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Class notes

  • Limbic system = emotions and memory. Of late, it has been questioned as to whether it is a separate system

Introduction: the limbic system

  • The limbic system is located on the medial surface of the cerebral hemispheres
    • Limbic comes from limbus (L. border) - border of the telencephalon and the diencephalon
    • It is the primitive system of the brain that existed before the rest of the cortex grew
  • Structures
    • Cingulate gyrus
    • Hippocampus
    • Olfactory bulb (associated) - early mammals are dominated by olfaction and touch
  • Functions
    • Involved in the processing of emotions
      • Partially overlaps with the brain’s emotional system – which is very complex

Early concepts of emotion

  • James-Lange (green)
    • There are two aspects to the emotional experience
      • Emotional expression (behavioural, autonomic) and emotional experience (feelings)
      • It was thought that expression caused experience
        • Ie: you are sad because you cry
        • Ie, sensory information went to the expression area and then to the experience area
  • Cannon-Bard (yellow)
    • Emotional experience is parallel to emotional expression
      • Based on two observations
        • There is no emotion if there is damage to the cortex
        • When emotional, the hypothalamus is active (in charge of physiological responses)
      • Thus, it was thought that sensory information went to the cortex causing experience before projecting to the hypothalamus causing expression
  • Papez
    • Merged the two theories
      • Thought that emotional experience and expression were intricately related
  • Ie: sensory goes to the cortex, and projects to the hippocampus and then the hypothalamus
    • Thus, it is projected back to the cortex via the thalamus
    • Process
      • Sensory stimulus (which is neutral) is detected by the sensory areas of the cortex
        • The cortex feeds this to the cingulate gyrus which sends it to the hippocampus
        • The hippocampus integrates the information and projects it to the hypothalamus via the fornix
        • The hypothalamus causes the response
    • This then feeds the information back to the cortex via the anterior nucleus of the thalamus
      • This transmits to the cingulate gyrus and then to the cortex causing feeling
      • So, in a way, we are sad because we cry, but the cortex detects this first (but doesn’t know that we are sad)
    • This loop is called the Papez circuit, and is compatible with both James-Lang and Cannon-Bard theories
      • But you also have feedback, where the hypothalamus can feed back to the cortex giving you emotions based on physiology
    • Cingulate gyrus (via cingulum) --> hippocampus --> fornix (underneath C.C.) --> hypothalamus --> thalamus (anterior nucleus) --> cortex

McLean circuit

McLean made it the limbic system an accumbensome idea! LoLoLoL

  • Further refined Papez and added the concept of the limbic lobe
  • Thus it was thought the limbic system was for emotional processing
    • Added the amygdala (emotion-laden memories), septal area (thin wall on the anterior horn of the lateral ventricle - medial to the NAcc, and in front of the hypothalamus), and the parahippocampal gyrus
      • Also added the orbitofrontal cortex (important for integrating visceral sensory information; appreciation of food), the dorsomedial nucleus of the thalamus (projects up to prefrontal cortex; important in judgement) and the nucleus accumbens (for reward; where the putamen meets the caudate nucleus)

Modern knowledge

  • The limbic system still loosely thought to be involve in emotional function
    • Mclean’s and Papez’s theories, however, may not be entirely true
  • All we know is that there are important structures:
    • The hippocampus – for memory
    • The amygdala – for emotions
      • In front and a little above inferior horn of lateral ventricle; by the hippocampus (it forms the anterior wall of the inferior horn of the lateral ventricle (while hippo forms the medial wall)
        • Medial part = olfactory input (similar embryology to cortex)
        • Basolateral part (deeper; similar embryology to striatum)
      • Amygdala is to emotions what the lentiform nucleus is to motor control

