neuroanat_martin15

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Martin Ch. 15: The Limbic System

FUNCTIONAL ANATOMY OF THE LIMBIC SYSTEM (see fig. 15-1)

Subcortical structureso Hippocampal formation: important for converting short to long term memory, and spatial

memory

oAmygdala: important for emotions and behavioral responses to emotions

There is a strong connection between the olfactory system and the limbic systemo Olfactory bulbs project to a portion of the amygdala and piriform cortex

Components of the limbic system are C-shaped(see fig. 15-1, 15-2)

C-shaped telencephalic components of the limbic system:o Limbic association cortexo Hippocampal formation and its efferent pathway, the fornixo Part of the amygdala and one of its output pathways, stria terminalis

The limbic association cortex is located on the medial surface of the frontal, parietal, and temporal lobes

Major association areas of the cortex = parietal temporal occipital area, prefrontal associationcortex, limbic association cortex

Limbic association cortex receives input from higher-order sensory areas and from otherassociation cortices

Limbic association cortex sends info to the hippocampal formation and amygdala

o Amygdala receives modality specific informationo Hippocampus receives integrated sensory info from multiple modalities

Cingulate and parahippocampal gyri form a ring of cortex that encircles the corpus callosum,diencephalons, and midbrain

o Cingulum: collection of axons deep within cingulate and parahippocampal gyri, corticalassociation fibers course in cingulum and terminate in parahippocampal gyrus

Rostral to cortical ring are medial orbital gyri of frontal lobe and cortex of temporal lobe Lateral boundary of limbic association cortex = collateral sulcus

The hippocampal formation plays a role in memory consolidation

Case of H.M.: medial temporal lobe was ablated to ameliorate temporal lobe epilepsy,hippocampal formation was lesioned and he lost the ability to convert short term memory to long

term

Hippocampal formation = subiculum, hippocampus proper, dentate gyrus (these portions from acylinder which during development is rolled up like a jelly-roll) (see fig. 15-4)

Major input to hippocampal formation = entorhinal cortex (part of limbic association cortex) Output pyramidal neurons are located in hippocampus and subiculum, axons collect on surface of

hippocampal formation and form fornix

From subiculum axons synapse in mammillary bodies; mammilary body project to anterior nucleiof thalamus via mammillothalamic tract; circuit is completed because these fibers go back tocincgulate gyrus which in turn projects to hippocampal formation (part of network of bi-

directional connections once thought to be involved in emotion, now known to be more important

in memory)

Korsakoffs syndrome: thiamin deficiency (too much booze) leads to degeneration of mammillarybodies and profound memory loss

Axons from hippocampus terminate in septal nuclei, which then send cholinergic and GABAergicprojects via fornix back to hippocampal formationThe amygdaloid nuclear complex contains three major functional divisions

Main portion of amygdala is almond-shaped Stria terminalis (major output) and bed nucleus of stria terminalis are C-shpaed Other output path = ventral amygdalofugal pathway which is more straight Amygdala is key to our emotional experiences Contains three principal groups of nuclei: basolateral, central, and corticomedial

The basolateral nuclei are reciprocally connected with the cerebral cortex (see fig. 15-6 A)

Basolateral nuclei are largest division of amygdala


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Important in attaching emotional significance to stimulus Inputs = higher-order sensory cortical areas of temporal and insular lobes and association cortex Outputs= cortex (limbic association and prefrontal cortices, allocortex of hippocampal formation

which is important in learning emotional significance of complex stimuli), basal nucleus, thalamus(via ventral amygdalofugal pathway), and central amygdala nucleus (important in mediating

behavioral responses to emotional stimuli)

The central nucleus projects to brain stem and spinal autonomic nuclei (see fig. 15-6 B) Receives viscerosensory input from brain stem nuclei (particularly solitary nucleus and

parabrachial nucleus)

Projects to dorsal motor nucleus of vagus via ventral amydalofugal pathway; also projects to otherbrain stem nuclei and portions of reticular formation

Central nucleus regulates autonomic nervous system through hypothalamic projections Receives input from basolateral nucleus

The corticomedial nuclei are reciprocally connected with olfactory structures (see fig. 15-6 C)

Important in behaviors triggered by olfactory stimulus (ex. sexual responses) Projects to ventromedial nucleus of hypothalamus (mediates functions associated with appetitive

behaviors) via stria terminalis

There are connections between components of the limbic system and the effector systems

Complex polysynaptic pathways link the limbic system with the endocrine, autonomic, andsomatic motor systems (see fig. 15-7)

Endocrine:o Indirect connections exist between the amygdala and the periventricular hypothalamuso Projection from corticomedial amygdala to ventromedial nucleus which projects to

arcuate nucleus

Visceral:o Central amygdaloid nucleus projects directly to brain stem and spinal autonomic centerso Connections to lateral hypothalamus influence autonomic function

Behavioral:o There are direct and indirect projections from the hippocampus, septal nuclei, and

amygdala to lateral hypothalamus which influences the reticulospinal systemo Important in triggering stereotypic defensive reactionso Periaqueductal gray mediates motor behaviors typical for particular species; receives

inputs from central amygdala nucleus and hypothalamuso Limbic system also influences somatic motor functions through limbic loop of basal

gangliaAll major neurotransmitter regulatory systems have projections to the limbic system

