Neuroanatomy PDF

Isabella Ronchi Ñëürøåñåtømÿ The prosencephalic vesicle is going to divide into the diencephalon and telencephalon. The cerebral hemispheres need to be able to host a lot of neurons, therefore the telencephalic vesicles grow large. The two telencephalic vesicles gro...

Isabella Ronchi Ñëürøåñåtømÿ The prosencephalic vesicle is going to divide into the diencephalon and telencephalon. The cerebral hemispheres need to be able to host a lot of neurons, therefore the telencephalic vesicles grow large. The two telencephalic vesicles grow rostrally, bend dorsally, and then grow posteriorly and inferiorly, acquiring a C- shaped appearance. In the meantime, they envelop the diencephalic vesicle in the middle. The diencephalon is not any longer in series with the telencephalon, but it is in parallel. While they grow, they meet the bones of the skull. The lobes of the cortex take their name from the bones they are closer to the frontal lobe, the parietal lobe, the occipital lobe, and the temporal lobe. Because of the change in shape of the vesicles, the cavity (lateral ventricle) within each of the telencephalic vesicles, also acquire a C-shaped appearance. The lateral ventricles have a frontal horn, a central horn, and an inferior horn, which all converge to the atrium. Communication between the third ventricle and the lateral ventricles takes place at the level of the intraventricular foramen. The third ventricle then communicates with the fourth ventricle through the aqueduct of Sylvius. Phylogenetically speaking, the brain is not homogenous. In cyclostomata there is a primitive pallium, and the cortex is poorly developed. In amphibians, the cortex starts to differentiate into the archipallium and the paleopallium. In reptiles the archicortex changes its position and the paleocortex becomes bigger; the neopallium appears. In mammals the neocortex expands so much that the archicortex and paleocortex get squeezed medially and ventrally at the level of the telencephalic vesicles. The paleocortex corresponds to areas of the cortex that process olfactory information, so it was much more important for less developed mammals. In humans, small and located inferiorly and ventrally. Ontogenesis retraces phylogenesis. The oldest portion of the cortex, the hippocampus, is in the floor of the inferior horn of the fourth ventricle. At the initial phases of embryonic development, the oldest part of the cortex is located dorsally, but it moves ventral and medial into the temporal lobe leaving behind a trail of fibres (fornix) to maintain communication with the region of the basal forebrain and the mammillary nuclei of the  Isabella Ronchi hypothalamus. The choroid fissure, in which the choroid plexus is forming, acquires a C-shape within the concavity of the arch formed by the fornix. In the region of the basal forebrain is the origin of the corpus callosum, which keeps the two hemispheres in communication. The corpus callosum contains commissural fibres that run horizontally. Because of the bending of the two telencephalic vesicles, they surround the diencephalic vesicle, and they fuse with it. The cortex can now project directly to the brainstem without crossing the diencephalon. During this process of bending and fusion, the region of the telencephalon that proliferates gives rise to the ganglionic/basal eminence or corpus striatum. At some point, neurons of the cortex form connections with the neurons of the brainstem and the spinal cord, so their axons must pass through the ganglionic eminence. At the same time the thalamus starts to project to the cortex and the axons of neurons of the thalamus need to pass to the ganglionic eminence. This bundle of ascending and descending fibres forms the internal capsule, that crosses the corpus striatum. Part of the basal eminence remains medial (caudate nucleus) to the internal capsule and part remains lateral (putamen). The internal capsule has an anterior limb, a genu and a posterior limb. The retrolenticular division and the sublenticular division are portions of the interal capsule that pass below or behind the lenticular nucleus. With completion of development, only the ventral portion of the diencephalon (that corresponds to the hypothalamus) remains visible. Expansion of the cerebral hemispheres is not uniform. A portion of the wall of the telencephalic vesicle as the other: it is the insula or hidden lobe. The cortex adjoining it grows over it, making it invisible from outside. The portions of the parietal, temporal and frontal lobes that join together to cover it are called opercula. The putamen is located -  Isabella Ronchi Further expansion of the cerebral hemispheres leads to formation of sulci that separate lobes and gyri of the cortex. The three major sulci are: the central sulcus (of Rolando) separates