Limbic system

Japanese: 大脳辺縁系 - だいのうへんえんけい（英語表記）limbic system

The limbic system plays an extremely important role in the basis of various human activities, such as memory, emotion, olfaction, and the maintenance of homeostasis. It is composed of various cortical and subcortical structures and has a long evolutionary history.

Anatomically, it is located inside the cerebral cortex and consists of the limbic cortex, which is invisible from the surface, the nucleus below it, and the nerve fiber connections that connect them. The limbic cortex consists of a circular structure surrounding the corpus callosum, and includes the parahippocampus, cingulate gyrus, medial orbitofrontal cortex, anterior insula, uncus, and temporal pole.

The hippocampal formation, which is connected to the parahippocampal gyrus by the dorsomedial pathway, includes the dentate gyrus, hippocampus, subiculum, etc. Areas located in the diencephalon include the hypothalamus, various thalamic nuclei including the anterior nucleus and mediodorsal nucleus, the habenula, and the mammillary body.

The amygdala, which has mutual connections with many of these areas, is also a core part of the limbic system. In addition, the olfactory cortex including the septal nucleus, the basal forebrain, the nucleus accumbens in the basal ganglia, and the ventral striatum including the caudate nucleus, putamen, and ventral part of the pallidum (globus pallidus) also make up part of the limbic system. Many of these areas also have strong mutual fiber connections with the brainstem.

Functionally, the limbic system is responsible for processing memory, emotions, motivation, and olfaction, as well as regulating homeostasis. First, the central areas in memory processing include the hippocampus and its surrounding areas, parts of the diencephalon such as the thalamus and mammillary bodies, and the basal forebrain. Many of these areas form a circuit in the medial limbic system that is densely packed with many neurons, called the Papez circuit, which runs from the hippocampus to the fornix to the mammillary bodies to the anterior thalamic nucleus to the cingulate gyrus to the hippocampus, and is deeply involved in the encoding (memorization) and retrieval (recall) of memories. In this circuit, the fornix refers to the fiber bundle (fiber connection) from the hippocampus to the mammillary body, and along with the mammilothalamic tract, which connects the mammillary body and thalamus, the inferior thalamic peduncle, which is close to the mammillothalamic bundle and forms a fiber bundle in the anterior part of the thalamus, and the temporo-pulvinar bundle, which passes through the pulvinar in the posterior part of the thalamus, it is deeply involved in various memory processing.

Pathologically, when some of these areas are damaged, symptoms of organic amnesia are often observed, and this is classified as temporal lobe amnesia, which involves damage to the hippocampus and surrounding areas; Korsakoff syndrome, which involves damage to the mammillary bodies or dorsomedial thalamic nucleus; and basal forebrain amnesia, which involves damage to the basal forebrain.

Symptoms of limbic system damage generally do not include significant impairment of short-term or procedural memory, and intelligence and language are within normal ranges, while anterograde amnesia, which is memory impairment for new events that occurred after the onset of the disease, and retrograde amnesia, which is memory impairment for events that occurred before the onset of the disease, are often observed, although to varying degrees. Case HM (whose real name was not disclosed at the time), who played an important role in proposing the two-store memory model of short-term and long-term memory, can be classified as having temporal lobe amnesia. The central areas involved in processing related to emotions and motivations include the amygdala, medial frontal orbital area, anterior insular cortex, and basal ganglia. Some of these areas form a circuit in the lateral limbic system called the Yakovlev circuit, which runs from the amygdala to the dorsal medial thalamic nucleus, medial frontal orbital area, uncinate fasciculus, anterior temporal lobe cortex, and amygdala, and are involved in various emotional processing. Furthermore, many of these areas are positioned as the core of neural networks known as the social brain and moral brain, which provides insight into how emotions, motivations, reward processing, and other factors are deeply involved in sociality and ethical judgment.

The amygdala is involved in primary emotions, that is, innate emotional responses that are necessary when detecting stimuli that may pose a threat to us, and is thought to play a role in controlling arousal. Experiments using monkeys have shown that bilateral temporal lobe resection, including the amygdala, results in extremely poor emotional and defensive responses, and abnormalities in sexual and eating behaviors. This condition is called Klüver-Bucy syndrome. Causes of amygdala damage in humans include encephalitis, amygdala resection (lobectomy) for the treatment of epileptic seizures, and Urbach-Wiethe disease, and case reports have shown that the amygdala is involved in learning and memory linked to emotions and rewards, as well as understanding facial expressions. These results are also supported by functional brain imaging studies using functional magnetic resonance imaging (fMRI).

