Functions of the Rhinencephalon, Amygdala, and Hypothalamus

The Rhinencephalon and Limbic System

The rhinencephalon is a structure related to the sense of smell. In humans, it has evolved into an area responsible for instinctive motor activity, autonomic and visceral functions, behavior, emotions, and memory. It forms the beginning of the limbic circuitry, a complex formation that occupies the inner surface of the cerebral hemispheres, with its end at the cingulum.

Klüver-Bucy Syndrome

First described in 1930, Klüver-Bucy syndrome results from a bilateral temporal lobectomy. Its symptoms include:

• Visual agnosia
• Hyperorality
• Hyperphagia
• Hypersexuality
• Flattening of emotions (emotional blunting)

MacLean's Theory on the Limbic System

MacLean's theory suggests that the limbic system is primarily related to the integration of behaviors such as aggression and fear. Through these experiences, animals can develop more intense and effective responses and new reflexes that increase their adaptive capacity.

The Amygdaloid System

The amygdaloid system is constituted by:

• Temporal amygdalae
• The hypothalamus
• The periaqueductal gray
• The midbrain ventral tegmental area

The amygdala is comprised of numerous nuclear structures that can be grouped into the Medial nuclei, Cortico-baso-lateral nuclei, and the Central nucleus.

Hypothalamic Regions and Nuclei

The medial and lateral areas of the hypothalamus are separated by the fornix and the mammillothalamic tract. The main areas include the Lateral area, Medial area, Tuberal region, and Mammillary region.

Lateral Hypothalamic Area

This area includes two large nuclei:

• Lateral preoptic nucleus: The superior telencephalic portion.
• Lateral hypothalamic nucleus: Stimulation results in hunger, while its injury causes anorexia.

Medial Hypothalamic Area

This area includes the periventricular area surrounding the third ventricle. It is divided into four regions from anterior to posterior: Preoptic, Supraoptic, Tuberal, and Mammillary.

Preoptic Region

This is the anterior telencephalic portion. It contains the medial preoptic nucleus, which regulates the release of gonadotrophic hormones from the anterior pituitary. It also contains the sexually dimorphic nucleus, whose development depends on testosterone levels.

Supraoptic Region

This region contains several important nuclei:

• Suprachiasmatic nucleus: Receives direct input from the retina, controlling circadian rhythms.
• Anterior nucleus: Plays a role in regulating temperature. It stimulates the parasympathetic autonomic nervous system (ANS). Its destruction produces hyperthermia.
• Paraventricular nucleus (PVN): Produces ADH (antidiuretic hormone), oxytocin, and CRH (corticotropin-releasing hormone). It regulates water balance via the supraopticohypophyseal tract. Its destruction results in diabetes insipidus.
• Supraoptic nucleus (SON): Synthesizes ADH and oxytocin.