The internal organization of the pallidum in mammals.

A survey of morphologic data on the pallidum indicate that this structure contains at least two types of efferent neurons probably related to its GABA mediated inhibitory and Substance P mediated excitatory action on target cells. In addition, there are short-axoned neurons with presynaptic dendrites forming triadic synapses of unknown significance. Characterization of afferent axon terminals suggest that the majority originate in the striatum, are gabaergic and inhibitory. Enkephalins may coexist in these afferents. Other striatal fibers are excitatory and may contain Substance P. Possible afferents from the subthalamic nucleus form inhibitory synapses and their action may be mediated by glycine. Brain stem afferents from the substantia nigra, pars compacta (dopaminergic), raphe nuclei (serotoninergic) and n. tegmentalis pedunculopontinus (transmitter unknown) are excitatory on pallidal cells on morphologic and/or physiologic grounds.