Limbic System: Anatomy

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 Olfactory pathways Set of nerve fibers conducting impulses from olfactory receptors to the cerebral cortex. It includes the olfactory nerve; olfactory bulb; olfactory tract; olfactory tubercle; anterior perforated substance; and olfactory cortex. Olfaction: Anatomy:
  • Input to the primary olfactory cortex Olfactory cortex Basal forebrain and medial part of temporal lobe areas that receive synaptic inputs from the olfactory bulb. Olfaction: Anatomy via the uncus Uncus Cerebral Cortex: Anatomy
  • Direct input to the amygdala: Olfactory emotions are associated with behavior.
  • Indirect input to the hippocampus: olfactory component of memory Memory Complex mental function having four distinct phases: (1) memorizing or learning, (2) retention, (3) recall, and (4) recognition. Clinically, it is usually subdivided into immediate, recent, and remote memory. Psychiatric Assessment
  • Hippocampal to hypothalamic: mediates visceral responses to odors
  • Orbitofrontal cortex: olfactory perception Perception The process by which the nature and meaning of sensory stimuli are recognized and interpreted. Psychiatric Assessment 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 Memory Complex mental function having four distinct phases: (1) memorizing or learning, (2) retention, (3) recall, and (4) recognition. Clinically, it is usually subdivided into immediate, recent, and remote memory. Psychiatric Assessment and spatial navigation
• Amygdala:
  • Located in the medial temporal lobe Temporal lobe Lower lateral part of the cerebral hemisphere responsible for auditory, olfactory, and semantic processing. It is located inferior to the lateral fissure and anterior to the occipital lobe. Cerebral Cortex: Anatomy
  • Function: uses memory Memory Complex mental function having four distinct phases: (1) memorizing or learning, (2) retention, (3) recall, and (4) recognition. Clinically, it is usually subdivided into immediate, recent, and remote memory. Psychiatric Assessment 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 Heart rate The number of times the heart ventricles contract per unit of time, usually per minute. Cardiac Physiology)
• Septal nuclei of the limbic system: reciprocal connections from the olfactory bulb Olfactory bulb Ovoid body resting on the cribriform plate of the ethmoid bone where the olfactory nerve terminates. The olfactory bulb contains several types of nerve cells including the mitral cells, on whose dendrites the olfactory nerve synapses, forming the olfactory glomeruli. The accessory olfactory bulb, which receives the projection from the vomeronasal organ via the vomeronasal nerve, is also included here. Olfaction: Anatomy, hippocampus, and amygdala
• Cingulate gyrus: modulation of the ANS ANS The ans is a component of the peripheral nervous system that uses both afferent (sensory) and efferent (effector) neurons, which control the functioning of the internal organs and involuntary processes via connections with the CNS. The ans consists of the sympathetic and parasympathetic nervous systems. Autonomic Nervous System: Anatomy and development of motivation
• Mammillary bodies: mediate affective and sexual behavior

Functions

• Mediates numerous functions including:
  • Emotion
  • Memory Memory Complex mental function having four distinct phases: (1) memorizing or learning, (2) retention, (3) recall, and (4) recognition. Clinically, it is usually subdivided into immediate, recent, and remote memory. Psychiatric Assessment
  • Attention Attention Focusing on certain aspects of current experience to the exclusion of others. It is the act of heeding or taking notice or concentrating. Psychiatric Assessment
  • 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: Anatomy
  • ANS ANS The ans is a component of the peripheral nervous system that uses both afferent (sensory) and efferent (effector) neurons, which control the functioning of the internal organs and involuntary processes via connections with the CNS. The ans consists of the sympathetic and parasympathetic nervous systems. Autonomic Nervous System: Anatomy function
  • Feeding behavior
  • Mating behavior
• The limbic system function can be remembered using the 5 “Fs”:
  • Feeding (hunger and satiety)
  • Forgetting ( memory Memory Complex mental function having four distinct phases: (1) memorizing or learning, (2) retention, (3) recall, and (4) recognition. Clinically, it is usually subdivided into immediate, recent, and remote memory. Psychiatric Assessment)
  • Fighting (emotional response)
  • Family (sexual reproduction and maternal instincts)
  • Fornicating (sexual arousal)

Papez Circuit

The Papez circuit is an interconnected network within the brain Brain The part of central nervous system that is contained within the skull (cranium). Arising from the neural tube, the embryonic brain is comprised of three major parts including prosencephalon (the forebrain); mesencephalon (the midbrain); and rhombencephalon (the hindbrain). The developed brain consists of cerebrum; cerebellum; and other structures in the brain stem. Nervous System: Anatomy, Structure, and Classification 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: Anatomy, and cingulate gyrus.

• The Papez circuit is a modulator circuit involved in several functions, including:
  • Fear
  • Anxiety Anxiety Feelings or emotions of dread, apprehension, and impending disaster but not disabling as with anxiety disorders. Generalized Anxiety Disorder
  • 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 Nucleus Within a eukaryotic cell, a membrane-limited body which contains chromosomes and one or more nucleoli (cell nucleolus). The nuclear membrane consists of a double unit-type membrane which is perforated by a number of pores; the outermost membrane is continuous with the endoplasmic reticulum. A cell may contain more than one nucleus. The Cell: Organelles 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: Anatomy → cingulate gyrus → hippocampus
  • The circuitry allows for memories to be stored based on the emotion produced by a stimulus.

