Harvey J. Karten

THE ORGANIZATION OF THE AVIAN TELENCEPHALON AND SOME SPECULATIONS ON THE PHYLOGENY OF THE AMNIOTE TELENCEPHALON

There have been numerous surveys of the morphology of the avian telencephalon and attempts to compare it to the various divisions of the telencephalon of other vertebrates. These comparisons were often tenuous, based upon apparent resemblance to structures in mammals. Time does not permit a complete survey of all structures discernible within the forebrain, and the present report is a resume of only some of the recent findings on the avian telencephalon. The major portion of the avian telencephalon consists of a mass of grey matter in the lateral wall of the hemisphere subdivided into several regions, all ending with the suffix “-striaturn,” and with a small region of clearly laminated pallium. As il consequence of the relatively small amount of pallium in birds, most suggestions concerning the nature of the avian telencephalon have derived from the striatal appearance of the cell masses in the forebrain. FIGURE 1 is a Nissl-stained section of a transverse section through the pigeon telencephalon showing the location of the paleo-, neo-, and various subdivisions of the hyperstriatum. It is worth recalling, however, the original reason for designating these areas as striatum. Striatum was simply a descriptive term indicating that these regions consisted of large masses of grey matter with bundles of myelinated axons passing through them, giving them a striated appearance. However, this simple descriptive feature provided no information about the differences between different masses of grey matter within the telencephalon, their connections, or participation in any functional system, and in no way provided any defining characteristics of the cells that would permit comparing them to cell masses in other vertebrates. There was no logical reason to expect that all striatal structures were necessarily comparable, except that they consisted of masses of neurons, glia and axons. Even more puzzling was the striking discrepancy between the relatively small amount of pallium and large volume of striatum in sauropsids, and the apparently inverse ratios of large areas of pallium and only moderate amounts of striatum in mamma1s.t

The ascending auditory pathway in the pigeon (Columba livia) II. Telencephalic projections of the nucleus ovoidalis thalami

Summary 1. The nucleus ovoidalis of the pigeon is a major diencephalic terminus of the brachium of the inferior colliculus (nucleus mesencephali lateralis, pars dorsalis). 2. The fiber degenerations following stereotaxic lesions of the nucleus ovoidalis were studied with the Nauta-Gygax and Fink-Heimer methods for degenerating axons and boutons terminaux. 3. Efferent axons of the nucleus ovoidalis form the medial portion of the fasciculus prosencephali lateralis and pass through the medial part of the paleostriatum augmentatum to enter the overlying capsula occipitalis interna (CIO). 4. Some scattered ovoidalis axons terminate on the intercalated neurons of the CIO, and in the medial caudal neostriatum. By far the major distribution of the ovoidal projection, however, takes place in a sharply demarcated region of the medial caudal neostriatum, consisting of small, densely packed neurons. This region corresponds to Field L of Rose. 5. This study, taken in conjunction with an earlier report on the projections of the avian inferior colliculus, indicates the existence of a discrete and sharply segregated auditory projection pathway leading from the inferior colliculus over the thalamic nucleus ovoidalis to the forebrain. 6. The relationships between avian and mammalian ascending auditory pathways are discussed.

The organization of the ascending auditory pathway in the pigeon (Columba livia) I. Diencephalic projections of the inferior colliculus (nucleus mesencephali lateralis, pars dorsalis)

Abstract 1. An experimental-anatomical study of the projection from the inferior colliculus (MLD) in the pigeon was performed by the aid of the Nauta-Gygax method. The resulting observations can be summarized as follows. 2. A massive efferent fiber system — the brachium of the inferior colliculus—ascends from the MLD, passing medial to the brachium of the superior colliculus and dorsal to the ectomammillary nucleus. 3. The majority of its fibers are distributed to the ipsilateral nucleus ovoidalis via the prominent tractus nuclei ovoidalis. 4. Other fibers from the brachium of the inferior colliculus enter the dorsal supraoptic decussation (DSOD), terminating in passage on neurons of the nucleus of the DSOD, and join the contralateral tractus nuclei ovoidalis to end in the nucleus ovoidalis of the opposite side. 5. Degenerating axons were followed to the contralateral MLD and, via a descending path, to the ipsilateral trapezoid body. 6. No axonal degeneration was seen in nucleus ovoidalis after lesions of the spinal cord, nuclei cuneatus and gracilis, nucleus solitarius, trigeminal nucleus, cerebellar nuclei, retina or optic tectum. 7. It is concluded that nucleus ovoidalis represents a specific diencephalic auditory relay nucleus, possibly comparable to the ventral division (Morest) of the mammalian medial geniculate nucleus.

A General Profile of the Vertebrate Brain, with Sidelights on the Ancestry of Cerebral Cortex

The most elementary tenet of the theory of evolution is that animal specification followed a temporal sequence such that one order of species developed from another, and in time gave rise to one or more further orders. The reconstruction of the “tree of evolution,” one of the most constantly pursued goals of biology, is attended by numerous difficulties, foremost among which is the circumstance that existing forms of life represent little more than “leaves on the ends of branches” of a tree, the trunk and limbs of which have long been extinct. Virtually all extant animals appear to be specialized forms that have diverged in greater or lesser degree from any of the identified or presumed mainlines of evolution. The identification of such “mainlines,” furthermore, is often highly uncertain, the more so because several vertebrate classes appear to have evolved not from one, but from several ancestors. Modern amphibians, for example, are suspected of representing several developmental lines originating from various piscine forms.

