NATIONAL QUALIFICATIONS CURRICULUM SUPPORT
Human Biology
Unit 3: Neurobiology and
Communication
A Case Study into the
Limbic System
Student’s Notes
[HIGHER]
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Acknowledgement
Learning and Teaching Scotland gratefully acknowledges this contribution to the National
Qualifications support programme for Human Biology.
The publisher gratefully acknowledges permission to use the following sources: Figure 1 brain
connectivity graph from On the relationship between emotion and cognition by Luiz Pessoa
from Nature, Vol 9 February 2008, p. 152,
http://www.nature.com/nrn/journal/v9/n2/full/nrn2317.html, Reprinted by permission from
Macmillan Publishers Ltd: Nature Reviews Neuroscience 9, 148-158 (February 2008) © 2008
http://www.nature.com/nrn/index.html; Diagram c freezing, from fear conditioning induces
associate long term potentiation in the amygdala by Rogan Staubli and LeDoux from Nature,
Vol 390, December 1997, p. 605,
http://www.nature.com/nature/journal/v391/n6669/full/391818b0.html, Reprinted by
permission from Macmillan Publishers Ltd: Nature 391, 818 (19 February 1998 © 1998
http://www.nature.com/nature/index.html; Diagram from
http://www.dkimages.com/discover/previews/740/68987.jpg © Dorling Kindersley
Every effort has been made to trace all the copyright holders but if any have been inadvertently
overlooked, the publishers will be pleased to make the necessary arrangements at the first
opportunity.
© Learning and Teaching Scotland 2011
This resource may be reproduced in whole or in part for educational purposes by educational
establishments in Scotland provided that no profit accrues at any stage.
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A CASE STUDY INTO THE LIMBIC SYSTEM (H, HUMAN BIOLOGY)
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Contents
Background
4
Structure of the limbic system
4
Activity 1
6
Fear regulation and the amygdale
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Connectivity of the amygdale
7
Activity 2
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Circuitry of fear
9
Activity 3
10
An experimental paradigm for studying fear
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Activity 4
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A CASE STUDY INTO THE LIMBIC SYSTEM (H, HUMAN BIOLOGY)
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STUDENT’S NOTES
Student’s notes
Background
This case study aims to provide a deeper understanding of the limbic system
using the regulation of fear as an example.
The limbic system describes a set of inner brain struct ures that extend down
through the core of the brain to the upper part of the brainstem.
The functions of the limbic system include:
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emotional response
learning and memory
hormone regulation
movement
sensory perception.
Structure of the limbic system
The thalamus is a large collection of nuclei located on top of the brainstem
near the centre of the brain. It relays sensory information to and from the
cerebral cortex. The thalamus is also involved in motor signalling to the
cerebellum and regulation of consciousness, sleep and alertness.
Lying just below the thalamus is the hypothalamus, which links the nervous
system with the endocrine system. It is a small structure with an important
role in homeostatic control, body temperature regulation, hormone re lease,
hunger, thirst and sexual behaviour.
The hippocampus has the shape of a curved tube and plays an important role
in memory and spatial navigation.
Where the hippocampus ends, the amygdala begins. This cluster of nuclei is
involved in emotional response and memory.
Surrounding the amygdala and hippocampus is the parahippocampal gyrus.
This portion of cortex helps to modify memory and emotional response.
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A CASE STUDY INTO THE LIMBIC SYSTEM (H, HUMAN BIOLOGY)
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STUDENT’S NOTES
Another region of limbic cortex called the cingulate gyrus modifies
behaviour such as aggression. It sweeps round over the fornix, extending
towards the hippocampus.
Just below the cingulated gyrus is the corpus callosum. It connects limbic
structures on the right side of the brain with those on the left, and helps
communication across the two hemispheres of the brain.
The fornix is an arched pathway of nerve fibres transmitting information
from the hippocampus and other limbic areas to the mamillary body.
The septum pellucidum is a thin sheet separating the fornix from the corpus
callosum.