the frontal lobe from the parietal lobes, the lateral sulcus (of Sylvius) separates the temporal lobe from the others, the calcarine sulcus is in the occipital lobe. On the medial surface the limbic lobe separated by the adjoining lobes by several sulci and grooves. The limbic lobe includes areas of the frontal, parietal and temporal lobes. It consists of the cingulate gyrus and the parahyppocampal gyrus. The parahyppocampal gyrus terminates with a U-shaped region called uncus. The cingulate gyrus is in close relationship with the corpus callosum. In the frontal lobe, the precentral gyrus contains the primary motor cortex. The most anterior part of the In this area, there are the superior, middle and inferior frontal gyri. The superior frontal gyrus contains the premotor cortex, for orientation, eye and head movements. The inferior frontal gyrus is divided into an opercular part, a triangular part and an orbital part. A lesion in the frontal lobe results in loss of motor function and changes in personality. In the parietal lobe, posterior to the central sulcus, is the postcentral gyrus, that contains the primary somatosensory cortex. The rest of the parietal lobe is divided into the superior perietal lobule and inferior parietal lobule, which is subdivided into the supramarginal gyrus and the angular gyrus. The superior parietal lobule is important for body scheme, while the inferior parietal lobule is important for language. In the temporal lobe there are the superior, middle and inferior temporal gyri. The superior temporal gyrus contains the primary auditory cortex, surrounded by the secondary  Isabella Ronchi auditory cortex. A lot of the temporal cortex is involved in understanding what we see. The occipital lobe is all devoted to vision. On the medial aspect of the occipital lobe there is the calcarine sulcus and the cortex around it is the primary visual cortex. The calcarine sulcus divides the cortex around it into the cuneus superiorly and the lingual gyrus inferiorly. On the ventral surface of the frontal lobe there are orbital gyri, that are in relationship with the orbital cavities. The straight gyri are also in relation with the orbital cavity. There also olfactory bulbs. The anterior and posterior perforated substances are perforated to allow passage of the deep vessels that enter the brain parenchyma from the circle of Willis. The hippocampus is located medially in the temporal lobe. It stretches on the floor of the inferior horn of the lateral ventricle in continuity with the structures of the basal forebrain and mammillary bodies of the hypothalamus through the fornix, a system of white matter. The fornix is located below the corpus callosum. The hippocampus is coiled on itself, because originally it was dorsal and lateral and was squeezed ventral and medial. Medially and dorsally to the hippocampus, there is a choroid plexus in the floor of the inferior horn of the lateral ventricle. The hippocampus (cornu Ammonis) belongs to the hippocampal formation, which contains the dentate gyrus and the subiculum. The hippocampus is important for memory and learning. The subicular cortex is important to receive all the information coming from different areas of the cortex and to feed it to the dentate gyrus and hippocampus. The hippocampus is a very sensitive region of the brain. It is damaged very early in Alzheimer. Damage can be caused by hypoxia, encephalitis, and medial temporal lobe  Isabella Ronchi epilepsy. Bilateral hippocampal damage causes , a memory disorder characterised by inability to retain new information (anterograde amnesia) and a less severe defect in recall of old memories (retrograde amnesia). Hippocampal amnesia affects more severely episodic memory and less semantic memory. Semantic memory is memory for facts (e.g., Kabul is the capital of Afghanistan), while episodic memory is memory for personally experienced events (e.g., remembering where you parked your car this morning). The amygdala is a large deep d in front of the hippocampus and is close to structures related to olfaction and belonging to the limbic system. Together with the limbic lobe, the amygdala belongs to the limbic system, important for emotions, learning, behaviour, learning From the amygdala there is the origin of a bundle of white matter, the stria terminalis, that continues dorsally to the thalamus and then to the forebrain. The Bed nucleus of the stria terminalis is involved in homeostasis, behaviour, fear, defensive responsive, mood disorders The amygdala is involved in mood disorders, that is dysregulation of the feelings of happiness and sadness. Individuals affected by unipolar depressions have increased blood flow in the amygdala and prefrontal cortex. The amygdala modulates the autonomic responses based on learning and previous