Limbic system

The word limbic means that it lies on the medial rim of the brain between the cerebral cortex and the hypothalamus. So, it consists of a number of structures with looped connections that project into the

hypothalamus .

The cingulate gyrus and parahippocampal gyrus are in continuity with one another around the splenium of the corpus

callosum .

The cingulate gyrus projects to the parahippocampal gyrus via the fibers of the cingulum ( Association fibers connecting frontal ; parietal ; parahippocampal gyrus

and the adjacent temporal gyri ).

The principal structures of the limbic system are linked by a series of

connections ( The Papez circuit ).

The powerful input to the limbic system is from the neocortical association areas .

So, it appears important to intellectual; judgmental; predictive faculties and the planning ( regulation ) of behavior ( 1 ) .

Memory, emotional aspects of behaviour and perceptuospatial function . ( 2 ) .

The storage of previous sensory experiences. Recall objects seen or music heard in the past ( 3 ) .

Planned behaviour

Information can reach supramodal status and meaning

Entry of information to the limbic system is either directly to amygdala or indirectly to

hippocampal formation , via the entorhinal area.

The amygdala appears to provide an affective connotation to experience and social stimuli .

The hipocampus is concerned with converting recent memory to long term memory ( permits a

link to previous experience ).

The limbic system , via the hpothalamus and its connections with the outflow of the autonomic nervous system and its control of the endocrine system ( pituitary gland ) is able to influence emotional behavior . Like reactions to fear ;

anger ; sexual behavior.

Also, it is able to influence motor responses through projections to nucleus accumbens ,

which forms a part of the basal ganglia .

Entorhinal area ( E A ) is the cranial extension of the parahippocampal gyrus

amygdaloid

Hippocampal Formation

It consists of the hippocampus , the dentate gyrus and part of the parahippocampal gyrus .

The hippocampus is formed by an infolding of the inferomedial part of the temporal lobe into the lateral ventricle .

The dentate gyrus lies between the parahippocampal gyrus and the hippocampus. Anteriorly, it continues into the uncus .

The hippocampal formation receives afferents from the inferior temporal cortex via the entorhinal area .

Also, receives fibers from the contralateral entorhinal area and hippocampus via the fornix and hipoocampal commissure ( consists of transverse fibers linking the

posterior columns of the fornix on each side ) .

The principal efferent pathway from the hippocampus is the fornix .

It is C- shaped fascicle of fibers that links the hippocampus with the mammillary

body of the hypothalamus .

The efferent fibers which converge on the ventricular surface of the hippocampus are called fimbria. They pass posteriorly then superiorly to become continuous with the crus of the fornix which curves forward beneath the splenium of corpus callosum.

The 2 crura unite in the middle line to form the body which divides into 2 columns . These curve downwards forming the anterior border of the interventricular foramen then enter the hypothalamus. Most fibers terminate in

the mammillary body.

The mammillary body projects to A. nuclear group of the thalamus via the mammillothalamic tract and to the brain

stem via the mammillotegmental tract .

The A. nuclei of the thalamus have major connections with the cingulate gyrus .

Amygdala

It lies near the anterior temporal pole , between the inferior horn of the lateral ventricle & the lentiform nucleus .

It receives afferents from the inferior temporal association cortex ; the olfactory tract and the septum. Also, it receives catecholamine and serotonin-containing

projections from the brain stem in the medial forebrain bundle .

The ventral amygdalofugal path ( consists of a diffuse array of fibers passing inferior to the lentiform nucleus through the A. perforating substance to reach the

hypothalamus) .

The principal efferent ,project into the stria terminalis which runs in the wall of the lateral ventricle following the curvature of the caudate nucleus, to terminate in the hypothalamus.

Septum

It lies beneath the rostral part of the corpus callosum .

It interconnects with the amygdala and projects to the hypothalamus via the medial forebrain bundle . It also, connects to the monoaminergic nuclei in the brain stem . It dose so via fibers that project to the habenular nuclei (epithalamus)

of the diencephalon and constitute the stria medullaris thalami.

The habenular nuclei project via the fasciculus retroflexus to the interpeduncular nuclei which project to the brain stem as well as the hypothalamus .

The areas that are typically included in limbic system in cortical regions are hippocampus and neocortex which includes insular cortex; orbital frontal cortex;

sucallosal gyrus; cingulate gyrus and parahipocampal gyrus .

The subcortical portions are olfactory bulb; hypothalamus; amygdala; septal nuclei and anterior nucleus of the thalamus.

Its input is formed of limbic cortex, amygdala and hippocampus.

Its output are the septal nuclei and hypothalamus .

Limbic lobe disorder

Alcohol abuse, with deficiency of thiamine in diet , lead to capillary haemorrhages in the limbic system .The patient falls into confusion and coma ( Wernicke’s encephalopathy)

If partial recovery occur , there is failure to remember previous experience ( Retrograde amnesia ) . Or to learn new facts ( Anterograde amnesia – unaffected memory for remote past events before the lesion developed ) . This state is called

Korsakoff’s psychosis .

A similar amnesic syndrome occurs when bilateral, surgical temporal lobectomy including the hippocampal formations.

Temporal lobe lesion near the amygdala and hippocampi can lead to complex experiences of smell , mood , and memory .

Surgical ablation of the amygdala has eliminated uncontrollable rage reaction in some psychotic patients.