Hippocampus and Limbic System PDF

HIPPOCAMPUS AND LIMBIC SYSTEM DR NADINE RAMPF | PHD [email protected] LECTURE OUTLINE Overview 04 Cortical structures 08 Hippocampal formation 09 Hippocampus 10 Papez circuit 14 Subcortical structures 15 Amygdala...

HIPPOCAMPUS AND LIMBIC SYSTEM DR NADINE RAMPF | PHD [email protected] LECTURE OUTLINE Overview 04 Cortical structures 08 Hippocampal formation 09 Hippocampus 10 Papez circuit 14 Subcortical structures 15 Amygdala 16 Septal area 18 Fibre bundles 19 Fornix 21 02 OBJECTIVES Identify, name and describe the macroscopic anatomy, position and relationships of the elements of the limbic system, and where applicable, the role of each Differentiate between cortical and non-cortical components of the limbic system Identify and name the components involved in the Papez memory circuit 03 LECTURE OUTLINE Overview Cortical structures Hippocampal formation Hippocampus Papez circuit Subcortical structures Amygdala Septal area Fibre bundles Fornix 04  Cingulate Gyrus Fornix Hypothalamus INTRODUCTION The term limbic system is applied to the parts of the cortical Thalamus and subcortical structures that form a ring around the upper end of the brainstem Formerly called rhinencephalon because of its association to olfaction The limbic cortex is phylogenetically the oldest part of the cerebral cortex Consist of primitive cortical tissue (allocortex) which consists of only three layers and surrounds the hilum of the cerebral hemisphere Septal area Second ring of transitional cortex (juxta-allocortex) between Olfactory Bulb the allocortex and the neocortex and consists of three to six Amygdala layers Mammilary Body The cortical tissue of remaining non-limbic portion of the Hippocampus hemisphere is the neocortex which is made up of six layers and most highly developed in man The limbic system plays a vital role in elaboration of emotional behaviour, drive, and memory 05 FUNCTIONS Emotional aspects of behaviour together with visceral responses accompanying these emotions, particularly the reactions of fear and anger and emotions associated with sexual behaviour which are necessary for: Self preservation: Procuring of food and eating Preservation of the species: Sex and rearing young ones Learning and retention of recent memory Integration of olfactory, visceral and somatic impulses reaching the brain The visceral responses following activities in limbic system are expressed through the hypothalamus by way of autonomic nervous system Because of visceral responses to activities in the limbic system, it is also known as visceral brain 06 COMPONENTS The structures forming the limbic system are interposed between the superolateral surfaces of the diencephalon and the inferomedial surfaces of the two cerebral hemispheres CORTICAL STRUCTURES Limbic lobe consisting of the: Cingulate gyrus Isthmus Parahippocampal gyrus Uncus Hippocampal formation which includes the: Hippocampus (cornu ammonis) Dentate gyrus Gyrus fasciolaris Indusium griseum SUBCORTICAL STRUCTURES Amygdaloid nuclear complex Septal region and nuclei Olfactory areas Hypothalamus esp. mammillary bodies Anterior nucleus of thalamus The cingulate gyrus is a ‘satisfaction centre’ of brain and associated with the feeling of satisfaction after a meal or after sexual intercourse 07 LECTURE OUTLINE Overview Cortical structures Hippocampal formation Hippocampus Papez circuit Subcortical structures Amygdala Septal area Fibre bundles Fornix 08 HIPPOCAMPAL FORMATION Components Hippocampus Dentate gyrus Gyrus fasciolaris Indusium griseum Medial and lateral longitudinal striae 09 HIPPOCAMPUS a.k.a Ram’s Horn or Ammon’s Horn/Cornu Ammonis Area of the cerebral cortex which has rolled into the floor of the inferior horn of the lateral ventricle during fetal life In the adult brain, it forms a longitudinal elevation in the floor of the inferior horn of the lateral ventricle and is continuous medially with the subiculum and parahippocampal gyrus Its anterior extremity is expanded and bears few grooves and intervening ridges Posteriorly it gradually narrows and ultimately ends beneath the splenium of the corpus callosum The ventricular surface is covered by a thin layer of white fibres called the alveus The fibres of alveus originate in the hippocampal cortex, course towards the medial border of hippocampus where they converge to form a narrow strip of white matter, the fimbria of the hippocampus Phylogenetically, the hippocampus represents the archicortex and consists of three layers: Superficial molecular layer Middle pyramidal cell layer Deep polymorphic cell layer 10  WHO WAS PATIENT H.M.? and what happens when you remove the hippocampus? https://www.youtube.com/watch?v=KkaXNvzE4pk 11 CONNECTIONS Afferents Hippocampus receives fibres mainly from the entorhinal area (area 28) Efferents The fornix is the main efferent tract of the hippocampus The fibres leaving the hippocampus pass: To the opposite hippocampus through the commissure of the fornix/hippocampal commissure To the septal and anterior hypothalamic regions To the mammillary body which sends impulses to the cingulate gyrus through the anterior nucleus of the thalamus FUNCTIONS Formerly regarded as the part of the olfactory system but it has no direct connections with the sense of smell in man The integrative centre which influences endocrine and visceral functions and emotional states through its connections with hypothalamus, septal nuclei, and the cingulate gyrus It was once considered as the seat of the soul It plays an important role in recent memory 12 OTHER COMPONENTS OF THE HIPPOCAMPAL FORMATION Dentate gyrus In the fetal brain develops as a further extension of the hippocampus and occupies the interval between the hippocampus and the parahippocampal gyri, lying deep to the fimbria It has a three-layered archicortex Its surface is toothed hence the name dentate gyrus Anteriorly the dentate gyrus runs medially across the inferior surface of uncus, i.e. tail of dentate gyrus Gyrus fasciolaris The posterior end of dentate gyrus is continuous with the splenial gyrus or gyrus