The nucleus accumbens in the basal ganglia is one of the areas that receive projections from the amygdala, and is known to be involved in reward- and pleasure-related processing, and its relationship with dopamine metabolism has been pointed out. In addition, the medial orbital frontal cortex and anterior insular cortex are known to be widely involved in controlling and regulating autonomic nervous activity and endocrine activity, generating subjective emotional experiences, and even decision-making based on them, in response to activity from the amygdala. Damage to these areas can cause problems in various processes related to emotions and motivations, which can lead to abnormalities in subjective body sensations (interoceptive sensitivity), inappropriate decision-making, and personality disorders. The medial orbital frontal cortex and anterior insular cortex, along with the cingulate gyrus and various parts of the thalamus that have strong fiber connections to the brainstem, are also involved in controlling and regulating homeostasis.

The olfactory cortex, which is responsible for the sense of smell, plays an important role in the recognition of odors, which is one of the functions of the limbic system. First, substances taken into the nasal cavity are detected by olfactory cells in the olfactory epithelium, and the signal is transmitted to the olfactory bulb via the olfactory nerve, and then transmitted to the olfactory cortex, where it is recognized as an odor. The olfactory cortex has particularly strong nerve fiber connections with the amygdala, and plays an evolutionarily essential role in avoiding dangerous situations. In addition to the various pathologies associated with local brain damage as mentioned above, each part of the limbic system is known to be closely related to the symptoms of various mental and neurological disorders, such as schizophrenia, anxiety disorders, panic disorders, obsessive-compulsive disorder, dementia, Parkinson's disease, and epilepsy. →Memory disorders →Sense of smell →Social brain →Basal ganglia [Umeda Satoshi]

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Japanese:

大脳辺縁系は，記憶，情動，嗅覚，生体の恒常性維持機能など，人間のさまざまな活動の根幹を担うきわめて重要な役割を担っている。さまざまな皮質および皮質下構造から成り，進化的にも古い歴史をもつ組織である。

　解剖学的には，大脳皮質cerebral cortexの内側に位置しており，表面からは見えない辺縁皮質limbic cortexとその下の核nucleus，およびそれらを相互につなぐ神経線維連絡から成り立っている。辺縁皮質は，脳梁corpus callosumを取り巻く円環構造から成り立っており，海馬傍回parahippocampus，帯状回cingulate gyrus，前頭眼窩野内側部medial orbitofrontal cortex，島皮質前部anterior insula，鉤uncus，側頭極temporal poleなどを含む。

　海馬傍回に背内側経路で連絡をもつ海馬体hippocampal formationには，歯状回dendate gyrus，海馬hippocampus，海馬支脚（海馬台）subiculumなどが含まれる。間脳diencephalonに位置する部位としては，視床下部hypothalamus，視床前核anterior nucleusや視床背内側核mediodorsal nucleusなどを含む視床のさまざまな核，手綱habenula，乳頭体mammilary bodyなどが挙げられる。

　これらの多くの部位と相互連絡をもつ扁桃体amygdalaも，辺縁系における中核的な部位である。さらに，中隔核septal nucleusを含む嗅皮質olfactory cortex，前脳基底部basal forebrain，大脳基底核basal gangliaにおける側坐核nuculeus accumbens，尾状核caudate・被殻putamen・淡蒼球pallidum（globus pallidus）の腹側部を含む腹側線条体ventral striatumも辺縁系組織の一部を構成している。これらの多くの部位は，脳幹brainstemとも相互の強い線維連絡をもっている。

　大脳辺縁系は，機能的には記憶，情動・動因，嗅覚に関連する処理やホメオスタシスの調整の役割を担っている。まず，記憶の処理における中心的な部位としては，海馬およびその周辺領域，視床や乳頭体などの間脳の一部，前脳基底部が挙げられる。これらの部位の多くは，パペッツ回路Papez circuitとよばれる，海馬→脳弓→乳頭体→視床前核→帯状回→海馬という多くの神経細胞が密集する内側辺縁系における回路を構成しており，記憶の符号化（記銘）および検索（想起）処理に深く関与している。この回路における脳弓fornixは，海馬から乳頭体への線維束（線維連絡）を意味し，乳頭体と視床を結ぶ乳頭体視床束mammilothalamic tract，乳頭体視床束と近接し，視床の前部において線維束を構成する下視床脚inferior thalamic peduncle，視床後部における視床枕pulvinarを経由する側頭視床枕束temporo-pulvinar bundleとともに，記憶のさまざまな処理に深くかかわっている。