Central Trigeminal Structures and the Lateral Hypothalamic Syndrome in the Rat

Extrahypothalamic lesions of central trigeminal structures produce a syndrome of aphagia, adipsia, finickiness, and food spillage. The similarity of these effects to the lateral hypothalamic syndrome and the location of trigeminal structures within the diencephalon suggest that some components of the lateral hypothalamic syndrome are due to incidental damage to trigeminal fibers of passage.

Peptides are in the eye of the beholder

Neuroactive peptides, previously identified in central neurons and thought to be neurotransmitters or modulators, have been recently described in ocular tissues. The uvea (iris, ciliary body and choroid) contains extrinsic nerve fibers immunoreactive for substance P and VIP; exogenous application of these peptides evokes physiological reactions in the eye. At least eight neuroactive peptides have been localized to retinal neurons, generally the intrinsic, local-circuit amacrine cells. In the best studied retina, that of the pigeon, the local- ization of each peptide within a unique amacrine cell type implies that the multiple structural types described by Cajal are functionally distinctive neurons. Physiological explorations suggest functions for two of the retinal peptides, substance P and enkephalin, in visual information processing, and a biochemical study indicates that at least one peptide, somatostatin, is synthesized within the retina.

Identification of serotonin within vital-stained neurons from leech ganglia.

Abstract— We have measured serotonin (5‐HT) within large and small neurosomata which are vitally stained by Neutral Red dye. A micro‐radioenzymatic technique which is sensitive to 50fmol of 5‐HT was employed on intact ganglia, 75 μm Retzius Cells (RZ) and a 10 μm ventro‐lateral cell (VL) taken from the leech Macrobdella decora. The stain does not affect the levels of 5‐HT in either ganglia or RZ. The VL cell body contains 5‐HT at concentrations of at least 100 mm. Microspectrofluorometry of all the ganglionic neurosomata which fluoresce following the Falck‐Hillarp formaldehyde condensation reaction detected rapidly‐fading emission peaks of 509–523 nanometers. We conclude that all seven fluorescent neurons in the leech ganglion very probably contain serotonin.

The tractus infundibuli and other afferents to the parahippocampal region of the pigeon.

While recent anatomical studies have contributed greatly to our understanding of the avian telencephalonU,l~,t4,1s, ?2, little is yet known about the dorsomedial forebrain, an area considered to be homologous with the mammalian hippocampus and related structures ~,~,4,8,16. We have now begun to examine the connections of this region in the pigeon in order to shed some light on the comparative anatomy of the hippocampal complex. Our first step was to establish a homologue to the fornix longus, which would then allow us to delineate the avian equivalents of structures known to be connected by that fiber system in mammals. Although the fornix longus has actually been demonstrated in birds, one proposed candidate has been the tractus infundibuli (IN) l, a distinct bundle of large, myelinated axons which courses through the ventral hypothalamus 7. The present study has focused on examining the origins and terminal projections of IN, along with some other neural associations of the dorsomedial telencephalon. To first determine the polarity of the IN and its locus of termination, lesions were placed stereotaxically la in the dorsal telencephalon, septum, dorsal anterior thalamus, or at the ventral portion of the di-mesencephalic junction. The animals were sacrificed 2 or 4 days later, perfused with saline and formalin, and prepared according to the method of Fink and Heimer ~ for degenerating boutons. In contrast to earlier descriptions 1,7, we found the IN to be an ascending system which projects to the dorsomedial forebrain. Following ventral tegmental lesions (Fig. 1), large degenerating axons of IN were seen to cross in the supramammiltary decussation (retroinfundibular commissure), travel anterolaterally until reaching the so-called nucleus mammillaris lateralis, then group into fascicles forming a distinct bundle ventral to the medial forebrain bundle. Rostral to the anterior commissure, at the level of the nucleus of the diagonal band of Broca, the fibers turn dorsally and

The Distribution of Primary Lagenar Fibers within the Vestibular Nuclear Complex of the Pigeon

The central projections of the lagenar branchof the cochleolagenar nerve of the pigeon were studied by the Fink-Heimer method in transverse, horizontal and sagittal sections of the brain stem. In addition to terminating within specialized areas associated with the cochlear nuclei, primary lagenar neurons distribute to a limited region of the ventral part of the lateral vestibular nucleus, the dorsolateral portion of the descending vestibular nucleus, the quadrangular division of the superior vestibular nucleus, and the medial vestibular nucleus. Direct lagenar projections are also traceable to the external cuneate nucleus and the processus lateralis cerebello-vestibularis. The gigantocellular (dorsal) division of the lateral vestibular nucleus, interstitial nucleus and tangential nucleus are free of terminal degeneration following extirpation of the lagenar ganglion.

Terminal distribution of retinal fibers in the tegu lizard (Tupinambis nigropunctatus)

SummaryThe retinal projections in the tegu lizard were traced using degeneration-silver methods. Bilateral projections were found to the dorsolateral geniculate and the posterodorsal nuclei. Unilateral, crossed projections were traced to the suprachiasmatic nucleus, the ventrolateral geniculate nucleus, the mesencephalic lentiform nucleus, nucleus geniculatus praetectalis, the ectomammillary nucleus, and the optic tectum. Some of these connections are distinctly different from those reported in other reptiles and suggest that important interspecific variations occur among reptiles.