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Cell types of the auditory neostriatum in the starling forebrain are described. This area in the caudal neostriatum is defined neurophysiologically by the appearance of auditory neurons. Through use of the rapid Golgi technique, four types of neurons are identified, mainly on the basis of their processes: Neurons with long descending axons and thick dendrites rich in spines (type 1), neurons with long ascending axons and thin dendrites poor in spines (type 2), short-axon neurons (type 3), and microneurons (type 4). The axons of the long neurons pass outside the confines of the auditory neostriatum. Among neurons of type 1; some of the long descending axons directed toward the lower brain centers enter the capsula interna occipitalis (CIO). The descending axons give off many collaterals within the auditory neostriatum. With neurons of type 2; most of the ascending axons cross the lamina hyperstriatica, enter the hyperstriatum ventrale, and arborize near its periventricular region. Some of the long ascending axons reach the overlying hyperstriatum ventrale, pars caudale (HVc, the vocal control area). Among neurons of types 3 and 4; the axons of short-axon neurons and of microneurons end with fine branches within the auditory neostriatum. The dendrites of long-axon neurons are oriented in specific directions, whereas those of short-axon neurons and of microneurons do not show a definite pattern of orientation. In the region of the auditory neostriatum that lies immediately adjacent to the midline of the brain, the first three types of neurons are arranged around the central core known as field L, which is composed of the microneurons and the terminal ramifications of auditory afferents. Laterally the microneurons, along with the fibers of the input tract, undergo a rostral shift to occupy a more peripheral position within the auditory neostriatum. The neurons of the auditory neostriatum are compared with those of the mammalian auditory cortex, and a functional classification of nerve cells into projection neurons, association neurons, and interneurons is proposed.

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