　病理学的には，これらの部位の一部が損傷を受けると，器質性健忘organic amnesiaの症状が認められる場合が多く，海馬およびその周辺部位の損傷を伴う側頭葉性健忘temporal amnesia，乳頭体や視床背内側核などの損傷を伴うコルサコフ症候群Korsakoff syndrome，前脳基底部の損傷を伴う前脳基底部健忘basal forebrain amnesiaなどに分類される。

　大脳辺縁系の損傷による症状としては，一般に短期記憶や手続き記憶の障害は顕著でなく，知能や言語なども正常範囲内である一方で，程度の差はあるものの，発症後に起こった新しい事象についての記憶障害である前向健忘anterograde amnesiaや，発症前に起こった出来事についての記憶障害である逆向健忘retrograde amnesiaが認められる場合が多い。短期記憶と長期記憶という記憶の二重貯蔵モデルtwo-store memory modelが提案されるうえで重要な位置づけとなった症例H.M.（当時，本名非公開）は，側頭葉性健忘に分類することができる。情動や動因に関連する処理を担う中心的な部位としては，扁桃体，前頭眼窩野内側部，島皮質前部，大脳基底核などが挙げられる。これらの部位の一部は，ヤコブレフ回路Yakovlev circuitとよばれる，扁桃体→視床背内側核→前頭眼窩野内側部→鉤状束→側頭葉皮質前部→扁桃体という外側辺縁系における回路を構成しており，情動にかかわるさまざまな処理に関与している。また，これらの部位の多くは，社会脳social brain，倫理脳moral brainとよばれる神経ネットワークの中核としても位置づけられており，情動や動因，報酬処理などがいかに社会性や倫理判断に深くかかわるかが理解できる。

　扁桃体は，一次感情，すなわちわれわれの身に危険が及ぶような刺激を察知する際に必要となる，生得的に備わった感情反応に関与する部位であり，覚醒度の制御にかかわる機能を担っているとされている。サルを用いた実験では，扁桃体を含む両側側頭葉切除により，情動反応や防衛反応がきわめて乏しくなり，性行動や食行動にも異常が認められることが明らかにされている。これは，クリューバー-ビューシー症候群Klüver-Bucy syndromeとよばれている。人間における扁桃体損傷の原因は，脳炎，てんかん発作の治療を目的とした扁桃体切除（ロベクトミー），ウルバッハ-ビーテ病Urbach-Wiethe diseaseなどであり，その症例報告から，扁桃体が情動や報酬と結びついた学習や記憶，顔表情の理解などに関与することが示されている。これらの結果は，機能的磁気共鳴画像（fMRI）を用いた脳機能画像研究によっても支持されている。

　大脳基底核における側坐核は，扁桃体から投射を受ける部位の一つであり，報酬や快楽に関連した処理を担う部位として知られ，ドーパミン代謝との関連が指摘されている。また，前頭眼窩野内側部や島皮質前部は，扁桃体の活動を受け，自律神経活動や内分泌活動の制御や調整をしたり，主観的な感動体験を生み出したり，さらにはそれらを基にした意思決定などにも幅広く関与することが知られている。これらの部位の損傷により，情動や動因にかかわるさまざまな処理に不具合が生じ，その結果として，主観的な身体感覚（内受容感覚interoceptive sensitivity）の異常，不適切な意思決定，人格障害などに発展する可能性が指摘されている。前頭眼窩野内側部や島皮質前部は，脳幹との強い線維連絡をもつ視床の各部位，帯状回とともに，ホメオスタシスの制御や調整にも関連する部位である。

　大脳辺縁系機能の一つである匂いの認識については，嗅覚機能を担う嗅皮質（嗅覚皮質）olfactory cortexが重要な役割を担っている。まず鼻腔に取り込まれた物質が，嗅上皮の嗅細胞によって感知され，その信号が嗅神経を通して嗅球olfactory bulbへと伝わり，それが嗅皮質に伝達されることで匂いとして認識される。嗅皮質は，とくに扁桃体と強い神経線維連絡をもっており，進化的にも危険な状況を回避するうえで必要不可欠な機能を担っている。辺縁系における各部位は，前述のような局所脳損傷に伴う各病態に加え，統合失調症，不安障害，パニック障害，強迫神経症，認知症，パーキンソン病，てんかんなど，さまざまな精神疾患や神経疾患の症状とも密接な関係があることが知られている。　→記憶障害　→嗅覚　→社会脳　→大脳基底核
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