Neuroanatomy Lesson 8: Anatomy of the Limbic System PDF

NEUROANATOMY Lesson 8: Anatomy of the Limbic System Overview of the Limbic System The limbic system, the group of structures in the brain that governs emotions, motivation, olfaction (sense of smell), and behaviour. The limbic system is also involved in the formation of lo...

NEUROANATOMY Lesson 8: Anatomy of the Limbic System Overview of the Limbic System The limbic system, the group of structures in the brain that governs emotions, motivation, olfaction (sense of smell), and behaviour. The limbic system is also involved in the formation of long-term memory. The structures of the limbic system are found deep inside the brain, immediately below the temporal lobes and buried under the cerebral cortex, just above the brainstem. The limbic system consists of several interconnected components, including the thalamus, hypothalamus, basal ganglia, cingulate gyrus, hippocampus, and amygdala. Overview of the Limbic System It was called the limbic system; arising from the Latin word limbus, meaning edge. The limbic system was originally called the rhinencephalon (a combination of Greek terms meaning “nose brain”) because it was believed to be associated with one’s sense of smell. It is now considered only one part of the brain to regulate visceral, autonomic processes. In general, the limbic system assists in various processes relating to cognition; including spatial memory, learning, motivation, emotional processing, and social processing. Embryology of the Limbic System The particular embryologic origins can separate the currently defined major structures of the limbic system.The limbic system forms from different components that rise from the mesencephalon, diencephalon, and telencephalon: The mesencephalic components are from visual, auditory, and somatosensory inputs processed in the region. The diencephalic components are the hypothalamus, anterior thalamic nuclei, and habenular commissure. The telencephalic components contain the cortical and subcortical regions; which are the olfactory bulbs, hippocampus, parahippocampal gyrus, fornix, columns of the fornix, mammillary body, septum pellucidum, amygdala, cingulate gyrus, entorhinal cortex. Embryology of the Limbic System The first areas of the brain to fully develop are the brainstem and midbrain; they govern the bodily functions necessary for life, called the autonomic functions. At birth, these lower portions of the nervous system are very well developed, whereas the higher regions (the limbic system and cerebral cortex) are still rather primitive. Higher function brain regions involved in regulating emotions, language, and abstract thought grow rapidly in the first three years of life. Components of the Limbic System The limbic lobe refers to a specific group of anatomical structures found in the region of the cortex on the medial aspect of cerebral hemisphere forming a rim around the corpus callosum. It also includes cingulate and parahippocampal gyri. These structures also have interdependent functional similarities associated with the formation of memories and the expression of a variety of emotions. Components of the Limbic System There is no universal agreement on every component of the limbic system because of its complex interconnectedness with different parts of the brain. However, the primary components of the limbic system are: 1. Thalamus 2. Hypothalamus 3. Basal ganglia 4. Cingulate gyrus 5. Hippocampus 6. Amygdala Components of the Limbic System The lymbic system is able to complete its functions through intricate connections to other systems of the brain. Traditionally, it has been divided into two groups: a cortical and a subcortical component. Primary components of the Limbic System: 1. Thalamus (Review) The thalamus is a paired structure located at the centre of the limbic system and is the confluence of many neural pathways connecting to the cerebral cortex. It serves as the main relay station for information such as sensory and motor signals from and to the rest of the brain, mainly the cerebral cortex, as well as from and to other parts of the limbic system. It transforms sensory information into a more manageable form for higher brain functions. Primary components of the Limbic System: 1. Thalamus (Review) The thalamus also maintains alertness and directs a person’s attention to sensory events. Damage to the thalamus can cause motor impairments, tremors, attention problems, insomnia, memory loss, vision loss or light sensitivity, and disorders of the motor systems. Severe damage can result in a coma. Primary components of the Limbic System: 2. Hypothalamus (Review) The hypothalamus is located just below the thalamus. Its most basic function is in homeostasis (maintaining a steady internal state). It controls autonomic functions such as hunger, thirst, body temperature, blood pressure , and heart rate. It is the interface between the nervous and endocrine systems, regulates sexual activity and also controls the body’s response to stress. Damage to the hypothalamus can lead to aggression, acute stress, hypothermia, hyperthermia, fatigue, weight gain or loss, and a high or low sex drive. Primary components of the Limbic System: 2. Hypothalamus (Review) – Fibers and Nuclei The hippocampohypothalamic fibers connect the hippocampus with the mammillary bodies via the fornix. This pathway serves as the major output of the limbic system. There are also amygdalohypothalamic fibers that journey from the amygdaloid complex, travels caudal to the lentiform nucleus via the stria terminalis and enters the hypothalamus.  