The Limbic System

Overview of the Limbic System

• The limbic system consists of gray and white matter structures located deep within the cerebrum.
• It mediates olfaction, emotional responses, behavioral activities, and memory.

Gray Matter Structures of the Limbic System

• Includes the limbic lobe, hippocampal formation, amygdala, hypothalamus, thalamus, septal area, and habenula.

Limbic Lobe

• This is a C-shaped structure composed of the cingulate gyrus and the parahippocampal gyrus, located on the medial side of the brain.
• The cingulate gyrus participates in memory, emotional processing, and autonomic nervous system regulation.
• The parahippocampal gyrus, found in the temporal lobe, primarily handles memory processing.

Hippocampal Formation

• It sits medial to the parahippocampal gyrus and includes the dentate gyrus, subiculum, and hippocampus proper (cornu ammonis).
• The dentate gyrus functions as the input gateway to the hippocampus, receiving afferent (incoming) information.
• The subiculum and hippocampus proper serve as the output channels from the hippocampus, sending efferent (outgoing) information.

Amygdala (Amygdaloid Body)

• Essential for processing emotions, behaviors, and emotional reactions to smells, located above the hippocampal formation.
• It comprises two main nuclei, the corticomedial nuclear group (for olfaction) and the basolateral nuclear group (for other emotions and behaviors).

Hypothalamus

• Located in the diencephalon.
• Features multiple nuclei, notably the mammillary bodies and autonomic nervous system nuclei (posterior for sympathetic, anterior for parasympathetic functions).

Thalamus

• Also located in the diencephalon.
• Contains nuclei such as the anterior nucleus (involved in the Papez circuit for memory) and the medial dorsal nucleus.
• The medial dorsal nucleus receives input from the amygdala and connects to the prefrontal cortex, impacting emotions and behavior.

Septal Area and Habenula

• The septal area is located within the septum pellucidum and paraterminal gyrus.
• The habenula is a component of the epithalamus.
• Connected by the stria medullaris, the septal area and habenula participate in reward pathways and emotional responses.

White Matter Structures of the Limbic System

• These structures facilitate connections between limbic nuclei
• They include the fornix, stria terminalis, ventral amygdalofugal pathway, stria medullaris thalami, mammillothalamic tract, medial forebrain bundle, and mammillotegmental tract.

Fornix

• Connects the hippocampus to the septal area and the mammillary bodies of the hypothalamus.
• It has a role in memory pathways.

Stria Terminalis

• Connects the amygdala to the septal area and hypothalamus.
• This tract runs dorsally underneath the corpus callosum.

Ventral Amygdalofugal Pathway

• Connects the amygdala to the septal area, hypothalamus, and mediodorsal nucleus of the thalamus.
• It courses ventrally.

Stria Medullaris Thalami

• It links the septal area to the habenula.

Mammillothalamic Tract

• Connects the mammillary bodies of the hypothalamus to the anterior nucleus of the thalamus.

Medial Forebrain Bundle

• Extends from the prefrontal cortex through the lateral hypothalamus to the reticular formation in the brainstem.
• This is a two-way communication channel from the prefrontal cortex to the lateral hypothalamus, reaching the reticular formation of the brainstem.

Mammillotegmental Tract and Mammillary Peduncles

• The mammillotegmental tract links the mammillary bodies to the ventral tegmental area.
• Mammillary peduncles connect the ventral tegmental area back to the hypothalamus and mammillary bodies.
• The ventral tegmental area, located in the midbrain, is rich in dopamine.

Functions of the Limbic System: Olfaction

• Olfactory stimuli activate olfactory epithelial cells, then olfactory nerves, which transmit signals through the cribriform plate to the olfactory bulb.
• From the olfactory bulb, action potentials travel down the olfactory tracts, which split into medial and lateral olfactory striae.
• The lateral olfactory striae send information to the parahippocampal gyrus (for memory of smells) and the amygdala (for emotions associated with smells).

Functions of the Limbic System: Memory and Learning

• The Papez circuit is critical for memory and learning processes.
• The circuit involves the hippocampus (specifically the subiculum), fornix, mammillary bodies, mammillothalamic tract, anterior nucleus of the thalamus, and cingulate gyrus.
• The anterior nucleus of the thalamus relays information to the cingulate gyrus, which then sends it to the parahippocampal gyrus, and finally back to the hippocampus (dentate gyrus).
• The entorhinal cortex is the connection between the parahippocampal gyrus and the dentate gyrus.
• The cingulate gyrus also communicates with the prefrontal cortex, which integrates memory with thoughts and decision-making.

Functions of the Limbic System: Emotions and Behaviors

• The amygdala is the central structure for processing emotions and behaviors.
• Communication occurs between the amygdala and the cerebral cortex (including the prefrontal cortex), temporal lobe, insula, and posterior association area.
• The amygdala communicates with the septal area and hypothalamus, via the stria terminalis and fornix, to produce emotional and behavioral responses.

Behaviors Influenced by the Limbic System

Feeding Behaviors

• The amygdala affects feeding behavior through communication with the ventromedial nucleus (satiety) and the lateral hypothalamic nucleus (hunger) in the hypothalamus.
• Activation of the lateral hypothalamic nucleus can occur to promote comfort eating when experiencing sadness.

Sexual Behaviors

• The amygdala interacts with the periventricular nucleus (releasing oxytocin) and the medial preoptic nucleus (releasing gonadotropin-releasing hormone).
• This interaction influences sexual activity and drive.

Motivational Behaviors

• This system has an influence on motivation and goal-directed actions.
• During drug abuse, euphoria-inducing stimuli activate communication between the amygdala and the hypothalamus/septal area.
• The ventral tegmental area, a dopamine-rich region, sends dopaminergic neurons to the nucleus accumbens and prefrontal cortex.
• The pathway between the ventral tegmental area and prefrontal cortex constitutes the mesocortical pathway.
• The connection between the ventral tegmental area and nucleus accumbens is the mesolimbic pathway.
• Dopamine release in these areas generates a sense of reward, reinforcing substance abuse behaviors.

Emotional Responses

• The posterior hypothalamic nucleus sends descending axons via the hypothalamospinal tract to innervate preganglionic neurons in the thoracolumbar spinal cord region.
• Autonomic responses to fear, including increased liver glucose production, heart rate, blood pressure, and respiratory rate, are mediated via this tract.
• The hypothalamus releases corticotropin-releasing hormone (CRH), which stimulates the anterior pituitary to produce ACTH, which in turn stimulates the adrenal gland to produce cortisol (the stress hormone).

Clinical Relevance of the Limbic System

Kluver-Bucy Syndrome

• A rare neurological condition resulting from bilateral temporal lobe damage, particularly affecting the amygdala.
• Symptoms include placidity (lack of emotional reaction), hyperphagia, hypersexuality, and amnesia.

Wernicke Encephalopathy

• Usually arises from vitamin B1 deficiency.
• Damages the mammillary bodies, disrupting memory pathways.
• It is characterized by confabulation, ataxia, and ophthalmoplegia.

Wernicke-Korsakoff Syndrome (Wernicke's Encephalopathy)

• Usually due to vitamin B1 deficiency.
• Causes confabulation (vivid imagination to fill memory gaps).
• Also affects the middle cerbellar peduncles (ataxia)
• It also affects the third and sixth nerve nuclei to cause ophthalmoplegia