Amygdala

  • Located in the temporal lobe infront of the hippocampus
    • Thought to be involved in emotions (particularly bad emotions)
    • Comes from Latin (almond)
  • Anatomy
    • Corticomedial nuclei
      • Olfactory input
    • Basolateral nuclei
      • All other input (from the cortex)
    • Central nuclei
      • All output
        • Connections
    • Inputs
      • Olfactory bulb
      • Prefrontal cortex (especially the orbito-frontal cortex which is involved in social behaviours)
      • Septal area
      • Hypothalamus – internal systems
      • Hippocampus – consolidated memories
      • Temporal cortex
    • Outputs
      • Orbito-frontal cortex
      • Septal area
      • Hypothalamus – expression of emotions (mainly)
      • Temporal cortex
      • NOT the hippocampus
  • Several pathways from amygdala to other places
    • Ventral amydalofugal pathway
    • Stria terminalis (same curve to fornix; kind of sitting on top of fornix)
      • These two pathways allow the amygdala to influence emotional expression
  • Function (tell by putting an electrode in and giving it some juice)
    • Expression/experience of emotions
    • Arguments for (what we know):
      • Lesions cause reduced emotionality
        • Animals become tame and placid – can’t recognise dangers, or make associations for danger (light, sound + electric shock)
        • Humans can’t identify anger vs fear; key region for PTSD/anxiety etc; evolved fear of snakes in primates: Did Steve Irwin have no amygdala (? :O)
        • Also, emotional tagging of memories doesn't work with a removed amygdala. Emotional tagging is good because it lets you stay away from dangerous situations.
      • Stimulation causes increased vigilance and defensive responses
        • Animals – fear and rage
        • Humans – fear and anxiety
      • Functional imaging
        • When shown the facial expression of fear, recognition corresponds to left amygdala activation

Hippocampal formation

  • Made up of old cortex (archicortex); nb there's also paleocortex and neocortex (which has developed greatly in humans)
    • Has been pushed medially and inwardly by the expansion of the neocortex
  • Hippo sits posteriorly to the amygdala.
  • Hippocampal formation = hippocampus + adjacent transitional cortex between hippocampus and parahippocampal gyrus. The hippocampus is cortex, but has only 3 layers
  • Anatomy
    • Hippocampal gyrus
    • Dentate gyrus
    • Subiculum
    • 3 layers (instead of the 6 in the cortex)
  • Path
    • Sensory information travels into the formation and reaches the dentate gyrus
  • It is projected from here to the hippocampal gyrus and then back down to the subiculum
  • From here, it is projected via the fornix to the hypothalamus and to the cortex
    • Function not very well understood, pathway can be seen however
  • Function
    • The consolidation of short term memory into long term memory (declarative memory)
  • Memory is declarative (facts and events) or nondeclarative (procedural, classical conditions)
  • Classical conditions – skeletal musculature (e.g. "I can play violin"), emotional responses (e.g. "holy crap I saw a snake"). Sometimes you can also remember events because of emotional responses - not always a neat division
    • If we lose the hippocampus, we can get anterograde amnesia
      • Because the hippocampus is important for consolidation and declarative memory
  • Input to hippocampus via amygdala, olfaction, parahippocampal gyrus, and the entire neocortex (i.e. things you see/hear etc are remembered)
  • The hippocampus keeps the information for some time, then sends it back.
    • Outputs to - parahippocampal gyrus, amygdala and hypothalamus (via fornix)
  • So the hippocampus needs some emotional stimulation alongside your inputs


  • H.M. – removal of hippocampus to treat epilepsy
  • Everyday is a new day
  • Can remember childhood and learn new skills
  • Moods and personalities are unchanged
  • Can learn new motor information, but not emotional or declarative memory
  • Cardiac arrests etc can cause hippocampal necrosis
    • Other important things
      • The hippocampus is sensitive to ischaemia and oxygen deprivation
      • Early Alzheimer’s has neurodegeneration of the hippocampus
      • Electrical stimulation of the hippocampus can cause déjà vu

Additional info

  • Cingulate gyrus gets its name from the cingulum, a fibre bundle within the cingulate gyrus.

Official notes

Limbic system: a system for processing emotions and consolidating memories. The word limbic (limbus = border, edge) originates from Broca's limbic love (1870) = cingulate gyrus + hippocampal formation, found in all mammals and present in reptiles.

Theories

  • James-Lange (1880): experience of emotions is secondary to expression of emotions.
  • Cannon-Bard (1920): experience and expression of emotions occur at the same time.
    • Cortex -> experience. hypothalamus -> expression.
  • Papez (1930): proposes a loop connecting hypothalamus, thalamus, cortex and hippocampus.
    • Papez circuit = cingulate gyrus (collects inputs from neocortex) -> entorhinal Cx -> hippocampus --> hypothalamus (mamillary bodies) --> anterior nucleus of thalamus --> cingulate gyrus. Papez loop unifies J-L and C-B theories.
  • MacLean (1950): popularises Papez circuit, adds parahippocampal gyrus, amygdala and septal area and calls this emotion system "limbic system" after Broca's limbic lobe. Some connected structures such as orbitofrontal cortex, DM thalamus and nucleus accumbens may also be included in limbic system. The concept of limbic system as an emotional system is useful but is challenged by recent studies of the functional organisation of its different components e.g. the amygdala and hippocampus.

