The mediodorsal nucleus of the rat thalamus has been divided into medial, central and lateral segments on the basis of its structure and axonal connections, and these segments have been shown by experiments using the autoradiographic method of demonstrating axonal connections to project to seven distinct cortical areas covering most of the frontal pole of the hemisphere. The position and cytoarchitectonic characteristics of these areas are described. The medial segment of the nucleus projects to the prelimbic area (32) on the medial surface of the hemisphere, and to the dorsal agranular insular area, dorsal to the rhinal sulcus on the lateral surface. The lateral segment projects to the anterior cingulate area (area 24) and the medial precentral area on the dorsomedial shoulder of the hemisphere, while the central segment projects to the ventral agranular insular area in the dorsal bank of the rhinal sulcus, and to a lateral part of the orbital cortex further rostrally. (The term "orbital" is used to refer to the cortex on the ventral surface of the frontal pole of the hemisphere.) A ventral part of this orbital cortex also receives fibers from the mediodorsal nucleus, possibly its lateral segment, but the medial part of the orbital cortex, and the ventrolateral orbital area in the fundus of the rhinal sulcus receive projections from the paratenial nucleus and the submedial nucleus, respectively. All of these thalamocortical projections end in layer III, and in the outer part of layer I. The basal nucleus of the ventromedial complex (the thalamic taste relay) has been shown to have a similar laminar projection (layer I and layers III/IV) to the granular insular area immediately dorsal to, but not overlapping, the mediodorsal projection field. However, the principal nucleus of the ventromedial complex appears to project to layer I, and possibly layer VI, of the entire frontal pole of the hemisphere. The anteromedial nucleus does not appear to project to layer III of the projection field of the mediodorsal nucleus, although it may project to layers I and VI, especially in the anterior cingulate and medial precentral areas. A thalamoamygdaloid projection from the medial segment of the mediodorsal nucleus to the basolateral nucleus of the amygdala has also been demonstrated, which reciprocates an amygdalothalamic projection from the basolateral nucleus to the medial segment. The habenular nuclei also appear to project to the central nucleus of the amygdala. These results are discussed in relation to the delineation and subdivision of the prefrontal cortex in the rat, and to amygdalothalamic and amygdalocortical projections which are described in a subsequent paper (Krettek and Price, '77).

The efferent fiber connections of the nuclei of the amygdaloid complex with subcortical structures in the basal telencephalon, hypothalamus, midbrain, and pons have been studied in the rat and cat, using the autoradiographic method for tracing axonal connections. The cortical and thalamic projections of these nuclei have been described in previous papers (Krettek and Price, '77b,c). Although the subcortical connections of the amygdaloid nuclei are widespread within the basal forebrain and brain stem, the projections of each nucleus have been found to be well defined, and distinct from those of the other amygdaloid nuclei.The basolateral amygdaloid nucleus projects heavily to the lateral division of the bed nucleus of the stria terminalis (BNST), to the caudal part of the substantia innominata, and to the ventral part of the corpus striatum (nucleus accumbens and ventral putamen) and the olfactory tubercle; it projects more lightly to the lateral hypothalamus. The central nucleus also projects to the lateral division of the BNST and the lateral hypothalamus, but in addition it sends fibers to the lateral part of the substantia nigra and the marginal nucleus of the brachium conjunctivum. The basomedial nucleus has projections to the ventral striatum and olfactory tubercle which are similar to those of the basolateral nucleus, but it also projects to the core of the ventromedial hypothalamic nucleus and the premammillary nucleus, and to a central zone of the BNST which overlaps the medial and lateral divisions. The medial nucleus also projects to the core of the ventromedial nucleus and the premammillary nucleus, but sends fibers to the medial division of the BNST and does not project to the ventral striatum. The posterior cortical nucleus projects to the premammillary nucleus and to the medial division of the BNST, but a projection from this nucleus to the ventromedial nucleus has not been demonstrated. Projections to the "shell" of the ventromedial nucleus have been found only from the ventral part of the subiculum and from a structure at the junction of the amygdala and the hippocampal formation, which has been termed the amygdalo-hippocampal area (AHA). The AHA also sends fibers to the medial part of the BNST and the premammillary nucleus.Virtually no subcortical projections outside the amygdala itself have been demonstrated from the lateral nucleus, or from the olfactory cortical areas around the amygdala (the anterior cortical nucleus, the periamygdaloid cortex, and the posterior prepiriform cortex). However, portions of the endopiriform nucleus deep to the prepiriform cortex project to the ventral putamen, and to the lateral hypothalamus.

