The limbic system(Physiology)

The limbic system is a group of gray matter and white matter structures lodged deep within the cerebrum that are involved in four functions: olfaction, emotional responses, behavioral activities and memories.

Anatomical structures:

–Limbic lobe: has two main components that are cingulate gyrus (involved in memory and emotional processing and autonomic nervous system) and Para hippocampal gyrus (primarily involved in memory processing).

– Hippocampal formation: composed of three parts: dentate gyrus (afferent information, subiculum (efferent information) hippocampus proper (efferent information)

– Amygdala: which is involved in emotions and emotional behaviors and emotional responses to smell. The Amygdala has two components: cortico medial nuclear group (involved in olfaction) and basolateral nuclear group (all other emotions and behaviors not related to olfaction).

– Hypothalamus: the most important nuclei of hypothalamus in relation with limbic system are mammillary bodies, and autonomic nervous system nuclei (posterior sympathetic anterior parasympathetic)

– Thalamus: there are two main nuclei involved in the limbic system (anterior nucleus of the thalamus which is involved in papez circuit and the other one is called the mediodorsal nucleus which is involved in the circuit connecting the amygdala to prefrontal cortex “emotions and behaviors)

Septal area:

Habenula (part of epithalamus)

Habenula and septal area are connected via a structure called stria medullaris and are involved, particularly in reward pathway and emotional responses as well.

Pathways connecting different structures of the limbic system:

Fornix:

Connects hippocampus to hypothalamus (mammillary bodies) and the septal area.

Striae terminalis: connects the amygdala to the hypothalamus and septal area.

Ventral amygdalofugal pathway : connects the amygdala to the hypothalamus and septal area and also to the mediodorsal nucleus of the thalamus

Striae Medullaris thalami: connects septal area to habenula.

Mamillo-thalamic tract: connects mammillary bodies to the anterior nucleus of the thalamus.

Medial forebrain bundle:  two-way connection that connects the prefrontal cortex to the reticular formation in the brain stem running through the hypothalamus.

Mamillo tegmental tract: connects mammillary bodies of hypothalamus to ventral tegmental area

Mamillary peduncles: connects the ventral tegmental area to the hypothalamus

Functions of limbic system:

-Olfaction: Olfactory epithelial cells, olfactory bulb, action potential in the olfactory tracts, the olfactory tracts bifurcate in medial olfactory striate and lateral olfactory striate which goes into the Para hippocampal gyrus (memory) and to the amygdala (emotions of smell).

-Memory (learning): papez circuit:

Subiculum → Fornix → Mamillary bodies → Mamillo thalamic tract → anterior nucleus of the thalamus → cingulate gyrus →Para hippocampal gyrus → entorhinal cortex → dentate gyrus → subiculum.

Hippocampus (dentate gyrus, which is the receiving portion and subiculum and hippocampus proper which are the leaving portion),

Cingulate gyrus sends also the information to the prefrontal cortex and the purpose of communicating with the prefrontal cortex is to have our memory be involved with our thought and decision making.

-Emotions: particularly the emotional responses, and it’s also involved in behaviors and there are three types of behaviors that the limbic system is involved in :

a) Emotional responses: that means (fear, rage, anger, sadness)

b) Behaviors (feeding behaviors, sexual behaviors, motivational behaviors)

The Amygdala is the center (epicenter) for emotions and behaviors in the limbic system. How does it basically know that we are fearful, angry, or sad? In fact, the limbic system is in communication with our cerebral cortex.

Prefrontal cortex: is involved in thought processing (reasoning, judgment, decision making, personality

Temporal lobe: is involved in multiple functions (smell, taste, visceral sensations, and other areas which are involved in auditory association areas and all are in communication with the amygdala)

Posterior association area: receives information from three different areas: somatosensory association cortex, visual association cortex, and auditory association cortex, and then communicates that with the amygdala, so the amygdala receives informations from all of these structures. And from the cerebral cortex communicating with the amygdala, it will be possible to have enough information to send information to two regions via striae terminalis to septal area and hypothalamus.

The feeding is also controlled by amygdala imagine the sadness amygdala will send information to the nuclei of hypothalamus the ventro-medial nucleus is involved in satiety and the other nucleus is called lateral hypothalamus nucleus and is involved in hunger so the amygdala is responsible for feeding aspect of the organism.

The sexual behavior is also controlled by amygdala:

Depending your psychological situation amygdala feel if you are in sexual mood or not and if yes it will intervene by two nucleus the first one is paraventricular nucleus which releases oxytocin which is commonly secreted in females for uterine contraction and milk ejection, in males it is engaged in sexual orgasm and sexual drives itself. And the other one is Medial Preoptic nucleus which releases Gonadotropin releasing Hormone and GNRH releases in male testosterone which is also responsible for increased sex drive.

Motivational behavior:

We know that amygdala connects with septal area and hypothalamus via the striae terminalis or Ventral medio amygdalo fugal pathway and from the septal area or from the hypothalamus they both can communicate with the ventral tegmental area located in the brainstem, in the ventral tegmental area there is a lot of dopamine (many dopaminergic neurons are located there) and ventral tegmental area sends neurons to Nucleus Accumbens and prefrontal cortex so the amygdala in some conditions (drug abuse for example)  will sends the information to the septal area and to hypothalamus and these in turn will send informations to the ventral tegmental area and from the ventral tegmental area the dopaminergic neurons will connect with nucleus Accumbens and prefrontal cortex. The pathway from the ventral tegmental area to the prefrontal cortex is called the meso cortical pathway and the pathway from the ventral tegmental area to the nucleus Accumbens is called the mesolimbic pathway. The areas receiving the dopaminergic neurons are engaged in reward sensation

Fear:

In fear induced (as emotion) the amygdala will connect via the stria terminalis and ventral amygdalofugal pathway with hypothalamus (posterior hypothalamic nucleus) and the posterior hypothalamic nucleus sends its descending axons downwards via what is called the hypothalamospinal tract and innervates the preganglionic neurons(sympathetic) located within the thoraco lumbar region of the spinal cord  and from there the sympathetic motor neurons will go to the target organs and the final result will be:

Sympathetic activation: cf autonomic nervous system

And the Paraventricular nucleus of hypothalamus which will secrete the cortico-tropine releasing hormone (CRH) which will stimulate the secretion of ACTH by pituitary gland, which in turn will activate the cortisol secretion in adrenal gland (cortisol is called the stress hormone)

Corticotropin-releasing hormone (CRH), a 41 amino acid neuropeptide, discovered in 1981 by Wiley Vale is likely involved in all three types of stress-response. Behavioral responses may involve CRH present in the cerebral cortex and amygdala. Autonomic responses are controlled in part by brainstem fibers descending from the locus coeruleus, which receives CRH-containing fibers from the amygdala and paraventricular nucleus. Hormonal responses center on activation of the hypothalamic–pituitary–adrenal (HPA) axis, which is initiated by CRH present in the paraventricular nucleus of the hypothalamus (PVH).