The amygdaloid complex, located in the medial temporal lobe,is structurally diverse and comprises 13 nuclei. These arefurther divided into subdivisions that have extensive internuclearand intranuclear connections. These nuclei and subnuclei aredistinguished on the basis of cytoarchitectonics, histochemistry,and the connections they make (105, 208). The architectonicorganization and connectivity of the amygdala have been extensivelyreviewed (4, 48, 159, 208). Although these reviews havemostly concentrated on the rat amygdala, it has also been studiedin the monkey (8) and cat (213). These studies reveal thatwhile there are many similarities between species, there arealso clear differences in the organization and the relativesizes of the different amygdaloid nuclei. Because functionalstudies of the amygdala have largely been carried out in therat, in this review we mostly concentrate on results obtainedin this species. We use the nomenclature introduced by Priceet. al (213) with some modifications (159) (see below). Inthis classification, amygdala nuclei are divided into threegroups (Fig. 1): 1) the deep or basolateral group, which includesthe lateral nucleus, the basal nucleus, and accessory basalnucleus; 2) the superficial or cortical-like group, which includesthe cortical nuclei and nucleus of the lateral olfactory tract;and 3) the centromedial group composed of the medial and centralnuclei. Finally, there is a separate set of nuclei that do noteasily fall into any of these three groups and are listed separately.These include the intercalated cell masses and the amygdalohippocampalarea.
A. Basolateral Nuclei
The basolateral or deep nuclei comprise the lateral nucleus(LA), the basal nucleus (B), which is sometimes called thebasolateral nucleus (BLA), and the accessory basal nucleus(AB), which is also known as the basomedial nucleus (Fig. 1).Often, these three nuclei are collectively referred to as thebasolateral complex (91). The LA is located dorsally in theamygdala where it abuts the basal nucleus ventrally. It is bordered laterally by the external capsule and medially by thecentral nucleus. It has three subdivisions: the smaller celleddorsolateral subdivision, the larger celled ventrolateral subdivision,and the medial subdivision. The basal nucleus is located ventralto the LA and is subdivided into the rostral magnocellularsubdivision and the more caudal intermediate and parvicellularsubdivisions. The AB is found ventral to the basal nucleus and lies adjacent to the amygdalohippocampal area (AHA). Itis comprised of the magnocellular subdivision, the intermediatesubdivision, and the parvicellular subdivision (208, 210).
B. Cortical-like Nuclei
The second group is the superficial or corticomedial nuclei(150, 213) (Fig. 1). Although these superficial structuresare called nuclei, many have cortical characteristics sincethey are located at the surface of the brain and have a layeredstructure (213). They comprise the nucleus of the lateralolfactory tract (NLOT), bed nucleus of the accessory olfactorytract (BAOT), the anterior and posterior cortical nucleus (CoAand CoP, respectively), and the periamygdaloid cortex (PAC).The BAOT is at the very rostral part of the amygdala whereit is bordered laterally by the CoA. The CoA is a layered structurelocated lateral to the NLOT rostrally and the medial nucleuscaudally. The CoP is also three layered and is located in themost caudal parts of the amygdala where it borders the AHA dorsallyand the PAC laterally. The PAC is found ventral to the basalnucleus and is subdivided into three subdivisions: the periamygdaloidcortex, the medial division, and the sulcal division (208,213).
C. Centromedial Nuclei
The centromedial nuclear group is found in the dorsomedial portionof the amygdaloid complex and consists of the central (CeA),medial (M), and the amygdaloid part of the bed nucleus of striaterminalis (BNST; Fig. 1). Traditionally these nuclei werepooled with the cortical nuclei. However, it has recently beensuggested that the central, medial, and BNST have histochemicaland developmental characteristics that are distinct from thecortical nuclei (see below). Thus, as initiated by McDonald(159), we will also separate this group from cortical nuclei.The CeA is located dorsomedially in the rostral part of theamygdala, bordered laterally by the basolateral complex, dorsallyby the globus pallidus, and medially by the stria terminalis.The CeA has four divisions: the capsular subdivision (CeC),lateral subdivision (CeL), intermediate subdivision (CeI),and medial subdivision (CeM) (92, 149). The medial nucleusis found near the surface bounded medially by the optic tract.It begins at the level of the NLOT and extends caudally. Ithas four subdivisions: rostral, central (dorsal and ventral),and caudal.
D. Other Amygdaloid Nuclei
The final group of nuclei comprising the remaining amygdalaareas are the anterior amygdala area (AAA), the amygdalo-hippocampalarea (AHA), and the intercalated nuclei (I) (8, 210, 213).The AHA is the most caudal of the amygdaloid nuclei and iscomprised of the medial and lateral subdivisions. The intercalatednuclei are small groups of neurons found in clusters withinthe fiber bundles that separate the different amygdaloid nuclei(173).
E. Extended Amygdala
Although the above classification has been adopted by many,several authors have suggested that a different classificationis more appropriate. Initially, based on the known connectionsof the amygdala, Alheid and Heimer and co-workers (4, 5) arguedthat the centromedial amygdala should be extended rostrallyand medially. They pointed out that the amygdala innervatesthe BNST and the caudodorsal regions of the substantia inominata(ventral pallidum). Furthermore, these two regions have similarefferent connections to the descending projections of the amygdala.Thus they argued that these regions are part of the amygdaloidcomplex. By including these regions they suggested that thecentromedial complex should be termed the "extended amygdala."More recently, Swanson and Petrovich (267, 269) have arguedthat the nuclei of the amygdaloid complex are a structurallyand functionally heterogeneous group that have been arbitrarilygrouped. They suggest that these nuclei should be divided intofour functional systems. These systems would be the frontotemporal,autonomic, main olfactory, and accessory olfactory systems.In this classification, the basolateral nuclei, which embryologicallyare cortical-like nuclei, receive afferents from similar sourcesand contain cells resembling cortical neurons (see below) frompart of the frontotemporal (cortical-like) system. The centralnuclei, embryologically striatal in origin (214), containcells morphologically similar to those in the striatum (214,269) and make many connections with regions involved in autonomiccontrol comprise the autonomic system. Finally, the corticalnuclei, and the medial nucleus, which are the major targetof olfactory projections (see below), are part of the main andaccessory olfactory systems.
This grouping of the nuclei naturally divides the amygdaloidcomplex into distinct functional systems and fits well withthe development of the structures. The classification thatwe have described above is largely in agreement with this proposal,with the basolateral nuclei constituting the frontotemporalgroup, the centromedial nuclei forming the autonomic group,and the cortical-like nuclei constituting the two olfactorygroups.