experiences (especially emotionally charged). It also has a role in anxiety, PTSD, aggressive behaviour, alcoholism, and substance abuse. Klüver- Bucy syndrome is a rare disease due to bilateral lesions of mesial/anterior temporal lobe structures. It leads to memory loss, emotional changes, extreme sexual behaviour. Placidity, indifference It can be caused by trauma, encephalitis, temporal lobe epilepsy, Alzheimer The amygdala was a popular target during the era of psychosurgery, specifically for the treatment of intractable aggression. At the time, psychiatrists believed on prisoners. Brodmann areas to a functional classification. The white matter in the cerebral hemispheres is organised in: o Commissural fibres form the corpus callosum, which keeps the two hemispheres in communication. The corpus callosum connects homologous areas of the two hemispheres. Cranially and caudally, there are two areas of the corpus callosum called forceps minor and forceps major respectively. In between these two there are the radiated fibres. The induseum griseum is a strip of greyish matter,  Isabella Ronchi animals it corresponds to the dorsal hippocampus). In a sagittal view, the corpus callosum can be divided (from posterior to anterior) into the splenium, the trunk, the rostrum and the genu. Between the genu and the anterior column of the fornix there is a thin layer of tissue called septum pellucidum. The septum pellucidum forms the medial wall of the anterior horn of the lateral ventricle. The basal forebrain is located anteriorly and ventrally with the respect to the septum septal nuclei. The anterior commissure is anterior to the fornix. It can be divided into an anterior division and a posterior division. The anterior division connects the two olfactory tracts and olfactory cortexes. The posterior division connects middle gyri of the temporal lobes and the amygdalae and contains fibres from the stria terminalis. When the fibres of the fornix reach the area of the anterior commissure, they slip into pre- commissural fibres, directed to the septal nuclei and into post- commissural fibres that reach the mammillary bodies (of the hypothalamus) and the anterior nuclei (of the thalamus). The hippocampal formation is in relationship with the basal forebrain, which has to do with behaviour and decision making. Between the crura of the fornix there is a commissural system that keeps in communication the hippocampus on the right with the hippocampus on the left (commissure hippocampi). The septal nuclei receive fibres but also project cholinergic axons towards the fornix and the amygdala. In some degenerative pathologies, like Al cholinergic nuclei in the forebrain. The cholinergic projection to the hippocampus is reduced, basal nucleus of Maynert projects to the hippocampus, but also gives rise to an extensive projection to the neocortex. Another important nucleus is the nucleus accumbens, and corresponds to the ventral striatum. The nucleus accumbens is important for the reward systems. The two hippocampal formations are kept in contact by the commissure of the fornix. o Associative fibres connect areas of the same hemispheres. They can be short or long, depending on how far the two gyri to be connected are.  Isabella Ronchi o Projecting fibres are organised in capsules. In the brain there are the extreme capsule, between the insular cortex and the claustrum; the external capsule between the putamen and the claustrum and the internal capsule. The latter is made of fibres running from the thalamus to the cortex and from the cortex to the thalamus, brainstem and spinal cord. The section of the internal capsule that goes from the thalamus to the cortex is called corona radiata. The basal ganglia are a complex of nuclei located deep in the cerebral hemispheres. The basal ganglia are the caudate nucleus, the putamen and the pallidus. The caudate and the putamen are also called the striatum. The putamen and pallidus together are collectively the lenticular nucleus. All three together they are called corpus striatum. The oldest part of the corpus striatum corresponds to the nucleus accumbens (or ventral striatum) and is located in the basal forebrain. The caudate nucleus has an important relationship with the thalamus: the head of the caudate nucleus is located anterior to the thalamus, while the body of the caudate is located dorsal to the thalamus. The tail of the caudate nucleus runs below the thalamus and laterally. The basal ganglia are important for posture and movement control. The neocortex is organised in 6 layers, numbered from I to VI going from superficial to deep. The cerebral neocortex is largely excitatory. There are two main categories of neurons: pyramidal cells (75-80%) and nonpyramidal cells (or stellate cells). Pyramidal cells are characterised by an apical