Amygdala

  • Located beneath uncus at tip of inferior horn of lateral ventricle
  • Divided into corticomedial (CM) and basolateral (BL) groups of nuclei and central (Ce) nucl.
  • Input/outputs:
    • CM - inputs from olfactory tract and olfactory Cx. CM projects to BL.
    • BL - many inputs: from thalamus (auditory, visual, somatosensory), hypothalamus, neocortex (prefrontal and temporal), hippocampus and reticular formation (monoaminergic cell group) and CM. BL processes information and projects to Ce.
    • Ce - projects to (i) hypothalamus and brainstem (expression of emotions) and (ii) prefrontal Cx, temporal Cx, septal are and DM thalamus (experience of emotions).
  • Main fibre bundles:
    • Stria terminalis: parallel to fornix follows lat. ventr; runs from amygdala to septal area and hypothalamus (sits between caudate and thalamus).
    • Ventral amygdalofugal pathway: more direct route for connections with hypothalamus, thalamus (MD) and brainstem
  • Function: controls the expression and experience of innate (e.g. angry faces) as well as learned emotional responses (e.g. live wires). Emotional memories are formed in the amygdala
  • Stimulation:
    • in animals: arousal, fear and anger responses
    • In humans: feelings of fear and anxiety
  • Lesion/ablation (bilateral)
    • in animals: placid, fearless, and unable to learn danger signals (conditioned fear = emotional memory)
    • in humans: placid, unable to recognise facial expression of fear and anger and difficulty in forming emotional memories

Hippocampal formation

  • Located in the temporal lobe in the medial wall of the inferior horn of the lateral ventricle. Curled end of cortex due to pressure of the expanding neocortex. Continuous with parahippocampal g.
  • Divided into 3 parts:
    • Hippocampus (hippocampal g): 3 layered archicortex: molecular (facing dentate g), pyramidal and polymorphic (facing ventricle) layers.
      • Pyramidal layer divided into 3 regions: CA 1 (adjacent to subiculum), CA2 and CA3 (adjacent to dentate gyrus). CA = cornu Ammonis.
    • Dentate g: 3 layered archicortex also, but pyramidal layer replaced by granular layer
    • Subiculum: transitional zone between 6 layered entorhinal Cx (neocortex) and 3 layered archicortex. Hippocampal formation also has 2 small sulci: 1) Hippocampal sulcus: between subiculum and dentate g. 2) Fimbriodentate sulcus: fimbria and dentate gyrus.
  • Input/outputs: main input is entorhinal Cx (ant. part of parahippocampal gyrus). Entorhinal cortex is the interface between neocortex and hippocampal formation. Other inputs: amygdala, hypothalamus, septal area and contralateral hippocampus (last 3 via fornix). Also receives inputs from monoaminergic cell groups of reticular formation. Main output is also entorhinal Cx. Other outputs are also amygdala, hypothalamus, septal area and contralateral hippocampus (last 3 via fornix).
  • Fibre bundle associated with hippocampal formation:
    • Fornix forms as a continuation of the fimbria. Each fornix has a crus (part posterior to thalamus), commissure (beneath splenium), body (above thalamus), and column. Column divides into a precommissural part (-->septal area, anterior hypothalamus) and postcommissural part (--> mammillary body)
  • Function: Consolidation of memories (i.e. from short term to long term memories) of facts and events (declarative memory).
  • Lesion/ablation (bilateral): In human (case of H.M.) -> anterograde amnesia (cannot remember things seen or done since lesion but can remember things before, e.g. childhood); however, can learn new tasks (procedural memory intact); no change in personality.
  • Other observations: CA1 sensitive to hypoxia -> results in memory deficits. Evidence of hippocampal degeneration in early stages of Alzheimer's disease.
    • Stimulation of hippocampus -> feelings of deja vu

Bilateral removal of temporal lobes: Kluver-Bucy syndrome

  • Carried out experimentally in monkeys in the 1930s
  1. Monkeys become fearless and placid, absence of emotions (amygdala)
  2. Learning difficulties due to difficulty remembering (hippocampus)
  3. Visual agnosia - failure to recognise objects visually (inferotemporal Cx - visual association)
  4. Hyperorality - repeatedly explore objects with mout (due to 3 and 2)
  5. Males become hypersexual