fasciolaris Indisium griseum Gyrus fasciolaris continues as a thin layer of grey matter over the corpus callosum as the indusium griseum The indusium griseum is vestigial grey matter and contains two delicate longitudinal bands of white matter fibres buried in it, the medial and lateral longitudinal striae The subiculum Transition zone between three-layered archicortex and six-layered neocortex and receives input from the hippocampus 13 PAPEZ CIRCUIT Begins and ends with the hippocampal formation Includes the following limbic structures: Mammillary body/nucleus The anterior nucleus of the thalamus Cingulate gyrus Entorhinal cortex The route followed by the circuit of Papez: Hippocampal formation Mammillary nucleus The anterior nucleus of the thalamus Cingulate gyrus Entorhinal cortex The structures included in the Papez circuit and their connections constitute a harmonious mechanism that is essential for memory and emotion A modified version of the Papez circuit (Paul D. Maclean) also includes the amygdala 14 LECTURE OUTLINE Overview Cortical structures Hippocampal formation Hippocampus Papez circuit Subcortical structures Amygdala Septal area Fibre bundles Fornix Mammilothalamic tract 15 AMYGDALA Almond-shaped mass of grey matter underlying the rostral part of the parahippocampal gyrus on the anteriormost part of the roof of the inferior horn of lateral ventricle Continuous posteriorly with the tail of caudate nucleus and stria terminalis Connections Main afferents from primary olfactory regions Stria terminalis forms the main efferent tract Takes a circuitous route along with (but not functionally related to) the tail of caudate nucleus in close relation to the lateral ventricle until the level of anterior commissure, where majority of its fibres terminate in the septal area and anterior portion of the hypothalamus Other fibres join the anterior commissure and are distributed to the contralateral amygdaloid body Some fibres run caudally to reach the habenular nucleus through stria medullaris thalami Functions Control somatic responses to internal needs, drives or instincts Also believed to play an important role in smell-mediated sexual behaviour Stimulation produces excitability, fear and rage Bilateral damage reduces fear and increases sexual activity 16  THE AMYGDALA Fear, Anxiety and Aggression https://www.youtube.com/watch?v=IyLGLxfPRCs 17 SEPTAL REGION Located on the medial aspect of the frontal lobe beneath the genu and rostrum of corpus callosum and in front of the lamina terminalis The septal region includes paraterminal and parolfactory gyri The cerebral cortex in this region is called septal area The septal area has been shown to be a pleasure zone of brain in rat 18 LECTURE OUTLINE Overview Cortical structures Hippocampal formation Hippocampus Papez circuit Subcortical structures Amygdala Septal area Fibre bundles Fornix 19 FIBRE BUNDLES Fibres tracts/bundles of the limbic system: Fornix Mammillothalamic tract Stria medullaris thalami Stria terminalis Medial forebrain bundle Anterior commissure Cingulum Diagonal band (of Broca) 20 FORNIX The fornix is a large bundle of mainly projection fibres that connects the hippocampus with the mammillary body It constitutes the sole efferent system of the hippocampus On the medial surface of the cerebral hemisphere, it is seen as an arched prominent bundle of white fibres below the corpus callosum, along the lower border of septum pellucidum There is one fornix in each cerebral hemisphere but the two are so closely related/fused beneath the middle of the body of the corpus callosum that they are usually described as a single structure 21  Body FORNIX Column of of fornix fornix Origin, Course and Distribution of its Fibres Crus of fornix Arise mainly from the pyramidal cells of the hippocampus and form a thin layer of white fibres on its ventricular surface called the alveus The fibres of ‘alveus’ collect on the medial margin of the hippocampus to form a Commissure narrow strip of white matter, the fimbria, lying flat over the dentate gyrus of fornix The fimbria becomes a rounded band, the crus of fornix as it arches upwards, medially and forwards underneath the splenium of the corpus callosum The two crura, one of each hemisphere, curving over the thalamus, converge and unite in the midline beneath the trunk of the corpus callosum to form the body of the fornix Mammillary Anteriorly, the body of the fornix divides into two columns, the columns of the fornix Bodies Each column of fornix arches downwards towards the anterior commissure and forms the anterior boundary of the interventricular foramen Then it curves posteriorly through the hypothalamus to end in the mammillary body These fibres being located posterior to anterior commissure are referred to as the postcommissural fornix Some fibres of the column pass in front of the anterior commissure to end in the Fimbria of Hippocampus septal area and anterior hypothalamic region, etc. to constitute the precommissural Hippocampus fornix 22  Body FORNIX Column of of fornix fornix The Hippocampal Commissure of the Fornix Crus of fornix The two crura are interconnected by fibres passing from one to another These crossing fibres interconnect the two hippocampi and form the Commissure of fornix commissure of fornix/hippocampal commissure Some fibres of fornix pass above the splenium of the corpus callosum to end in the cingulate gyrus of the same side and constitute the dorsal fornix made up of association fibres Most of the fibres of the fornix are made up of projection fibres Mammillary connecting the hippocampus with the mammillary body Bodies Thus, fornix consists of three types of white fibres: Projection fibres Commissural fibres Association fibres Clinical Correlation Fimbria of Hippocampus Hippocampus Bilateral transection of the fornix may cause a clinical condition called ‘acute amnestic syndrome’ in which an individual is unable to consolidate short-term memory into long-term memory 23  THANK YOU FOR YOUR ATTENTION Content for this lecture: Chapters 9 and 13 | Snell's Clinical Neuroanatomy, 8th Edition