The striatal efferents in the globus pallidus and in the substantia nigra.

The radial fibers in the globus pallidus are the striatal efferents. Evidence now indicates that they arise from medium-sized neurons in the striatum. During their transit of the globus pallidus, they give off collaterals in both segments of the globus pallidus and undergo reduction in caliber. It has not been possible to observe the same radial fiber emitting collaterals in both segments of the globus pallidus, and the prospects of ever doing so are not good. The radial fibers in the medial segment of the globus pallidus continue into the substantia nigra by way of the "comb" bundle. The divergence and convergence in the striatal efferent systems is considerable. The afferent plexuses of fine fibers bearing bouton en passant endings, which completely ensheath the long dendrites in the globus pallidus and the substantia nigra and make "longitudinal axodendritic connections," are derived from the convergence of branches off the collaterals of a number of different radial fibers in the globus pallidus and the convergence of "comb" bundle fibers in the substantia nigra. The synaptic endings on these fibers constitute most of the endings in the globus pallidus. They are the endings with large, egg-shaped, synaptic vesicles and form symmetrical synapses.