Primary components of the Limbic System: 2. Hypothalamus (Review) – Fibers and Nuclei There are several nuclei that make up the hypothalamus, that are closely related to the limbic system. – The preoptic nucleus regulates the secretion of gonadotropin releasing hormone (GnRH), which is important for sexual development. – The lateral nucleus modulates the feeding impulse (lesions associated with this nucleus has been associated with anorexia nervosa). – The dorsomedial and ventromedial nuclei are involved in the regulation of satiety, fear and sexual activity.  Primary components of the Limbic System: 2. Hypothalamus – Anterior Wall (subcallosal gyrus) On the anterior wall of the hypothalamus, anterior to the lamina terminalis and the anterior comissure it is found the subcallosal gyrus, that i is a relatively small gyrus. The subcallosal gyrus, is inferior to the rostrum (first part) of the corpus callosum and posterosuperior to the orbitofrontal cortex the cerebrum. The area also corresponds with sections of Broadman areas 24 and 32, and area 25. It is believed to be involved in depression. Primary components of the Limbic System: 3. Basal ganglia (Review) The basal ganglia are structures located at the top of the midbrain. They help to regulate voluntary movements, from running down a street to maintaining balance and posture. The basal ganglia are also involved in cognitive and emotional behaviour and are linked with the formation of habits and addictions. Damage to these structures can cause tremors, involuntary muscles movements, and abnormal posture and is associated with movement-based disorders such as Parkinson disease and Huntington disease. Primary components of the Limbic System: 4. Cingulate gyrus The cingulate gyrus is part of the limbic cortex, the outermost region of the limbic lobe. This area is believed to help regulate emotions, behaviour (primarily aggression), and pain as well as control autonomic motor function. Its frontal portion links sights and smells with memories of previous emotions. Primary components of the Limbic System: 4. Cingulate gyrus It is a “C” shaped structure that is divided into a prelimbic and an infralimbic cortex, an anterior cingulate and a retrosplenial cortex. The cingulate cortex commences ventral to the rostrum of the corpus callosum, curves rostrally then follows the genu of the corpus callosum to progress posteriorly to blend with the precuneus of the parietal lobe. Primary components of the Limbic System: 4. Cingulate gyrus The cingulate gyrus is separated from the corpus callosum by the callosal sulcus (inferiorly) and from the medial frontal gyrus and paracentral lobule by the cingulate sulcus superiorly. The cingulate sulcus is continuous with the marginal sulcus, which separates the paracentral lobule from the precuneus. It is believed that the cingulate gyrus is strongly associated with the perception of neuropathic pain and nociception. Cortical components (Limbic lobe): Parahippocampal gyrus It is located medial to the rhinal sulcus (an anterior continuation of the collateral sulcus) and the lateral occipitotemporal gyrus, lateral to the uncus (a hook-shaped limbic structure located on the anteromedial aspect of the parahippocampal gyrus, in the mediobasal region of the temporal lobe, lateral to the mammillary bodies, posterior perforated substance and cerebral crus -crus cerebri-, and just anterior to the lateral geniculate body).  Cortical components (Limbic lobe): Parahippocampal gyrus The area corresponds with several Brodmann areas such as the entorhinal cortex (area 27, 28), and areas 35, 36, 48 and 49. The parahippocampal gyrus provides a path of communication between the hippocampus and all cortical association areas through which afferent impulses enter the hippocampus. Area 48 Area 49 Cortical components (Limbic lobe): Orbitofrontal cortex The inferior surface of the frontal lobe rests on the roof of the orbit in the anterior cranial fossa: the orbitofrontal cortex. The olfactory bulb and tract can be found running along the olfactory sulcus. The olfactory sulcus separates the straight gyrus of the orbitofrontal cortex from the medial orbital gyrus. The orbitofrontal cortex perceives smell, which can also be involved in the formation of memories. Cortical components of the Limbic System: Hippocampal formation (cortical components) The hippocampal formation is an umbrella term used in reference to a specific cluster of structures. These structures are: – The hippocampus – The dentate gyrus – The subicular complex – The entorhinal cortex. Primary components of the Limbic System: 5. Hippocampal formation (cortical components) The hippocampus is the region of the brain associated with memory. The hippocampus is a bundle of grey matter, residing in the floor of the temporal horn of the lateral ventricle; that resembles a ram’s horn. It is a paired structure with two parts, each shaped like a seahorse and curving back in its respective lobe from the corresponding amygdala. The hippocampus is thought to be principally involved in storing long-term memories and in spatial processing and navigation. Damage to this region can lead to memory impairment and anterograde amnesia. People with Alzheimer disease have shown cell loss in the hippocampus. Hippocampal formation (cortical components): the dentate gyrus The dentate gyrus is a serrated grey matter structure that is found medial to the hippocampus and lateral to the parahippocampal gyrus as it travels along the floor of the temporal horn of the lateral ventricle. It extends anteriorly into the uncus and continues superomedially with the fimbria of the hippocampus and becomes the indusium griseum (a thin grey matter structure that covers the dorsal surface of the corpus callosum).  