dendrite that extends to the layers above the cell body. The axon of pyramidal neurons abandons the cerebral cortex (corticofugal axon), but it leaves a recurrent collateral. All pyramidal cells are excitatory. Nonpyramidal cells are mostly located in the granular layers (II and IV). Among nonpyramidal cells, there are inhibitory and excitatory interneurons. Excitatory nonpyramidal cells are spiny, while inhibitory nonpyramidal cells are smooth. Nonpyramidal neurons are important because they modulate the activity of the pyramidal cells. The pyramidal neurons develop from the ventricular zone, while nonpyramidal neurons originate from migration from the ganglionic eminence. Interkinetic nuclear migration in the neuroepithelium leads to a proliferation of progenitor cells, which leave the proliferation cycle and progressively differentiate into neurons or glial cells. When they exit the  Isabella Ronchi proliferation cycle, they move to their final position using apical radial glia. progenitor cells themselves. The differentiating neurons organise in three layers around the lumen of the neural tube. The innermost layer is the ventricular layer (ependymal layer). The majority of the neuron cell bodies cluster in the mantle layer, while the axons are found prevalently in the marginal layer, the outermost. In the cortex, instead, there are six layers. At some point, there is a first wave of migration, meaning that some differentiating cells migrate more superficially in the preplate region, that eventually will become layer I. The axons from the preplate region are located in the intermediate zone, which will become the white matter. In the second wave of migration, other neurons leave the replicative cycle and move to the cortical plate. They split the preplate into two compartments, a marginal zone and a subplate region. The cortical plate correspond two layers V-VI. The later born neurons will have to cross the cortical region to form layers IV, III and II. An additional layer, the subventricular zone, forms for proliferating cells. This process is known as inside-out formation of the cortical layers. Apart from layering, correct expansion of the cortex also has to take place for proper development. The process of gyrification in upper vertebrates is promoted by basal radial glial cells, which impose a tangential migration pathway to neurons and consequent tangential expansion and folding. The basal radial glia add in a fan-like manner to the pre-existing scaffold. The process of migration of differentiating neurons can be disrupted, leading to a defective development of the cortex. Migration takes place thanks to a link between the nucleus, the microtubule network and the centrosome to enable movement. A microtubule cage surrounds the nucleus and links with the centrosome using the microtubule network. The lissencephaly 1 protein functions as a bridge. The nonpyramidal cells reach the layer of the cortex from a different source, the ganglionic eminence. They tangentially migrate within the cortical layers and arrange erall microcircuitry of the cortex cannot function. In primates, cortical interneurons originate also from the ventricular zone. The layers in the neocortex do not have the same thickness in different regions of the cortex, and also different regions of the cortex do not have the same thickness. For instance, sensory information that reaches the cortex from the thalamus end up in layer IV. Layer IV of the primary sensory cortex is much thicker than in the primary motor cortex. Layer V is much thicker in the primary motor cortex, and it contains giant pyramidal cells and is important for the pyramidal tract. Agranular are the areas of the cortex where there are only few granule cells (nonpyramidal cells), while granular contain prevalently nonpyramidal cells.  Isabella Ronchi Homotypical means that all the six layers are represented and well distinguishable, while heterotypical are regions of the neocortex where some of the layers are not as distinguishable. During mammals phylogenesis the paleopallium and archipallium decreased in size, while the neopallium became dominant occupying almost the whole surface of the brain. The human cortex can be divided into: isocortex (the neocortex) and allocortex (paleopallium and archipallium). The paleocortex is the oldest cortical area of the telencephalon which contains 3 to 5 layers of neuronal cell bodies. It includes the olfactory bulb, olfactory tubercle and the piriform cortex. All those cortical and non-cortical areas which are related to the sense of smell are summarised as the rhinencephalon or olphactory brain. The archicortex is constituted by 3 to 4 layers of neurons and contains the hippocampal formation. It includes the dentate gyrus, the cornu Ammonis and the subiculum. The archicortex