucleus, especially in the lateral hypothalamus and subthalamus, contribute to the pallido‐TPc pathway; that entopeduncular neurons projecting to TPc are most numerous in the ventral part of the nucleus; and that of the neocortical motor fields, both areas 4 and 6 project to TPc. An autoradiographic study of the efferent connections of TPc showed that the major connections were directed toward more rostral structures. Small deposits of labeled amino acids centered in TPc invariably elicited bilateral labeling of the pars compacta of the substantia nigra and the subthalamic nucleus, the densest label being on the ipsilateral side. Other structures variably or more sparsely labeled were: the posterior and lateral hypothalamus; centre median complex; an intermediate part of the lateral thalamic nucleus; central gray substance; nucleus parabrachialis pigmentosus (retro‐rubral nucleus); the raphe nuclei linearis intermedius, dorsalis, and magnus; nucleus reticularis tegmenti pontis; the pontine and medullary reticular for‐mation including the nuclei pontis oralis and caudalis; and the nucleus reticularis gigantocellularis, especially its ventral supraolivary part. These observations demonstrate that TPc shares some connections with a wider tegmental region, but that, in addition, TPc is characterized by a set of afferent and efferent connections that differentiate it from the surrounding teg‐mentum and affiliate it with the basal ganglia. By comparison with findings in the monkey and rat, these experiments further suggest that the connections of TPc may vary across mammalian species. The location and fiber connections of TPc indicate that it is probably distinct from the physiologically defined mesencephalic locomotor region, but suggest the hypothesis that TPc, as a part of an extrapyramidal tegmental loop‐circuit, may be involved in the production of simple resting tremors.