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 Neurons The basic cellular units of nervous tissue. Each neuron consists of a body, an axon, and dendrites. Their purpose is to receive, conduct, and transmit impulses in the nervous system. Nervous System: Histology 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: Anatomy and certain subcortical regions, resulting in gross atrophy Atrophy Decrease in the size of a cell, tissue, organ, or multiple organs, associated with a variety of pathological conditions such as abnormal cellular changes, ischemia, malnutrition, or hormonal changes. Cellular Adaptation and degenerative changes Degenerative Changes Spinal Stenosis. The clinical features are memory Memory Complex mental function having four distinct phases: (1) memorizing or learning, (2) retention, (3) recall, and (4) recognition. Clinically, it is usually subdivided into immediate, recent, and remote memory. Psychiatric Assessment impairment, loss of executive function and judgment Judgment The process of discovering or asserting an objective or intrinsic relation between two objects or concepts; a faculty or power that enables a person to make judgments; the process of bringing to light and asserting the implicit meaning of a concept; a critical evaluation of a person or situation. Psychiatric Assessment, 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: Anatomy: begins in a small area on the roof of the nasal cavity Nasal cavity The proximal portion of the respiratory passages on either side of the nasal septum. Nasal cavities, extending from the nares to the nasopharynx, are lined with ciliated nasal mucosa. Nose Anatomy (External & Internal), which is covered with specialized mucosa. From this specialized area, the olfactory nerve Olfactory nerve The 1st cranial nerve. The olfactory nerve conveys the sense of smell. It is formed by the axons of olfactory receptor neurons which project from the olfactory epithelium (in the nasal epithelium) to the olfactory bulb. Nose Anatomy (External & Internal) transmits the sensory Sensory Neurons which conduct nerve impulses to the central nervous system. Nervous System: Histology perception Perception The process by which the nature and meaning of sensory stimuli are recognized and interpreted. Psychiatric Assessment of smell Smell 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: Anatomy via the olfactory pathway, which is composed of olfactory cells and olfactory bulb Olfactory bulb Ovoid body resting on the cribriform plate of the ethmoid bone where the olfactory nerve terminates. The olfactory bulb contains several types of nerve cells including the mitral cells, on whose dendrites the olfactory nerve synapses, forming the olfactory glomeruli. The accessory olfactory bulb, which receives the projection from the vomeronasal organ via the vomeronasal nerve, is also included here. Olfaction: Anatomy, tractus and striae olfactoriae, and the primary olfactory cortex Olfactory cortex Basal forebrain and medial part of temporal lobe areas that receive synaptic inputs from the olfactory bulb. Olfaction: Anatomy 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 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 and development begin from early childhood, representing a complex process of events that leads to the final development of sexual orientation Orientation Awareness of oneself in relation to time, place and person. Psychiatric Assessment and behavior. Sexual behavior and interactions include several changes that are quite different between males and females. 
• Wernicke encephalopathy Encephalopathy Hyper-IgM Syndrome and Korsakoff syndrome Korsakoff syndrome An acquired cognitive disorder characterized by inattentiveness and the inability to form short term memories. This disorder is frequently associated with chronic alcoholism; but it may also result from dietary deficiencies; craniocerebral trauma; neoplasms; cerebrovascular disorders; encephalitis; epilepsy; and other conditions. Wernicke Encephalopathy and Korsakoff Syndrome: occur due to a severe thiamine Thiamine Also known as thiamine or thiamin, it is a vitamin C12H17N4OSCl of the vitamin B complex that is essential to normal metabolism and nerve function and is widespread in plants and animals Water-soluble Vitamins and their Deficiencies 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 Encephalopathy Hyper-IgM Syndrome, oculomotor dysfunction, and gait ataxia Gait ataxia Impairment of the ability to coordinate the movements required for normal ambulation (walking) which may result from impairments of motor function or sensory feedback. This condition may be associated with brain diseases (including cerebellar diseases and basal ganglia diseases); spinal cord diseases; or peripheral nervous system diseases. Friedreich Ataxia. Korsakoff syndrome Korsakoff syndrome An acquired cognitive disorder characterized by inattentiveness and the inability to form short term memories. This disorder is frequently associated with chronic alcoholism; but it may also result from dietary deficiencies; craniocerebral trauma; neoplasms; cerebrovascular disorders; encephalitis; epilepsy; and other conditions. Wernicke Encephalopathy and Korsakoff Syndrome is a severe presentation of Wernicke encephalopathy Encephalopathy Hyper-IgM Syndrome and is attributed to chronic thiamine Thiamine Also known as thiamine or thiamin, it is a vitamin C12H17N4OSCl of the vitamin B complex that is essential to normal metabolism and nerve function and is widespread in plants and animals Water-soluble Vitamins and their Deficiencies 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 An acquired cognitive disorder characterized by inattentiveness and the inability to form short term memories. This disorder is frequently associated with chronic alcoholism; but it may also result from dietary deficiencies; craniocerebral trauma; neoplasms; cerebrovascular disorders; encephalitis; epilepsy; and other conditions. Wernicke Encephalopathy and Korsakoff Syndrome presents with personality changes, anterograde and retrograde amnesia Retrograde amnesia Loss of the ability to recall information that had been previously encoded in memory prior to a specified or approximate point in time. This process may be organic or psychogenic in origin. Organic forms may be associated with craniocerebral trauma; cerebrovascular accidents; seizures; dementia; and a wide variety of other conditions that impair cerebral function. Wernicke Encephalopathy and Korsakoff Syndrome, and confabulation Confabulation Memory fabrication to make up for memory lapses Wernicke Encephalopathy and Korsakoff Syndrome.