LIMBIC SYSTEM

Dr. Amani A. Elfaki

Associate Professor

Department of Anatomy

Learning Objectives

• Define the limbic system

• Identify the parts of the limbic system

• Describe the circulation of the limbic systemand its connections

• Mention the functional parts of the limbicsystem

Definition

• The term limbic refers to deep portions of thecerebrum that form a ring around the diencephalon.

• The limbic system includes nuclei and tracts along theborder between the cerebrum and diencephalon.

• The term limbic system was loosely used to include agroup of structures that lie in the border zonebetween the cerebral cortex and the hypothalamus.

Definition

• Now it is recognized, as the result of research, thatthe limbic system is involved with many otherstructures beyond the border zone in the control ofemotion, behavior, and drive. It also appears to beimportant to memory.

• The limbic system is a functional grouping ratherthan an anatomical one.

LIMBIC SYSTEM AND ASSOCIATED STRUCTURES OF THE RIGHT HEMISPHERE

Functions of the limbic system

1) Establishing emotional states.

2) Linking the conscious, intellectual functions of the cerebralcortex with the unconscious and autonomic functions of thebrain stem.

3) Facilitating memory storage.

Motivational System

• The sensory cortex, motor cortex, andassociation areas of the cerebral cortex enableyou to perform complex tasks, it is largely thelimbic system that makes you want to dothem.

• For this reason, the limbic system is alsoknown as the motivational system.

Structures of the Limbic System

• Anatomically, the limbic structures include thesubcallosal, the cingulate, and the parahippocampalgyri, the hippocampal formation, the amygdaloidnucleus, the mammillary bodies, and the anteriorthalamic nucleus.

• The alveus, the fimbria, the fornix, themammillothalamic tract, and the stria terminalisconstitute the connecting pathways of this system.

THE COMPONENTS OF THE LIMBIC SYSTEM IN THE DIENCEPHALON

COMPONENTS OF THE LIMBIC SYSTEM IN THE CEREBRUM

Hippocampal Formation

• The hippocampalformation consists ofthe hippocampus,the dentate gyrus,and theparahippocampalgyrus.

• Is located in thetemporal lobe ofeach cerebral cortex,medial to the inferiorhorn of the lateralventricle.

Hippocampus

• The hippocampus is a curved elevation of graymatter that extends throughout the entire length ofthe floor of the inferior horn of the lateral ventricle.

• Its anterior end is expanded to form the peshippocampus.

• It is named hippocampus because it resembles a seahorse in coronal section.

Hippocampal Formation

Dissection of the right cerebral hemisphere exposing the cavity of the lateral

ventricle, showing the hippocampus, the dentate gyrus, and the fornix

Coronal section of the hippocampus and related structures

Dentate gyrus

• The dentate gyrus is anarrow, notched band ofgray matter that liesbetween the fimbria of thehippocampus and theparahippocampal gyrus.

• Posteriorly, the gyrusaccompanies the fimbriaalmost to the splenium ofthe corpus callosum andbecomes continuous withthe indusium griseum.

Indusium griseum

• The indusium griseumis a thin, vestigial layerof gray matter thatcovers the superiorsurface of the corpuscallosum.

Dissection of both cerebral hemispheres showing the superior surface of the corpus callosum

Parahippocampal gyrus

• The parahippocampalgyrus lies between thehippocampal fissure andthe collateral sulcus andis continuous with thehippocampus along themedial edge of thetemporal lobe.

• The dentate gyrus andthe parahippocampalgyrus form the posteriorand inferior portions ofthe limbic lobe.

Amygdala

• The amygdalar nuclei, a major component of the limbicsystem, resemble almonds in shape and are located in the tipof the temporal lobe beneath the cortex of the uncus androstral to the hippocampus and the inferior horn of the lateralventricle.

• There are two main groups of these nuclei: the corticomedialand central and the basolateral. The corticomedial-centralgroup is relatively small and is phylogenetically older. Itmaintains connections with the phylogenetically older regionsof the central nervous system, such as the olfactory bulb,hypothalamus, and brain stem.

Amygdala

• The basolateral group is larger and phylogeneticallymore recent. It has extensive connections with thecerebral cortex.

• Several neurotransmitters have been demonstratedin the amygdala, including acetylcholine, gamma-aminobutyric acid (GABA), noradrenaline, serotonin,dopamine, substance P, and enkephalin.

CORONAL SECTION OF THE BRAIN SHOWING THE AMYGDALA AND ADJACENT STRUCTURES

AMYGDALA

A THREE-DIMENSIONAL RECONSTRUCTION OF THE LIMBIC SYSTEM, SHOWING THE RELATIONSHIPS AMONG THE MAJOR COMPONENTS

Fornix

• The fornix is a tract of whitematter that connects thehippocampus with thehypothalamus.

• From the hippocampus, thefornix curves medially, meetingits counterpart from theopposing hemisphere.

• The fornix proceeds anteriorly,inferior to the corpus callosum,before curving toward thehypothalamus.

• Many fibers of the fornix end inthe mamillary bodies of thehypothalamus.

Anterior Commissure

• The anterior commissure is aband-like tract of white fibersthat crosses the midline tojoin both cerebralhemisphere.

• It contains two fibersystems: an interbulbarsystem, which joins bothanterior olfactory nuclei nearthe olfactory bulbs, and anintertemporal system, whichconnects the temporal lobeareas of both cerebralhemispheres.

Septal Area• The septal area, also called

the septal nuclei or septalcomplex, is an area of graymatter lying above thelamina terminalis andbelow the rostrum of thecorpus callosum, near andaround the anteriorcommissure.

• The septal area is a focalpoint within the limbicsystem, and is connectedwith the olfactory lobe,amygdala, hippocampus,and hypothalamus. Theseptal area is a "pleasurecenter" in the brain.

Connecting Pathways of the Limbic System• The connecting pathways of the

limbic system are the alveus, thefimbria, the fornix, themammillothalamic tract, andthe stria terminalis.

• The alveus consists of a thinlayer of white matter that lies onthe superior or ventricularsurface of the hippocampus.

• It is composed of nerve fibersthat originate in thehippocampal cortex. The fibersconverge on the medial borderof the hippocampus to form a

bundle called the fimbria.

DIAGRAM SHOWING SOME IMPORTANT AFFERENT AND EFFERENT CONNECTIONS OF THE LIMBIC SYSTEM

Connecting Pathways of the Limbic System

• The fimbria now leaves the posterior end of the hippocampus as the crusof the fornix. The crus from each side curves posteriorly and superiorlybeneath the splenium of the corpus callosum and around the posteriorsurface of the thalamus.

• The two crura now converge to form the body of the fornix, which isapplied closely to the undersurface of the corpus callosum . As the twocrura come together, they are connected by transverse fibers called thecommissure of the fornix.

• These fibers decussate and join the hippocampi of the two sides.

DIAGRAM SHOWING SOME IMPORTANT AFFERENT AND EFFERENT CONNECTIONS OF THE LIMBIC SYSTEM

Connecting Pathways of the Limbic System

• The mammillothalamic tract provides important connections betweenthe mammillary body and the anterior nuclear group of the thalamus.

• The stria terminalis emerges from the posterior aspect of the amygdaloidnucleus and runs as a bundle of nerve fibers posteriorly in the roof of theinferior horn of the lateral ventricle on the medial side of the tail of thecaudate nucleus.

• It follows the curve of the caudate nucleus and comes to lie in the floor ofthe body of the lateral.

MEDIAL ASPECT OF THE RIGHT CEREBRAL HEMISPHERE SHOWING STRUCTURES THAT FORM THE LIMBIC SYSTEM