Midbrain dopaminergic projections: originate from ventral tegmental area and substantia nigrapars compacta; course through MFB and nigrostriatal tract and synapse on neurons in prefrontal

association area, limbic association area, limbic association cortex in medial frontal lobe,cingulated gyrus, striatum, amygdala, and hippocampal formation; excessive dopamine

transmission in limbic structures may contribute to schizophrenia

Serotonergic projections: originate from dorsal and median raphe nucleus; course through MFB,dorsal longitudinal fasciculus, and medial longitudinal fasciculus and synapse on neurons inamygdala, hippocampal formation, striatum , and cerebral cortex; drugs that block serotonin

reuptake are effective in treating mood disorders

Noradrenergic projection: originates from locus ceruleus; influences entire cerebral cortex; maybeinvolved in depression

Cholinergic projection: originates from basal nucleus, medial septal nucleus, and nucleus of thediagonal band; targets = entire neuocortex, amygdala, and hippocampal formation; Alzheimers

begins with loss of basal forebrain cholinergic neurons

REGIONAL ANATOMY

Three components of the limbic system have a C-shape: the limbic association cortex, thehippocampal formation and the fornix, and the amygdala and the stria terminalis

The nucleus accumbens and olfactory tubercule comprise part of the basal forebrain (see fig. 15-8)

Limbic loop of basal ganglia:


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o input side = ventral striatum (nucleus accumbens, olfactory tubercles, and ventromedialparts of caudate and putamen) receives info from nuclear groups of amygdala

o output side= ventral pallidum which projects to medial dorsal nucleus of thalamus andthen to limbic association cortex

Olfactory tubercle: corresponds to the region of the anterior perforated substance wherepenetrating branches of middle and anterior cerebral arteries enter basal brain surface to supply

parts of basal ganglia and internal capsuleBasal forebrain cholinergic systems have diffuse limbic and neocortical projections (see fib. 15-9 and 15-

10)

Septal nuclei are adjacent to septum pellucidum, lateral nucleus is near ventricular surface, whilemedial nucleus is near septum pellucidum

Lateral septal nucleus receives projections from hippocampus and subiculum via fornix Medial septal nucleus receives input from lateral septal nucleus and projects to:

o Hippocampal formation via fornixo Midbrain via MFBo Habenula via stria medullaris

Medial septal nucleus uses acetylcholine Mediolateral organization of cortical projections from medial septal nucleus, nucleus of diagonal

band, and basal nucleus

oMedial portion: projects to hippocampal formation via fornix and to medial surface ofhemisphere via cingulum bundle

o Lateral portion: projects to lateral cortex via external capsuleThe cingulum courses beneath the cingulate and parahippocampal gyri(see fig. 15-9, 10,11)

Cingulum: pathway that connects regions of orbitofrontal gyri and cingulate gyrus withparahippocampal gyrus

Uncinate fasciculus: interconnects anterior portions of temporal lobe with medial orbital gyri offrontal lobe in more direct way

The three nuclear divisions of the amygdala are revealed in coronal section

Uncus= amygdala +rostral hippocampuso Uncal herniation: result of space-occupying lesion above cerebellar tentorium,

compresses mid brain leading to coma and death

Divisions of amygdala (see fig. 5-10)o

Corticomedial division merges with overlying cortex of medial temporal lobe;gets majorinput from olfactory bulb

o Bed nucleus of stria terminalis: C-shaped nucleus component of amygdala, connects withbrain stem autonomic and visceroafferent nuclei

The stria terminalis and the ventral amygdalofugal pathway are the two output pathways of the amygdala

Stria terminalis carries output from amygdala predominantly from corticomedial nuclei Stria terminalis and its bed nucleus are c-shaped Thalamostriate vein (drains portions of thalamus and caudate) runs with stria terminalis Major target of stria terminalis = ventral medial nucleus of hypothalamus (important in feeding) Ventral amygdalofugal pathway (other output from amygdala), projections of central and

basolateral nuclei course through this pathway; major targets:o Medial dorsal nucleus of thalamus: indirectly links basolateral amygdala with prefrontal

and orbito frontal cortices

oBasal forebrain:link amygdala with cortex indirectly

o Brain stem: contains parasympathetic preganglionic nuclei, receives projection fromcentral nucleus

The hippocampal formation is located in the floor of the inferior horn of the lateral ventricle

During development the hippocampal formation undergoes an infolding into the temporal lobe(see fig. 15-14)

The hippocampal formation is archicortex and has a laminar organization

Contains three principal cell layers Archicortex: type of allocortex (as opposed to neocortex)


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Dentate gyrus contains molecular (apical dendrites of granule cells), granule cell (projectionneurons of dentate gyrus), and polymorphic (interneurons) layers

Hippocampus and subiculum: layers are analogous to those of dentate, however, granule cell layeris replaced by pyramidal layer

o These pyramidal cells send axons to distant subcortical targetsA sagittal cut through the mammillary bodies reveals the fornix and mammillothalamic tract

Fornix contains four anatomical parts

o Fimbria: collection of alvear (pyramidal cell axons of hippocampus and subiculum)axons which collect on medial side of hippocampal formation

o Crus: where axons are separate from hippocampal formationo Body: where the axons from oth sides join at midlineo Column: where axons descend toward their targets

Mammillothalamic tract: originates from both medial and lateral mammillary nuclei; projects toanterior thalamaic nuclei

Mammillotegmental tract: originates from lateral mammillary nucleus Fibers of fornix terminate in many parts like mammillary bodies, anterior thalamic nuclei, and

lateral septal nucleus

Nuclei in the brain stem link telencephalic and diencephalic limbic structures with the autonomic

nervous system and the spinal cord

See fig. 15-17

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