(image via John Whalen's lab)
Amygdala
Human Amygdala
The amygdala is a subcortical, telencephalic structure bounded dorsally by the lentiform nucleus, laterally by the claustrum, and medially by the diencephalon. It generally lies ventral to the putamen, though it is shifted caudally relative to this structure. The human amygdala, as described by de Olmos, can be segregated into three separate divisions, which will be described in turn.
Laterobasal amygdaloid nuclear complex - This ovular structure constitutes the bulk of the amygdala and is further subdivided into five nuclei/areas. The lateral amygdaloid nucleus is the main sensory input area and is the most lateral of these nuclei; it makes up the bulk of the rostral part of the nuclear complex, tapering toward the caudal end. It is bounded laterally by the claustrum, from which it is separated by the external capsule; rostrally by the endopiriform nucleus; caudally by the lateral ventricle; and dorsally by the amygdalostriatl transition zone. The basomedial amygdaloid nucleus supplies output to the hippocampus, and the entorhinal and perirhinal cortices and is the most medial of these nuclei. It is apparent at the rostral extent, but becomes more prominent as one moves caudally. Between these two nuclei is the basolateral amygdaloid nucleus, bounded laterally by a lateral medullary lamina, and medially by a medial medullary lamina. Moving caudally through the nuclear complex, the ventral portion of the basolateral amygdala migrates medially, creating a “J” shaped nucleus. Medioventrally running along the rostral-caudal length of the basolateral nucleus is the paralaminar amygdaloid nucleus. This structure is more prominent at the rostral than the caudal end. Finally, at the rostral end of the nuclear complex is the amygdaloclaustral transition area, which, as the name implies, is the confluence of the claustrum and amygdala along with the endopiriform cortex. However, some do not distinguish this as a true amygdaloid structure.
Extended Amygdala – The extended amygdala as proposed by Alheid and Heimer (1988) was an attempt to separate the substantia innominata into anatomically related and connected structures. Specifically, it was proposed that the bed nucleus of the stria terminalis was connected to the central and medial amgydaloid nuclei. Presently, it is believed that the extended amygdala mediates reinforced behavior such as self stimulation (Waraczynski, 2006) and drug abuse (Koob, 2003).
The two primary nuclei of the extended amygdala are the central and the medial amygdaloid nuclei. The central nucleus is the main output nucleus of the amygdala and lies dorsal to and extends the length of the basolateral and basomedial nuclei of the nuclear complex discussed above. It is flanked medially by the anterior commissure, laterally by the amygdalostriatal transition area, and rostrally by the anterior amygdaloid area. The medial nucleus receives olfactory input and lies medial and slightly ventral to the central nucleus. Both of these areas are connected to separate areas of the bed nucleus of the stria terminalis via fibers through the sublentricular portion of the substantia innominata (called the sublentricular extended amygdala). The bed nucleus itself is a small collection of neurons collected along the midline extending rostrally from the amygdala to the posterior commissure.
Olfactory Amygdala – de Olmos further characterizes a third division of the amygdala, the olfactory amygdala. This division receives input from the olfactory bulb and olfactory cortex and projects mainly to the central and medial nuclei of the extended amygdala; it has been subdivided into two nuclei and two areas.
The two nuclei are the ventral and anterior cortical amygdaloid nuclei. The anterior portion borders the piriform cortex rostrally and the medial nucleus caudally. Ventral and caudal to this nucleus is the ventral nuclei, which is proximal to the lateral olfactory tract. The two areas of the olfactory amygdala are the amygdalopiriform transition and amygdalohippocampal areas. These lie between the amygdala and, respectively, the entorhinal cortex and the uncus of the hippocampus.
The amygdala is a subcortical, telencephalic structure bounded dorsally by the lentiform nucleus, laterally by the claustrum, and medially by the diencephalon. It generally lies ventral to the putamen, though it is shifted caudally relative to this structure. The human amygdala, as described by de Olmos, can be segregated into three separate divisions, which will be described in turn.
Laterobasal amygdaloid nuclear complex - This ovular structure constitutes the bulk of the amygdala and is further subdivided into five nuclei/areas. The lateral amygdaloid nucleus is the main sensory input area and is the most lateral of these nuclei; it makes up the bulk of the rostral part of the nuclear complex, tapering toward the caudal end. It is bounded laterally by the claustrum, from which it is separated by the external capsule; rostrally by the endopiriform nucleus; caudally by the lateral ventricle; and dorsally by the amygdalostriatl transition zone. The basomedial amygdaloid nucleus supplies output to the hippocampus, and the entorhinal and perirhinal cortices and is the most medial of these nuclei. It is apparent at the rostral extent, but becomes more prominent as one moves caudally. Between these two nuclei is the basolateral amygdaloid nucleus, bounded laterally by a lateral medullary lamina, and medially by a medial medullary lamina. Moving caudally through the nuclear complex, the ventral portion of the basolateral amygdala migrates medially, creating a “J” shaped nucleus. Medioventrally running along the rostral-caudal length of the basolateral nucleus is the paralaminar amygdaloid nucleus. This structure is more prominent at the rostral than the caudal end. Finally, at the rostral end of the nuclear complex is the amygdaloclaustral transition area, which, as the name implies, is the confluence of the claustrum and amygdala along with the endopiriform cortex. However, some do not distinguish this as a true amygdaloid structure.
