Kandel ER, Schwartz JH, Jessell TM 2000. Principles of Neural Science, 5th ed. McGraw-Hill, New York

The Limbic SystemN.Bèchet-Dept of Neuroscience

Kandel ER, Schwartz JH, Jessell TM 2000. Principles of Neural Science, 5th ed. McGraw-Hill, New York

Key Concepts What is the limbic system?

The Papez circuit

The hippocampus & memory formation

Kandel ER, Schwartz JH, Jessell TM 2000. Principles of Neural Science, 5th ed. McGraw-Hill, New York

Limbic System Not any one structure or neural circuit, rather:

Various brain structures that are related functionally and/or anatomically

Several interconnected nuclei & cortical regions

In a single sentence the function of this system: Control of self and species preservation This encompasses:

Emotion, level of arousal, motivation, reinforcing behaviors & memory

Kandel ER, Schwartz JH, Jessell TM 2000. Principles of Neural Science, 5th ed. McGraw-Hill, New York

Limbic System-Nomenclature On the medial aspect of each hemisphere the

cortex forms a ring

This ring coincides with the edge (limbus) of the cortex Hence the term “Limbic system” came about

Kandel ER, Schwartz JH, Jessell TM 2000. Principles of Neural Science, 5th ed. McGraw-Hill, New York

Limbic System-Nomenclature

Kandel ER, Schwartz JH, Jessell TM 2000. Principles of Neural Science, 5th ed. McGraw-Hill, New York

Limbic System-Components Still debated amongst those in neuroscience, however these structures are generally

accepted as forming part of the system: Insular cortex Olfactory bulb Hippocampus Amygdala Orbital frontal cortex (OFC) Hypothalamus Subcallosal gyrus Septal nuclei Cingulate gyrus Thalamic nuclei Parahippocampul gyrus Mammillary bodies

Kandel ER, Schwartz JH, Jessell TM 2000. Principles of Neural Science, 5th ed. McGraw-Hill, New York

Limbic System-Classification 2 accepted classification systems:

1)-Location of structure: Cortical structures

Cingulate gyrus Parahippocampul gyrus Subcallosal gyrus Insular cortex OFC Hippocampus

Collectively referred to as the “Limbic lobe”

Kandel ER, Schwartz JH, Jessell TM 2000. Principles of Neural Science, 5th ed. McGraw-Hill, New York

Limbic System-Classification 2 accepted classification systems:

1)-Location of structure: Sub-cortical structures

Amygdala Hypothalamus Thalamus Olfactory bulb Mammillary bodies Septal nuclei

Kandel ER, Schwartz JH, Jessell TM 2000. Principles of Neural Science, 5th ed. McGraw-Hill, New York

Limbic System-Classification 2 accepted classification systems:

2)-Function of structure: Input

Limbic lobe Amygdala

Output Hypothalamus Septal nuclei Amygdala

Kandel ER, Schwartz JH, Jessell TM 2000. Principles of Neural Science, 5th ed. McGraw-Hill, New York

Peripheral Feedback Theory In 1890 William James asked an important question:

What is the nature of fear? Do we run away from the bear because we are afraid or are we afraid because we are running away?

According to him it was the latter, he proposed: The conscious experience of fear is a consequence of

emotional responses These bodily changes that occur in the process of running

away

We are afraid because we run….

Kandel ER, Schwartz JH, Jessell TM 2000. Principles of Neural Science, 5th ed. McGraw-Hill, New York

Peripheral Feedback Theory Each feeling has its own unique pattern

of emotional expression or bodily signature These are controlled by descending

pathways Feedback from these peripheral

responses onto the sensory cortex produces the conscious feeling

Kandel ER, Schwartz JH, Jessell TM 2000. Principles of Neural Science, 5th ed. McGraw-Hill, New York

Central Theory In 1920 two Neuroscientists, Cannon & Bard, established:

There is a maintenance of emotional responses after total removal of the cerebral cortices

This disproved the peripheral feedback theory That is to say the cortex is not necessary for expression of patterns of emotional

responses

Complete transection of above the hypothalamus: Found animal was still capable of expressing rage response Termed “sham rage”

Due to lack of input from cortical regions

Kandel ER, Schwartz JH, Jessell TM 2000. Principles of Neural Science, 5th ed. McGraw-Hill, New York