Hippocampal formation (cortical components): the subicular complex In a coronal section, the cornu ammonis (CA) is subdivided into three regions, CA1(adjacent to the subiculum), CA3 (proximal to the dentate gyrus and CA2 (between CA1 and CA3). The subicular complex is a region of the hippocampus (best appreciated in coronal section) that is made up of (from superficial to deep) a parasubiculum, presubiculum, and a subiculum. This complex contains pyramidal neurons that project to the entorhinal cortex and other parts of the hippocampal formation. Hippocampal formation (cortical components): the entorhinal cortex The entorhinal cortex (Brodmann 28) is made up of the anterior pole of the parahippocampal gyrus and the uncus and is preceded by the gyrus semilunaris. This cortex extends rostrocaudally from the anterior amygdala to parts of the hippocampal formation. It is a direct recipient of afferents stimulation from the olfactory bulb. Primary components of the Limbic System: 6. Amygdala Due to its close proximity to the hippocampus, the amygdala helps modulate memory storage, especially of memories associated with strong emotions. Damage to the amygdala can result in higher levels of aggression, irritability, poor emotional control, and an inability to recognize emotions. Damage can also affect a person’s ability to recognize fear, resulting in more risk- taking. Primary components of the Limbic System: 6. Amygdala – Anatomical characteristics The almond-shaped amygdala (amygdaloid body) is located anterosuperior to the temporal (inferior) horn of the lateral ventricle, inferior to the lentiform nucleus (putamen and globus pallidus interna and externa) and deep to the uncus. The apex of the tail of the caudate nucleus fuses and the amygdala merge in the roof of the temporal horn of the lateral ventricle. Primary components of the Limbic System: 6. Amygdala – Anatomical characteristics The amygdala can be subdivided into a large ventrolateral component and a smaller dorsomedial division. – The ventrolateral group has central and basolateral nuclei that link the corticomedial nuclei of the dorsomedial division to the entorhinal cortex. – The corticomedial nuclei receive sensory input from the olfactory bulb. Primary components of the Limbic System: 6. Amygdala – Anatomical characteristics From the posterior aspect of the amygdala, the stria terminalis emerges and pursues a concave pathway. It extends posteriorly along the ventral surfaces of the basal ganglia and thalamus. The stria terminalis travels superiorly in a posterior relation to the thalamus, it travels anteriorly along the dorsal or ventricular surface of the thalamus, between the thalamus and the caudate nucleus and rostral to the thalamostriate veins. These fibers allow communication between the amygdala and regions of the hypothalamus to regulate the fear and anxiety responses. Primary components of the Limbic System: 6. Amygdala – Uncus The uncus is the home of the amygdala. It is located on the inferior surface of the cerebrum, posteromedial to the temporal lobe, lateral to the posterior perforated substance and mammillary bodies, anterior to the lateral geniculate body and anterolateral to the mesencephalic midbrain.  Primary components of the Limbic System: 6. Amygdala – Uncus There are three major components of the uncus. – Posteriorly there is an intralimbic gyrus. – Anteriorly there is the uncinate gyrus – Between them: the tail of the dentate gyrus Primary components of the Limbic System: 6. Amygdala – Uncus The uncus is also related to two other gyri that are superficially related to the amygdala known as the gyrus semilunaris and the gyrus ambiens. The former is located medially and continuous with the lateral olfactory stria; while the latter is located laterally and is continuous with the lateral olfactory gyrus (thin grey matter covering of the lateral olfactory stria). Case Study in Amygdala – Agression https://www.youtube.com/shorts/FIe15oxGTDc Other components of the Limbic System: the Alveus The alveus is a thin veil of white matter covering the hippocampus, deep to the ependymal layer. The nerve fibers traveling through the alveus from the cornu ammonis unite on the medial surface to form the fimbria of the hippocampus. Other components of the Limbic System: the Alveus The fimbria continues their journey superomedially and become the fimbria of the fornix as the hippocampus terminates and the fornix begins ventrally to the splenium of the corpus callosum. It should be noted at this point where the crura of the fornix ascend posterior to the thalamus, they communicate with each other via the commissure of the fornix. The decussating fibers permit communication between the hippocampi of each side.  Other components of the Limbic System: the Habenular nucleus The Habenular nucleus lies deep to the Habenular commissure that resides in the suprapineal space (above the pineal gland and recess). This nucleus communicates with the rest of the limbic system via the stria medullaris thalami (along the midline of the roof of the third ventricle). The septum communicates superiorly with the septum pellucidum (separates the left and right lateral ventricles). It contains dorsal, medial, caudal and ventral groups of nuclei inferior to the septum pellucidum. Other components of the Limbic System: the Mammillary bodies The mammillary bodies are a pair of rounded structures found inferior to the floor of the third ventricle. They are posterior to the pituitary gland and the tuber cinereum (floor of the hypothalamus) and anterior to the posterior perforated substance and interpeduncular fossa. The mammillary bodies communicate with the limbic system via the postcommisural fornix (posterior fibers of the fornix) and by way of the mammillothalamic tract.  Circle of Papez In 1937, James Papez made a proposition that there had to be reciprocating interactions between the cerebral cortex and the hypothalamus in order for emotional behavior to be consciously perceived. This circuit involves communications between the entorhinal area, cingulate gyrus, mammillary nucleus, hippocampal formation and anterior thalamic nucleus.