The cytoarchitectonic structure and divisions of the amygdaloid complex are described in the rat and cat, with special reference to the axonal connections of each of the amygdaloid nuclei, and to interspecies variations and similarities. Several intra-amygdaloid connections are also described, based on autoradiographic experiments with small injections of 3H-amino acids into the individual nuclei. Although it has probably not been possible to determine all of the intra-amygdaloid projections from these experiments, the connections which have been shown are very specific. The lateral nucleus projects to the basomedial nucleus, the lateral part of the central nucleus, and the periamygdaloid cortex, while the basolateral nucleus projects only to itself, the medial part of the central nucleus, and the nucleus of the lateral olfactory tract. The basomedial nucleus projects to the cellular layer of the medial nucleus and the amygdalo-hippocampal area, while the molecular layer of these structures and of the posterior cortical nucleus, receives projections from the periamygdaloid cortex or from the endopiriform cortex. There are also interconnections between the medial and posterior cortical nuclei, and commissural connections between the posterior cortical nuclei of the two sides.Several descriptions of the cytoarchitectonic and nuclear subdivisions of the amygdaloid complex are available; the most prominent of these for the rat are descriptions by Gurdjian ('28) and Brodal ('47), while for the cat the description by Fox ('40) is probably the most widely used. However, these descriptions were based primarily on the appearance of the nuclei in normal material, without regard to their efferent or afferent connections, which were largely unknown at the time. Consequently, there are some discrepancies between the different descriptions, especially between the rat and cat, and it is necessary to make several modifications in order to correlate the nuclear subdivisions with recently obtained data on axonal connections (Leonard and Scott, '71; de Olmos, '72; de Olmos and Ingram, ,'72; price, '73; Scalia and Winans, '75; Broadwell, '75; Krettek and Price, '77b,c, '78).This description of the amygdaloid complex is presented in a n attempt to provide an account of the amygdala in the rat and cat which is consistent between the two species and with known axonal connections. It is COUpled with a description of several intra-amygdaloid connections which have been demonstrated as part of an extensive study of the efferent connections of each of the amygdaloid nuclei (Krettek and Price, '77b,c, '78). MATERIALS AND METHODSThe cytoarchitectonic observations presented here are based on a large number of brains of rats and cats, both normal and experimental. Most of these were embedded in paraffin, sectioned a t 20 p m or 25 pm, and stained with thionin. In addition, three rat brains were fixed in 10% formalin and embedded in celloiden; these were then sectioned a t 50 p m in the frontal, sagittal or horizontal planes and sta...

Projections are described from the basolateral, lateral and anterior cortical nuclei of the amygdaloid complex, and from the prepiriform cortex, to several discrete areas of the cerebral cortex in the rat and cat and to the mediodorsal thalamic nucleus in the rat. These projections are very well-defined in their origin, and in their area of laminar pattern of termination. The basolateral amygdaloid nucleus can be divided into anterior and posterior divisions, based on cytoarchitectonic and connectional distinctions. In both the rat and cat the posterior division projects to the prelimbic area (area 32) and the infralimbic area (area 25) on the medical surface of the hemisphere. The anterior division projects more lightly to these areas, but also sends fibers to the dorsal and posterior agranular insular areas and the perirhinal area on the lateral surface. Furthermore, in the cat the perirhinal area is divided into two areas (areas 35 and 36) and the anterior division projects to both of these and also to a ventral part of the granular insular area; this last area is adjacent to, but separate from the auditory insular area and the second cortical taste area. In most of these areas, the fibers from the basolateral nucleus terminate predominantly in two bands: one in the deep part of layer I and layer II, and a heavier band in layer V (in the rat) or layers V and VI (in the cat). The lateral amygdaloid nucleus projects heavily to the perirhinal area, and also to the posterior agranular insular area. These fibers terminate predominantly in the middle layers of the cortex, although the cellular lamination in these two areas is relatively indistinct. The anterior cortical amygdaloid nucleus and the prepiriform cortex both project to the infralimbic area and the ventral agranular insular area, and the anterior cortical nucleus also projects to the posterior agranular area and the perirhinal area. In all of these areas, the fibers from these olfactory-related structures terminate in the middle of layer I. In the rat, the two divisions of the basolateral nucleus also project to the medial segment of the mediodorsal thalamic nucleus, with the anterior division projecting mainly to the posterior part of this segment and the posterior division to the anterior part. The endopiriform nucleus, deep to the prepiriform cortex, projects to the central segment of the mediodorsal nucleus; this may constitute the major olfactory input into the mediodorsal nucleus, since little or no projection could be demonstrated from the prepiriform cortex itself. Projections to the mediodorsal nucleus have not been found in the cat.

Axonal projections are described from the lateral and basolateral nuclei of the amygdaloid complex, and from the overlying periamygdaloid and prepiriform cortices and the endopiriform nucleus, to the lateral entohinal area, the ventral part of the subiculum, and the parasubiculum in the cat and rat. All of these projections have well-defined laminar patterns of termination, which are complementary to those of other projections to the same structure. Based on these results, and on cytoarchitectonic distinctions, the lateral entohinal area has been divided into dorsal, ventral, and ventromedial subdivisions. The olfactory bulb and prepiriform cortex project to layers IA and IB, respectively, of all three subdivisions, but the lateral amygdaloid nucleus has a restricted projection to layer III of the ventral subdivision only. The periamygdaloid cortex projects to layer II of the ventromedial and adjoining parts of the ventral subdivisions. The ventral part of the subiculum receives fibers from the posterior division of the basolateral nucleus, which terminate in the cellular layer and the deep half to one-third of the plexiform layer. The periamygdaloid cortex and the endopiriform nucleus also project to the same part of the subiculum, but these fibers terminate in the outer part of the plexiform layer. None of these projections extend into the dorsal part of the subiculum. The posterior division of the basolateral nucleus also projects to the posterodorsal part of the parasubiculum ("parasubiculum a" of Blackstad, '56). These fibers end in the deeper part of the plexiform layer and the superficial part of the cellular layer.