Limbic System

The limbic system is a neuronal network that mediates emotion and motivation, while also playing a role in learning and memory. The extended neural network is vital to numerous basic psychological functions and plays an invaluable role in processing and responding to environmental stimuli.

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Editorial responsibility: Stanley Oiseth, Lindsay Jones, Evelin Maza

Table of Contents

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Structure and Function

The limbic system is a highly integrated system of structures with feedback mechanisms to appropriately balance the continuous stream of information that is received.

Structure

The limbic system is composed of several structures that produce subconscious responses to stimuli:

  • Olfactory pathways:
    • Input to the primary olfactory cortex via the uncus
    • Direct input to the amygdala: Olfactory emotions are associated with behavior.
    • Indirect input to the hippocampus: olfactory component of memory
    • Hippocampal to hypothalamic: mediates visceral responses to odors
    • Orbitofrontal cortex: olfactory perception of odors
  • Hippocampal formation:
    • Composed of the dentate gyrus, hippocampus proper, and entorhinal cortex
    • Location: floor of the inferior horn of the lateral ventricle
    • Inputs: dentate gyrus (input gate), amygdala, cingulate gyrus, and prefrontal cortex
    • Function: long-term memory and spatial navigation
  • Amygdala:
    • Located in the medial temporal lobe
    • Function: uses memory of stimuli to provide appropriate emotional responses upon reexposure to a given stimulus
  • Hypothalamus Hypothalamus The hypothalamus is a collection of various nuclei within the diencephalon in the center of the brain. The hypothalamus plays a vital role in endocrine regulation as the primary regulator of the pituitary gland, and it is the major point of integration between the central nervous and endocrine systems. Hypothalamus:
    • Communicates with the amygdala and prefrontal cortex
    • Controls the physiological aspects of emotion (perspiration, heart rate)
  • Septal nuclei of the limbic system: reciprocal connections from the olfactory bulb, hippocampus, and amygdala
  • Cingulate gyrus: modulation of the ANS and development of motivation
  • Mammillary bodies: mediate affective and sexual behavior

Functions

  • Mediates numerous functions including:
    • Emotion
    • Memory
    • Attention
    • Olfaction Olfaction The sense of smell, or olfaction, begins in a small area on the roof of the nasal cavity, which is covered in specialized mucosa. From there, the olfactory nerve transmits the sensory perception of smell via the olfactory pathway. This pathway is composed of the olfactory cells and bulb, the tractus and striae olfactoriae, and the primary olfactory cortex and amygdala. Olfaction
    • ANS function
    • Feeding behavior
    • Mating behavior
  • The limbic system function can be remembered using the 5 “Fs”:
    • Feeding (hunger and satiety)
    • Forgetting (memory)
    • Fighting (emotional response)
    • Family (sexual reproduction and maternal instincts)
    • Fornicating (sexual arousal)

Papez Circuit

The Papez circuit is an interconnected network within the brain that is associated with many emotional processes. The Papez circuit involves the hippocampus, mammillary bodies, thalamus Thalamus The thalamus is a large, ovoid structure in the dorsal part of the diencephalon that is located between the cerebral cortex and midbrain. It consists of several interconnected nuclei of grey matter separated by the laminae of white matter. The thalamus is the main conductor of information that passes between the cerebral cortex and the periphery, spinal cord, or brain stem. Thalamus, and cingulate gyrus.

  • The Papez circuit is a modulator circuit involved in several functions, including:
    • Fear
    • Anxiety
    • Sadness
    • Happiness
    • Sexual pleasure
  • Consists of a number of limbic structures connected in a loop to modulate the above-mentioned behaviors:
    • Hippocampus → mammillary bodies → anterior nucleus of the thalamus Thalamus The thalamus is a large, ovoid structure in the dorsal part of the diencephalon that is located between the cerebral cortex and midbrain. It consists of several interconnected nuclei of grey matter separated by the laminae of white matter. The thalamus is the main conductor of information that passes between the cerebral cortex and the periphery, spinal cord, or brain stem. Thalamus → cingulate gyrus → hippocampus
    • The circuitry allows for memories to be stored based on the emotion produced by a stimulus.
Circuitry of the papez loop

Circuitry of the Papez loop

Image by Lecturio.

