Week 5 - Hypothalamus and Limbic PDF

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Summary

Neuroanatomy Week 5 – Hypothalamus and Limbic System Limbic Lobe    Main functions - Links hypothalamus and association (primary motor and secondary motor areas  behaviour) o Reward – (pleasure); increased survival – eating or having sex o Punishment- (pain); decreased survival – fear of failure...

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Neuroanatomy Week 5 – Hypothalamus and Limbic System Limbic Lobe 

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Main functions - Links hypothalamus and association (primary motor and secondary motor areas  behaviour) o Reward – (pleasure); increased survival – eating or having sex o Punishment- (pain); decreased survival – fear of failure / depression / injury pain o Memories – (associated with reward / punishments) o Motivation and reinforcement change behavior o Learning and conditioning o Emotional drive on reactions to stimuli and when to invoke responses Cortical regions o Cingulate gyrus  Anterior cingulate – activated during experience of pain or feelings of depression  Caudal anterior cingulate – registers actual pain quality (how bad; scale 1-10)  Rostral anterior cingulate – what actions to take to deal with pain  Subcallosal – behavior to an anticipated threat (pain avoidance) o Orbitofrontal (smell) o Parahippocampal gyrus  Learning and memory o Insula Subcortical nuclei o Hippocampus (in uncus) o Amygdala (in uncus) o Septal nucleus o Thalamus (mediodorsal and anterior nuclei) o Mammillary bodies

The papez circuit      

Info from cingulate cortex  para-hippocampal cortex via cingulum Infor goes  para-hippocampal cortex to the hippocampus via performant pathway of entorhinal cortex Infor goes from hippocampus  mammillary bodies (hypothalamus) via fornix Infor goes from mammillary bodies  anterior thalamus via the mammillothalamic tract Info goes from anterior thalamus back  cingulate cortex via anterior limb of internal capsule Emotional states o 2 aspects – conscious feelings (cortical) and behavioral response (ANS / hormones)

How is information stored?  

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STM converted to LTM Central executive system – supervisory system controls flow of information to and from 2 perceptual systemso Phonological loop o Verbal memorization to hold data in working memory and facilitate transfer to LTm o Requires; Brocas and Wernicke o Refreshes information and stops It being lost o Visuo-spatial sketchpad o Holds information on what we see; located in frontal lobe o Uses temporary storage and manipulation of spatial and visual information Long term memories – (2 main types) o Procedural (implicit)

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Neuroanatomy o Long-term memories of doing things (skills/habits) or emotions associated with them e.g. driving o Subconscious behavioral/physiological response o Declarative (explicit) o Verbally remembering things, events, happenings, conscious recall o Semantic memory; remembering isolate facts or events o Episodic memory; remembering sequences of linked events LTM Storage o One memory = pattern of neuronal activity o Neurons can be involved in many different memories, but activated in different patterns for each LTM retrieval o Key regions involved; hippocampus, amygdala, anterior thalamic nucleus, temporal lobe and papez circuit

Amygdala – works with and enhances memory systems     

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Located in entorhinal cortex of uncus, anteromedial to hippocampus Links emotions to visceral and behavioral responses and memories Functions include fear, terror and panic, reward Works with and enhances memory systems (identifies if new stimuli are nice, painful, exciting, boring Basolateral nuclei o Attaches emotional significance to stimulus o Connects with association and limbic cortices Central nucleus o Gut feelings; link brainstem RD Centromedian o Olfactory input, links to hypothalamus Sensory inputs o Threat detection  fight or flight response o Inputs – limbic and association areas o Output – hypothalamus  RF  activates reticulo-spinal tracts  sympathetic NS to fight or run away o Decision whether to run or fight may involve the septal nuclei o Septal nuclei – reward / pleasure Fear conditioning o Survival instincts – pleasure / pain  hardwired from birth o Life associations are formed  rapid emotional responses  Fear – re-backs / brown snakes  Reward – chocolate addition

Reward systems  

Ventral striatum – made up of septal nucleus, nucleus accumbens and basal nucleus of meynert Nucleus accumbens o Part of basal ganglia o Receives dopamine from ventral tegmental area

Neuroanatomy Hypothalamus 

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Function o Anatomically part of diencephalon o Functionally part of limbic system o Maintains homeostasis (critical for life) via responding to internal and external stimuli o Regulates ANS and endocrine functions o Chief commander in ANS – regulated by brainstem and spinal cord reflexes Anatomical boundaries o Medial wall = 3V border, superior = IVF & hypothalamic sulcus o Rostrocaudal = A/P commissures & lamina terminalis, fornix o Lateral = internal capsule (posterior) o Inferior = mamillary bodies & median eminence & tuber cincerum o Blood Supply = branches of ACA, PCA and PCom (query ICA, ACom) Homeostasis promoted 2 ways o Neural (arousal vs R and D – fast)  Anterior group excites OSNS  Posterior groups excite SNS o Hormones (pituitary gland – HPA)  Flow – APG (releasing an inhibiting hormones)  Fast – {{G (neurohormones)  Used for long and short feedback loops Nuclei o Preoptic – modulates secretion of gonadotropin releasing hormone o Paraventricular and supraoptic – regulate osmolality (via antidiuretic hormone) o Anterior – thermoregulation via stimulation of parasympathetic nervous system o Dorsomedial and ventromedial – controls feeding impulse o Lateral – feeding regulation o Arcuate – release prolactin (inhibits lactation) o Mamillary and posterior nuclei – thermoregulation Tracts o Median forebrain bundle –  Crosses entire lateral area  Connects brainstem to forebrain o Fornix – largest tract; part of Papez circuit to hippocampus o Stria terminalis – major pathway to/from amygdala o Mammillothalamic tract – part of papez circuit o Dorsal longitudinal fasciculus – connections to PAG, RF and collateral input from DCML, STT, NTS o Hypothalamispinal tract – ANS output to IML of spinal cord o Tuberinofundibualr – arcuate nucleus to APG o Supraoptichypophysial – SO and PVN to PPG

