Lectures 24-25: Limbic System

Overview of the Limbic System

Limbic lobe

allocortex

Limbic system

limbic lobe plus subcortical nuclei &

pathways

Limbic Cortex

Parahippocampal gyrus

Piriform cortex (anterior)

Entorhinal cortex (posterior)

Piriform cortex covers amygdala

Entorhinal cortex covers hippocampal formation

Limbic Association Cortex

Includes temporal pole and

orbitofrontal cortex

Receives inputs from association

areas of neocortex, and then exchanges that information with allocortex

Frontotemporal dementia

degeneration of orbitofrontal cortex and temporal pole cortex

Patients exhibit compulsive behavior, disinhibition, memory deficits, inability to control emotions

Basal Nucleus of Meynert

Basal Nucleus of Meynert

source of cholinergic projections (acetylcholine)

within substantia innominata

limbic projections to amygdala and hippocampal formation

influences arousal, attention, memory

part of etiology of Alzheimer disease

Dorsal & Median Raphe Nuclei

Dorsal & Median Raphe Nuclei

source of serotonergic projections (serotonin)

component of reticular formation

limbic projections to amygdala, hippocampal formation, hypothalamus

influences sleep and arousal, feeding/satiety, and mood

SSRIs demonstrate role in depression, anxiety, OCD, & other mood disorders

Locus Ceruleus

Locus Ceruleus

source of noradregenergic projections (norepinephrine)

diffuse projections to limbic system

influences sleep and arousal, autonomic functions

SNRIs demonstrate interaction with serotonergic system to affect mood

Ventral Tegmental Area (VTA)

Ventral Tegmental Area (VTA)

source of dopaminergic projections (dopamine)

limbic projections to hypothalamus, septal n., n. accumbens, & ventral pallidum

projection to nucleus accumbens (mesolimbic reward pathway)

influences motivation, reward, reinforcement

part of etiology of addiction, substance abuse

Septal Nuclei

Septal Nuclei

Receives dopaminergic input from VTA via the medial forebrain bundle

Connections with hippocampal formation, amygdala, and hypothalamus

Reward, pleasure, and reinforcement center

Lesion in humans may produce “sham rage”

Nucleus Accumbens (Ventral Striatum)

Nucleus Accumbens (Ventral Striatum)

Receives dopaminergic input from VTA via the medial forebrain bundle

(mesolimbic reward pathway)

Expresses receptors for endogenous opiates

Connections with hippocampal formation, hypothalamus, amygdala, and basal nuclei (part of limbic loop)

Reward, pleasure, and reinforcement center

Involved in addiction pathologies

Ventral Pallidum

Ventral Pallidum

Located within substantia innominata

Relay from nucleus accumbens to dorsomedial n. of thalamus (part of limbic loop)

Dorsomedial nucleus of thalamus

Limbic Loop Overview

limbic loop involves disinhibition

of the dorsomedial nucleus of the

thalamus, which projects to limbic,

prefrontal and orbitofrontal

association cortices

The limbic loop functions to regulate

emotional and motivational drives

Like the motor loop, the limbic loop is

facilitated by dopaminergic inputs

dopaminergic inputs to the limbic

loop originate in the VTA and project

to the nucleus accumbens

Dysfunction of the limbic loop

and/or VTA dopaminergic

modulation is implicated in

schizophrenia, depression, and

other disorders

Limbic Loop

Hippocampal Formation

Hippocampal Formation Components

Hippocampal Formation Afferents and Efferents

Cornu Ammonis

Flow of signals through the hippocampal formation

Hippocampal Formation

H.M.

Resection included amygdala,

parahippocampal gyrus, and ant.

2/3 of hippocampal formation

profound anterograde amnesia

Normal immediate and short-term (working) memory of approx. 5 minutes

Retrieval of existing memory (explicit & implicit) largely unaffected (retrograde amnesia limited to about ~8 months)

Implicit (procedural) memory unaffected

Priming intact

Personality and general intelligence normal (IQ of 112, ~ pre-operative level)

Although bilateral hippocampal resections are no longer performed, the hippocampal formation is especially vulnerable to cerebral anoxia or severe

hypoglycemia

Fornix

connects the hippocampal formation

with the mammillary nuclei of the hypothalamus

Fornix

Fornix

Fornix

Fornix

cingulum

Papez Circuit

Papez circuit overview

Structures of the Papez Circuit receive signals regarding sensation, memories, emotion, internal state, executive cognition, etc.

Structures of the Papez Circuit also send output signals, either via physiological expression

(autonomic/endocrine) or conscious experience

Memory

Korsakoff syndrome (alcoholic dementia)

Disorder of the Papez Circuit

thiamine deficiency often as a

result of chronic alcoholic

malnutrition

Degeneration of hippocampal

formation, fornix, mammillary

nuclei, or anterior thalamic nuc.

Dementia, as well as deficits in

short-term memory and memory

consolidation

Retrograde amnesia variable

Prone to confabulation

Wernicke-Korsakoff syndrome

Amygdaloid Afferents

Corticomedial: Olfactory tract, hypothalamus (feeding centers), brainstem nuclei

Basolateral: Hippocampal formation (subiculum), dorsomedial n. of thalamus, diverse areas of cerebral cortex

Amygdaloid Efferents

Corticomedial: Stria terminalis to hypothalamus, septal nuclei, nucleus accumbens

Basolateral: Ventral amygdalofugal pathway ascending to hypothalamus, dorsomedial n. of thalamus, cerebral cortex; and descending to brainstem nuclei

Amygdala

Klüver-Bucy syndrome

S.M.

with rare autosomal recessive condition (Urbach-Wiethe disease)

Calcification and atrophy of the anterior medial temporal lobes

Bilateral extensive lesions of the amygdalae with no detectable damage to the hippocampal formation

No motor or sensory impairments; no deficits in intelligence or language

Profound inability to recognize and experience fear

Some deficits in consolidation of emotional memory

Normal abilities relative to other emotions (both positive and negative emotions)

S.M., and two other individuals with Urbach-Wiethe, experienced fear and panic in response to CO2 inhalation

Anterior choroidal artery supplies

hippocampal formation and amygdala

Anterior choroidal a. occlusion

Contralateral homonymous hemianopia

Possible contralateral hemiparesis/hemiplegia

Transient anterograde amnesia

Transient emotional instability

pericallosal branch of anterior cerebral artery supplies

Cingulate gyrus

posteromedial branches of posterior

communicating artery supplies

Mammillary nuclei

Focal occlusions affecting the cingulate gyrus or mammillary nuclei

can produce dementia and/or memory deficits (Papez Circuit)