Located at the end of the fornix is the mamillary body. This small nucleus is
important for relaying information to and from the fornix and thalamus.
The olfactory bulb protrudes outwards from the limbic system towards the
front of the brain and is concerned with the perception of odours. The
inclusion of the olfactory bulb in the limbic system explains why our sense of
smell can evoke a forgotten memory or an emotional response.
A CASE STUDY INTO THE LIMBIC SYSTEM (H, HUMAN BIOLOGY)
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STUDENT’S NOTES
Activity 1
Using the information given above, label the diagram below t o show the
structures of the limbic system.
Figure from: DK
Fear regulation and the amygdala
Fear is a distressing emotional response brought about by the perception of
threat. It is the ability to recognise aversive stimuli and respond accordingl y
with strategies of escape and avoidance.
Fear can range from mild caution to extreme phobia, and include emotional
states such as worry, anxiety, embarrassment, horror, panic and dread. Some
of the most common fears are of spiders, snakes, heights, e nclosed spaces,
water, bridges and needles.
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A CASE STUDY INTO THE LIMBIC SYSTEM (H, HUMAN BIOLOGY)
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STUDENT’S NOTES
The amygdala is essential for emotional memory and is a key structure in the
neurobiology of fear. Extensive research has characterised the role of the
amygdala in fear processing.
Monkeys with lesions to the amygdala show reduced levels of aggression and
fear together with increased tameness towards human handlers. Imaging
studies in humans have also implicated the amygdala in fear whereby patients
with damage to the amygdala cannot recognise the emotion of f ear when
presented with pictures of frightened faces.
Connectivity of the amygdala
The graph below maps the connectivity of the amygdala in the brain.
Brain connectivity graph of the amygdala
Analysis of brain connectivity by Young and colleagues reveals that the amygdala
(Amyg, centre of the graph) makes widespread projections all over the brain. In fact,
it connects with all but eight cortical areas included in the analysis. Figure from:
Pessoa, L. On the relationship between emotion and cognition. Nature Perspectives,
2008, volume 9. P152.
The amygdala is a highly connected region of the brain. This makes sense
given the complexity and importance of a fearful response. From initial
sensory arousal to being able to remember an event months or even years
later, fear involves multiple brain regions controlling autonomic and
conscious behaviour.
A CASE STUDY INTO THE LIMBIC SYSTEM (H, HUMAN BIOLOGY)
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STUDENT’S NOTES
The major brain regions connected to the amygdala and r esponsible for the
overall processing and expression of fear are:
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hypothalamus
thalamus
hippocampus
brainstem
prefrontal cortex
sensory cortex
Activity 2
Imagine you are about to sit an exam. For each major brain region connected
to the amygdala listed above, describe its role in the emotional response.
Brain structures
connected to the
amygdala
Hypothalamus
Functional contribution to emotional
response
Thalamus
Hippocampus
Prefrontal cortex
Sensory cortex
Brain stem
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A CASE STUDY INTO THE LIMBIC SYSTEM (H, HUMAN BIOLOGY)
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STUDENT’S NOTES
Circuitry of fear
When the brain receives an emotional stimulus, such as something fearful, it
is first detected by the thalamus. From there, two parallel pathways exist for
processing this information:
 the thalamo-amygdala pathway (or the short route)
 the thalamo-cortico-amygdala pathway (or the long route).
The short route provides a fast impression of the situation. No cognition is
involved because the cortex is bypassed. This pathway activates the amygdala
directly to generate an immediate emotional (eg fight or fl ight) response
before any perceptual information is integrated into conscious experience.
Information associated with the long route is processed by the cortex before
reaching the amygdala indirectly to provide a full assessment of the fearful
situation. Various sensory modalities of the perceived stimulus are processed
by the primary sensory cortex, including visual and auditory cues, to build up
an elaborate conscious representation of the fearful stimulus.