is basically organised in three layers: the polymorph layer, the granular layer and the molecular layer in the dentate gyrus. In the hippocampus, the granular layer is substituted by the pyramidal layer. The frontal horn of the lateral ventricles is delimited medially by the septum pellucidum, anteriorly by the genu of the corpus callosum, inferiorly by the rostrum of the corpus callosum and laterally by the head of the caudate nucleus. The central part of the lateral ventricles is delimited inferiorly by the dorsal surface of the thalamus, medially by the septum pellucidum and by the corpus callosum. The posterior horn extends llosum. The inferior horn is delimited inferiorly by the hippocampus, superiorly by the tail of the caudate nucleus and stria terminalis, medially by the choroid fissure. Communication between the third ventricle and the lateral ventricles takes place through the interventricular foramina of Monro. They are located on each side at the junction of the roof and anterior wall of the third ventricle. The foramen is bounded anteriorly by the junction of the column and the body of the fornix and the anterior pole of the thalamus posteriorly. The ventricles are filled with CSF, which appear black in MRI and CT scans, so they can be used as reference points. In other types of weighting (T2), the ventricles may also appear white. Sometimes in the choroid plexuses of the lateral ventricles there can be formation of cysts. They do not give any clinical problems. The diencephalon gives rise to the thalamus (and metathalamus), hypothalamus, epithalamus and subthalamus. During development the diencephalon remains medial to the internal capsule. The thalamus is made of two lateral ovoid structures, that come in contact with each other at the inter-thalamic adhesion. The most posterior and largest portion of the thalamus is the pulvinar region. Between the two thalamic bodies there is the tela choroidea of the fourth ventricle. Close to where the tela choroidea comes in contact with the medial side of the thalamus, there is a stripe of white matter called stria medullaris, through which the epithalamus communicates with other areas of the brain. The stria terminalis runs over the dorsal surface of the thalamus. The anterior pole of the thalamus is in contact with the columns (pillar) of the fornix. Between the thalamic ovoids posteriorly there is the epithalamus, made by the pineal gland and by a small region called trigone of the habenula. The trigone of the habenula gives rise to the stria terminalis and is part of the limbic system. The habenular commission keeps in communication the habenular nuclei on the right and on the left. Ventral to the pulvinar region there are the lateral geniculate bodies, that form the metathalamus. Below the pineal gland, where the cerebral aqueduct opens in the third ventricle, there is the posterior commissure, important for the consensual response of both eyes to a light stimulus. It keeps  Isabella Ronchi in communication the pretectal areas on both sides. Other fibres connect the two superior colliculi and the enter in the medial longitudinal bundle of the brainstem. Below the posterior commissure, where the Sylvian aqueduct opens in the third ventricle, there is a sub- commissural organ, which produces substances to keep the patency of the mesencephalic aqueduct and ventricular system. The sub-commissural organ secretes a protein that binds a compound that needs to be cleared from the CSF. If the mesencephalic aqueduct is closed or narrowed, fluid accumulates in the lateral ventricles and in the third ventricle, leading to hydrocephalus. The grey matter of the thalamus can be divided in many nuclei, based on their position with respect to the internal medullary lamina. They ovoids can be divided into an anterior, a medial and a lateral compartment. Within the core of the medial compartment there is a Y-shaped lamina of white matter called internal medullary lamina. Within the internal medullary lamina there are areas of grey matter that form the intralaminar nuclei. There is also an external medullary lamina, which envelopes the thalamus laterally and separates the rest of the nuclei from the reticular nucleus. Functionally speaking, all the nuclei of the thalamus can be divided into: o Specific nuclei: they project to a specific area of the cortex o Association nuclei: they project to associative areas of the cortex o Non-specific nuclei: they project to many areas (intralaminar nuclei). This is an old definition. The thalamic peduncles are bundles of white matter that connect the thalamus to the cortex. They are part of the corona radiata, the portion of the internal capsule between the thalamus and the cortex. The subthalamus is the continuation of the tegmentum of the midbrain below the thalamus.