Endocrine functions 

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Anterior pituitary gland o Releasing and inhibiting hormones via portal system (CRH, TRH, GHRH, PRL, PIF, GnRh o 6 hormones – ACTH, TSH, GH, PRL, LH, FSH o Nuclei – arcuate, DM, PVN Posterior pituitary gland o Paraventricular nucleus secretes oxytocin – breastfeeding o Supraoptic nucleus secretes ADH (vasopressin) – water homeostasis

Neuroanatomy

Limbic System & Circuits

NB. The purpose of these notes is only to provide an overview of the material that is covered in the lecture. They are not intended to replace attendance at lectures, nor are they carefully edited for grammar or expression.

Text reading (Nolte) – Chapter 23 At the completion of this lecture and associated reading, you should be able to:

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describe the components of the limbic system; discuss the functions of the limbic system; outline Papez circuit and associated function; describe the limbic functions associated with the amygdala and hippocampus; explain the consequences of alternations/lesions in limbic structures and circuitry.

Limbic functions include emotion, learning and memory. It involves multiple regions and highly integrated circuitry. Limbic dysfunction is strongly associated with psychiatric disorders. Several different pathways are collectively referred to as the limbic system. In the past it was considered a single pathway, but we now know that is not the case. The limbic “system” has a role in emotionally driven behaviours, learning and memory. Limbic – latin “limbus” means border – reflecting the way the “limbic lobe” encircles the diencephalon hence is a border between subcortical regions and cerebral cortex. There is no consensus on the structures that form the limbic system, but most authors would agree that the amygdala, hippocampus, limbic association cortices, thalamic & hypothalamic limbic nuclei are included. The hippocampus and amygdala are the two key structures that provide the basis for dividing the functional circuitry into those associated with learning and memory (hippocampus) and those associated with emotions (amygdala). The limbic association cortex includes the cingulate gyrus, isthmus, parahippocampal

Neuroanatomy gyrus, orbitofrontal cortex, and temporal pole. These regions receive highly processed information from other cortical areas, which is then fed into the two main limbic regions – hippocampus or amygdala. The hippocampus is divided into the dentate gyrus, the hippocampus proper (cornu ammonis regions), and the subiculum. It is concerned with memory and learning. The subiculum is a transitional area between the hippocampus and parahippocampal gyrus. The hippocampus is a 3 layered cortex: molecular layer, strata (granule or pyramidal cell layer), polymorphic layer. Most hippocampal output is via pyramidal cells. The granule cell layer sends output into pyramidal cells of cornu ammonus, then into pyramidal cells of subiculum. Subiculum mostly feeds into alveus which feeds into fimbria, and then into fornix. Papez circuit is a loop of communication between the hippocampus, fornix, mammillary bodies, anterior thalamic nuclei, cingulate gyrus, and parahippocampal gyrus. Information passes from mammillary bodies, to anterior nucleus of thalamus via mammillothalamic tract. Then to cingulate gyrus, then to parahippocampal gyrus and into hippocampus. Out from hippocampus into fornix, and back down to mammillary bodies. This circuitry is known as Papez circuit in honour of the person to first describe it. Although he originally thought it was involved in emotionally related behaviour, we now know it is more important in explicit (semantic) memory formation/consolidation (knowledge of facts/events). In Korsakoff’s syndrome (an alcohol related brain damage), the mammillary bodies are destroyed, which is reflected in the severe memory loss associated with this disease. There is also a circuit that runs between the hippocampus, fornix, septal nuclei, back to the fornix, and back to the hippocampus. This circuit is thought to be involved in highly motivated behaviour eg. reproductive behaviour. In rats, when given a choice between food & water and electrical stimulation of septal nuclei, they choose the stimulation. One interpretation of this is that the septal nuclei act as a “pleasure centre”, regulating highly motivated behaviour such as reproductive or feeding behaviour. The hippocampus not only projects to mammillary bodies and septal nuclei, but also projects back to the entorhinal cx and then on to limbic association cortices which in turn have widespread connectivity. Hippocampal circuitry & the prefrontal cortex has been implicated in schizophrenia. The amygdala is a collection of nuclei that project to the hypothalamus, brain stem, hippocampus & temporal cortex, limbic association cortices, and the basal forebrain. It is involved in emotional aspects of learning, and in the formation of emotionally charged memories. Examples of functions include: • recognition of affective meaning of facial expressions • recognition of emotional content of speech • mediate behavioural response to emotional stimuli • fear conditioning • regulation of behaviours triggered by olfactory stimuli • emotional memories Dysfunction in the amygdala, hypothalamus & ventral striatum has been implicated in depression, anxiety & substance abuse disorders....