Extended Amygdala – The extended amygdala as proposed by Alheid and Heimer (1988) was an attempt to separate the substantia innominata into anatomically related and connected structures. Specifically, it was proposed that the bed nucleus of the stria terminalis was connected to the central and medial amgydaloid nuclei. Presently, it is believed that the extended amygdala mediates reinforced behavior such as self stimulation (Waraczynski, 2006) and drug abuse (Koob, 2003).
The two primary nuclei of the extended amygdala are the central and the medial amygdaloid nuclei. The central nucleus is the main output nucleus of the amygdala and lies dorsal to and extends the length of the basolateral and basomedial nuclei of the nuclear complex discussed above. It is flanked medially by the anterior commissure, laterally by the amygdalostriatal transition area, and rostrally by the anterior amygdaloid area. The medial nucleus receives olfactory input and lies medial and slightly ventral to the central nucleus. Both of these areas are connected to separate areas of the bed nucleus of the stria terminalis via fibers through the sublentricular portion of the substantia innominata (called the sublentricular extended amygdala). The bed nucleus itself is a small collection of neurons collected along the midline extending rostrally from the amygdala to the posterior commissure.
Olfactory Amygdala – de Olmos further characterizes a third division of the amygdala, the olfactory amygdala. This division receives input from the olfactory bulb and olfactory cortex and projects mainly to the central and medial nuclei of the extended amygdala; it has been subdivided into two nuclei and two areas.
The two nuclei are the ventral and anterior cortical amygdaloid nuclei. The anterior portion borders the piriform cortex rostrally and the medial nucleus caudally. Ventral and caudal to this nucleus is the ventral nuclei, which is proximal to the lateral olfactory tract. The two areas of the olfactory amygdala are the amygdalopiriform transition and amygdalohippocampal areas. These lie between the amygdala and, respectively, the entorhinal cortex and the uncus of the hippocampus.
Rat Amygdala
One apparent difference between the rat and human amygdala is its orientation. Within the rat, the nuclei seem to be rotated about 70-80 degrees such that the central and medial nuclei lie medial to the laterobasal amygdaloid nuclear complex. Within this nuclear complex the nuclei are also rotated such that the basomedial nucleus lies more medial. In addition, the entire amygdala is more lateral in the rat than the human, though this may be due to the relatively smaller cortex giving the appearance of a more lateral structure.
There are differences in terminology. For example, the designations of basomedial and basolateral nuclei are used in rats, but often in humans they are called the basal accessory amygdala and basal nucleus respectively. Also, some structures are not readily apparent in both species. The medial nucleus while apparent in primates is not as prominent as it is in rodents. There are intercalated cell masses dispersed through the amygdala. However, while these exist in both species, they are not as well defined in rodents as in humans. While a distinction is made in the human brain between the medial and ventral cortical nuclei, Kruger only refers to a single cortical amygdaloid nucleus. (Paxinos, on the other hand does distinguish two cortical nuclei, but calls them the anterior and posterior cortical nuclei). Furthermore, neither rat atlas distinguishes perilaminar nucleus from the rest of the nuclear complex.
One apparent difference between the rat and human amygdala is its orientation. Within the rat, the nuclei seem to be rotated about 70-80 degrees such that the central and medial nuclei lie medial to the laterobasal amygdaloid nuclear complex. Within this nuclear complex the nuclei are also rotated such that the basomedial nucleus lies more medial. In addition, the entire amygdala is more lateral in the rat than the human, though this may be due to the relatively smaller cortex giving the appearance of a more lateral structure.
There are differences in terminology. For example, the designations of basomedial and basolateral nuclei are used in rats, but often in humans they are called the basal accessory amygdala and basal nucleus respectively. Also, some structures are not readily apparent in both species. The medial nucleus while apparent in primates is not as prominent as it is in rodents. There are intercalated cell masses dispersed through the amygdala. However, while these exist in both species, they are not as well defined in rodents as in humans. While a distinction is made in the human brain between the medial and ventral cortical nuclei, Kruger only refers to a single cortical amygdaloid nucleus. (Paxinos, on the other hand does distinguish two cortical nuclei, but calls them the anterior and posterior cortical nuclei). Furthermore, neither rat atlas distinguishes perilaminar nucleus from the rest of the nuclear complex.
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