Central Theory Proposed central theory for

emotional expression Sensory info processed at

thalamus and sent to: Hypothalamus

Emotional responses Cortex

Conscious feelings

Kandel ER, Schwartz JH, Jessell TM 2000. Principles of Neural Science, 5th ed. McGraw-Hill, New York

Papez-Circuit

In 1937 James Papez came up with his own theory: For the most part agreed with the Cannon/Bard

central theory However suggested the presence of intermediate

structures between the hypothalamus & cerebral cortex

Kandel ER, Schwartz JH, Jessell TM 2000. Principles of Neural Science, 5th ed. McGraw-Hill, New York

Papez Circuit Thalamus relays signals to:

Sensory cortex Projects to cingulate cortex and Hippocampus

Projects back to hypothalamus Hypothalamus

Projects back onto anterior thalamic nuclei (ATN) ATN project to cingulate cortex

Convergence of signals at the cingulate cortex accounts for the conscious experience of feelings

Kandel ER, Schwartz JH, Jessell TM 2000. Principles of Neural Science, 5th ed. McGraw-Hill, New York

Papez Circuit

Kandel ER, Schwartz JH, Jessell TM 2000. Principles of Neural Science, 5th ed. McGraw-Hill, New York

Hippocampus-Overview An elaboration of the cerebral cortex

Situated in the medial temporal lobe Cortex narrows into single layer of densely packed

pyramidal neurons Area of the brain where long term potentiation (LTP) was

first identified

Acts as a storage site for memories

Kandel ER, Schwartz JH, Jessell TM 2000. Principles of Neural Science, 5th ed. McGraw-Hill, New York

Hippocampus-Structure

Kandel ER, Schwartz JH, Jessell TM 2000. Principles of Neural Science, 5th ed. McGraw-Hill, New York

Hippocampus-Structure

Kandel ER, Schwartz JH, Jessell TM 2000. Principles of Neural Science, 5th ed. McGraw-Hill, New York

Hippocampus-Parahippocampal Structures

Parahippocampal gyrus Entorhinal cortex

Network for memory & spatial navigation Interface between hippocampus & neocortex Projects via perforant pathways

Direct Indirect

Perirhinal cortex Visual recognition of of complex objects Connections to all major sensory areas

Kandel ER, Schwartz JH, Jessell TM 2000. Principles of Neural Science, 5th ed. McGraw-Hill, New York

Hippocampus-Perforant Pathways

Direct pathway Projects directly to CA1 neurons in hippocampus Involved in consolidation

Conversion of short term memories into long term memories

Indirect pathway Projects to granule cells in dentate Granule cells project to CA3 region

Via mossy fiber pathways CA3 neurons project to CA1 neurons

Via Schaffer collaterals Indirect pathway is involved in complex spatial learning & memory

Kandel ER, Schwartz JH, Jessell TM 2000. Principles of Neural Science, 5th ed. McGraw-Hill, New York

Hippocampus-Projections CA1 back to parahippocampal gyrus

From here to neocortex

CA1 to left & right fornices Mammillary bodies (part of hypothalamus)

Mammillothalamic tracts Anterior thalamic nuclei

Neocortex

Kandel ER, Schwartz JH, Jessell TM 2000. Principles of Neural Science, 5th ed. McGraw-Hill, New York

Long Term Potentiation (LTP) What exactly is LTP?

Long lasting increase in the amplitude of EPSP’s Achieved by a tetanus (train of stimuli)

Similar forms of potentiation have been described in fear conditioning (amygdala)

Seen at all 3 synapses in hippocampus Lasts days to weeks Does not induce a single form of plasticity, either alters:

Presynaptic cell (Altering NT release) Postsynaptic cell (Altering NT sensitivity; formation of new synapses) Both

Kandel ER, Schwartz JH, Jessell TM 2000. Principles of Neural Science, 5th ed. McGraw-Hill, New York

Long Term Potentiation Non-associative LTP

Occurs at mossy fiber pathway Presynaptic process

Increases neurotransmitter release via Ca2+ mediated recruitment of vesicles & closure of K+ channels

Associative LTP Occurs in Schaffer collateral pathway Both pre & postsynaptic process

Mediated by calcium/calmodulin complex