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Clinical Relevance

  • Alzheimer disease Alzheimer disease As the most common cause of dementia, Alzheimer disease affects not only many individuals but also their families. Alzheimer disease is a progressive neurodegenerative disease that causes brain atrophy and presents with a decline in memory, cognition, and social skills. Alzheimer Disease: a chronic neurodegenerative disease due to the loss of neurons in the cerebral cortex Cerebral cortex The cerebral cortex is the largest and most developed part of the human brain and CNS. Occupying the upper part of the cranial cavity, the cerebral cortex has 4 lobes and is divided into 2 hemispheres that are joined centrally by the corpus callosum. Cerebral Cortex and certain subcortical regions, resulting in gross atrophy and degenerative changes. The clinical features are memory impairment, loss of executive function and judgment, impaired cognitive function, and behavioral changes. Diagnosis is based on clinical examination, neuropsychiatric testing, and imaging. There is no cure, but the management of symptoms with medications may slow disease progression.
  • Olfaction Olfaction The sense of smell, or olfaction, begins in a small area on the roof of the nasal cavity, which is covered in specialized mucosa. From there, the olfactory nerve transmits the sensory perception of smell via the olfactory pathway. This pathway is composed of the olfactory cells and bulb, the tractus and striae olfactoriae, and the primary olfactory cortex and amygdala. Olfaction: begins in a small area on the roof of the nasal cavity, which is covered with specialized mucosa. From this specialized area, the olfactory nerve transmits the sensory perception of smell via the olfactory pathway, which is composed of olfactory cells and olfactory bulb, tractus and striae olfactoriae, and the primary olfactory cortex and amygdala.
  • Sexual physiology Sexual physiology Sexual physiology and development begin in early childhood and represent a complex process of events that lead to the final development of sexual orientation and behavior. Sexual behavior and interactions include several changes that are quite different between males and females. Sexual Physiology: sexual physiology and development begin from early childhood, representing a complex process of events that leads to the final development of sexual orientation and behavior. Sexual behavior and interactions include several changes that are quite different between males and females. 
  • Wernicke encephalopathy Wernicke encephalopathy Wernicke encephalopathy is an acute, reversible condition that is caused by severe thiamine deficiency. This condition is most commonly seen in alcohol abusers. The classic triad of symptoms is encephalopathy, oculomotor dysfunction, and gait ataxia, although all 3 features are only present in one-third of patients. Wernicke Encephalopathy and Korsakoff Syndrome and Korsakoff syndrome Korsakoff syndrome Korsakoff syndrome is a severe and late neuropsychiatric manifestation of Wernicke encephalopathy. Korsakoff syndrome presents with personality changes, anterograde and retrograde amnesia, and confabulation. Some of these changes are irreversible. Wernicke Encephalopathy and Korsakoff Syndrome: occur due to a severe thiamine deficiency. It has an acute presentation and is reversible. This condition is most commonly seen in individuals with alcohol use disorder Alcohol use disorder Alcohol is one of the most commonly used addictive substances in the world. Alcohol use disorder (AUD) is defined as pathologic consumption of alcohol leading to impaired daily functioning. Acute alcohol intoxication presents with impairment in speech and motor functions and can be managed in most cases with supportive care. Alcohol Use Disorder. The classic triad of symptoms is encephalopathy, oculomotor dysfunction, and gait ataxia. Korsakoff syndrome Korsakoff syndrome Korsakoff syndrome is a severe and late neuropsychiatric manifestation of Wernicke encephalopathy. Korsakoff syndrome presents with personality changes, anterograde and retrograde amnesia, and confabulation. Some of these changes are irreversible. Wernicke Encephalopathy and Korsakoff Syndrome is a severe presentation of Wernicke encephalopathy Wernicke encephalopathy Wernicke encephalopathy is an acute, reversible condition that is caused by severe thiamine deficiency. This condition is most commonly seen in alcohol abusers. The classic triad of symptoms is encephalopathy, oculomotor dysfunction, and gait ataxia, although all 3 features are only present in one-third of patients. Wernicke Encephalopathy and Korsakoff Syndrome and is attributed to chronic thiamine deficiency and alcohol use disorder Alcohol use disorder Alcohol is one of the most commonly used addictive substances in the world. Alcohol use disorder (AUD) is defined as pathologic consumption of alcohol leading to impaired daily functioning. Acute alcohol intoxication presents with impairment in speech and motor functions and can be managed in most cases with supportive care. Alcohol Use Disorder. Korsakoff syndrome Korsakoff syndrome Korsakoff syndrome is a severe and late neuropsychiatric manifestation of Wernicke encephalopathy. Korsakoff syndrome presents with personality changes, anterograde and retrograde amnesia, and confabulation. Some of these changes are irreversible. Wernicke Encephalopathy and Korsakoff Syndrome presents with personality changes, anterograde and retrograde amnesia, and confabulation.

References

  1. Seeley, W.W., Miller, B.L. (2018). Alzheimer’s disease. In Jameson, J.L., et al. (Eds.), Harrison’s Principles of Internal Medicine. 20th ed. Vol 2. pp. 3108-3114.
  2. Wolk, D.A., Dickerson, B.C. (2020). Clinical features and diagnosis of Alzheimer disease. UpToDate. Retrieved August 3, 2021, from https://www.uptodate.com/contents/clinical-features-and-diagnosis-of-alzheimer-disease
  3. Keene, C.D., Montine, T.J., Kuller, L.H. (2018). Epidemiology, pathology, and pathogenesis of Alzheimer disease. UpToDate. Retrieved August 3, 2021, from https://www.uptodate.com/contents/epidemiology-pathology-and-pathogenesis-of-alzheimer-disease
  4. Forcier, M. (2020). Adolescent sexuality. UpToDate. Retrieved June 22, 2021, from https://www.uptodate.com/contents/adolescent-sexuality

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