The hippocampus is then recruited to support memory of the situation, in
particular the context in which an aversive experience is associated with. This
structure is required for learning about the danger of an object or situation
and explains why we do not repeatedly touch a hot plate, for example.
The amygdala itself is composed of several nuclei that have specific roles in
processing fear. The lateral nucleus is the major input, receiving sensory
information from the thalamus. The central nucleus is the major output of the
amygdala, sending information to other brain structures responsible for the
physical reactions associated with fear such as increased heart rate,
perspiration, tense muscles, etc.
A number of internal pathways have been identified by which information is
routed from the lateral nucleus to the central nucleus. Neurons of the lateral
nucleus project to intermediate nuclei, including the basal nucleus and the
medial nucleus. These structures then relay information to the central
nucleus. There is also a direct connection between t he lateral nucleus and the
central nucleus.
Each of these nuclei can be modulated by brain structures that can influence
the emotional response such as the hippocampus, the cortex and the
hypothalamus.
A CASE STUDY INTO THE LIMBIC SYSTEM (H, HUMAN BIOLOGY)
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STUDENT’S NOTES
Activity 3
Using the information above, complete the flow diagram below, showing how
an emotional response is regulated the by the amygdala and its connecting
structures. Highlight the short and long routes for processing information.
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A CASE STUDY INTO THE LIMBIC SYSTEM (H, HUMAN BIOLOGY)
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STUDENT’S NOTES
An experimental paradigm for studying fear
Classic conditioning is a form of learning that involves presentation of a
neutral stimulus with a biologically significant stimulus.
The neutral stimulus is any action that does not result in a behavioural
response and is called the conditioned stimulus (CS). The biological ly
significant stimulus evokes an innate behavioural response and is called the
unconditioned stimulus (US).
If the CS and the US stimulus are repeatedly paired, eventually the two
stimuli become associated and a behavioural response is evoked by the CS
alone. This is called the conditioned response (CR).
Classic conditioning was first demonstrated by Ivan Pavlov (it is also called
Pavlovian conditioning) when he conditioned his laboratory dogs to salivate
to the sound of a bell. He paired the presentati on of food (the UC – biological
response) with a bell (CS – neutral response) and after a few repetitions the
dogs began to salivate in response to the bell alone.
Classic conditioning has provided a model behavioural paradigm for studying
emotional learning. Fear conditioning is where an organism learns to predict
an aversive event.
During fear conditioning subjects rapidly acquire fear to a previously
innocuous stimulus. In rodents, for example, a neutral tone (CS) can be paired
with a mild electric shock (US) to generate a conditioned fearful response to
the tone alone.
Fear conditioning has been studied in numerous species and the neural
mechanisms are highly conserved across species. In humans, fear
conditioning is typically measured by monitor ing skin conductance responses.
In rodents, conditioned fear is often measured by the freezing response – a
length of time during which the animal is immobile and observing its
surroundings for further fearful stimuli.
A CASE STUDY INTO THE LIMBIC SYSTEM (H, HUMAN BIOLOGY)
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STUDENT’S NOTES
Activity 4
Study the diagram and information below and analyse the results for this
experiment.
Using this information, answer the questions to develop a method and
conclusion for the experiment:
 What are the CS, US and CR?
 What is happening at each stage in the experiment (testing, tr aining and
testing?
 Why is there a drop in the freezing response of the conditioned group
between sessions 6 and 7?
The conditioning chamber was constructed
of stainless-steel bars and kept within a
ventilated and temperature-regulated
acoustic isolation box. Stimulus delivery
and data collection were controlled by
adjoining electrical accessories. The
isolation box was equipped with a video
camera for recording behaviour.
Fear conditioning in the rat leads to associative conditioning of fear behaviour.
Image from: Rogan, Staubli and LeDoux, Fear conditioning induces associative
long-term potentiation of synaptic currents in vivo. Nature, 1997, volume 390, p605.
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A CASE STUDY INTO THE LIMBIC SYSTEM (H, HUMAN BIOLOGY)
© Learning and Teaching Scotland 2011