17. The Limbic System

function: responsible for learning, memory, emotions

· receives from sensory and association cortices

· influences ANS via hypothalamus

· influences rational behaviour: you need to have an emotional response to the sabertooth tiger, not just see it!

* the neocortex does not project to ANS directly

*all limbic system structures project directly/indirectly to hypothalamus (part of limbic system)

- only hypothalamic output to ANS: 1. reticulo-spinal tract, 2. hypothalamic-spinal tract

- ANS feeds back to limbic system: quadriplegics show gradual blunting of emotional affect

limbic associations

1. ANS (see above)

2. olfaction: smell is integrated with memory, emotion, sexuality; associated with: 1. septal nuclei; 2. amygdala; 3. temporal lobe nuclei

3. reticular formation: association with (deteriorates with Alzheimer’s, normal aging, Down’s, Parkinson’s, JCD, Pick’s, Korsakoff’s à emotional changes)

4. orbitofrontal cortex: lesions to this (or DM) à

a. loss of initiative (Alzheimer’s)

b. socially unacceptable behavior (Alzheimer’s)

c. severe depression (Huntington’s, Alzheimer’s)

d. lack appreciation of consequences

5. extrapyramidal system: tremor associated with emotional stress

6. limbic system structures show the lowest threshold for seizures (auras: fear, olfactory, stereotyped movements)

I. Anatomy

a. Hippocampal formation

i. Hippocampus (“sea horse”)

- old cortical area

- connected to mammillary bodies (hypothalamus) via fornix

ii. Dentate gyrus

iii. Subiculum

b. Limbic Lobe (“on the edge”)

i. Cingulate Gyrus

ii. Orbitofrontal Cortex

iii. Parahippocampal Gyrus

iv. Septal Nuclei

c. Limbic system

i. Hypothalamus

ii. Hippocampus

iii. Nuclei of Thalamus (AN, DM, LD)

iv. Cingulate Gyrus

v. Parahippocampal gyrus

vi. Septal nuclei

vii. Habenular nuclei

viii. amygdala

ix. some neo-cortical areas (frontal and temporal)

1. entorhinal cortex (Area 28)

d. Pathways

- integrate memory with emotion

i. Papez Circuit

- integrates emotion with explicit memory

ii. Dorsomedial nucleus (thalamus)

- receives from most limbic structures

- output à orbitofrontal cortex

- integrates emotion with implicit memory

II. Biochemistry

a. Neurotransmitters

i. Glutamate

- implicated in seizure focus (glutamate toxicity) can à bilateral (callosal) lesions (therefore it is important to control seizures)

- limbic system (entorhinal cortex, hippocampus, dentate gyrus)

ii. 5HT (serotonin)

- raphe nuclei (isolated reticular formation nuclei)

- effects: promotes a feeling of well-being and sleep; reduces aggression and compulsion; elevates pain threshold

iii. NE

- increases feelings of well-being; reduces compulsion; may induce anxiety

iv. Dopamine

- midbrain (substantia nigra, VTA)

- increases feelings of well-being; increases aggression, alertness, sexual excitement; reduces compulsion

v. ACh

- basal forebrain neurons (nucleus basalis of Meynert), septal nuclei

- associated with memory

vi. GABA

- integrative interneurons

- reduces anxiety and compulsion; elevates pain threshold

b. Classic Psychoactive drugs

i. Major tranquilizers (= anti-psychotics = neuroleptics)

1. Dopamine blockers

- control psychotic behaviour

- L-dopa (Parinson’s treatment) is contraindicated in individuals with mental instability

a. Phenothiazines

b. Butyrophenones

2. Atypical anti-psychotic drugs

- fewer extrapyramidal motor system effects

- dopamine antagonists

a. Clozapine

b. Risperidone

ii. Minor tranquilizers (= anti-anxiety drugs)

1. benodiazepines (valium)

- valium+alcohol à lethal respiratory combination

- also anti-convulsant

- (GABA enhancer)

iii. Anti-depressants

1. Monoamine oxidase (MAO) inhibitors

- increase endogenous levels of monoamines (NE, 5HT)

- also block tyramine (found in cheese) metabolism à hypertension à hemorrhagic stroke

2. Tricyclic anti-depressants

- operate on monoaminergic systems (NE, 5HT)

- potent anti-cholinergic (à dry mouth)

c. Designer drugs

i. Prozac

- SSRI (selective serotonin reuptake inhibitor)

- impacts “reactive depression”, obsessive “depressed” personalities

ii. Zoloft

- SSRI

- works on those for whom Prozac does not work

iii. Welbutrin

- unknown neurochemical mechanism

d. Social drugs

i. Amphetamine

- stimulates catecholaminergic neurons in limbic system and reticular formation

- may à paranoid psychosis (treat with anti-psychotic agents)

ii. Cocaine

- monoamines à euphoria, tolerance

- impacts: 1. MFB, VTA, nucleus accumbens septi

III. Neuroimmunology

- limbic system plays a critical role in development of coping skills (“emotional thermostat”)

- limbic system affects: ANS (hypothalamic projections à sweat with fear, etc.); reticular formation (à alert with fear)

STRESS

- physical and psychological threats treated similarly by limbic system

1. Endocrine response/Immune response

a. increased:

i. endorphins (blunt physical and mental pain)

ii. enkephalins

iii. ACTH release (fight or flight) à catecholamines (epinephrine and norepinephrine) and cortisol release by adrenals

- high cortisol decreases natural immune response (decrease: natural killer cells, T cells, DNA repair; autoimmune increase)

- high cortisol seen in depressed patients

2. Cardiovascular response (mediated by catecholamine increase)

- chronic stress damages heart muscle (non-ischemic), increases CAD, muscular spasms of artery walls

- increases platelet clumping

- increases cholesterol build-up