Limbic System

"Life is all memory, except for the one present moment that goes by you so quickly you hardly catch it going." – Tennessee Williams

From the reptilian to the mammalian brain.

In evolutionary terms, the hindbrain (brainstem) along with the cerebellum was firstly developed in early vertebrates and developed differently in several species since then to allow complex behaviors, including humans (Naumann et al., 2015; Chakraborty & Jarvis, 2015). As we have already discussed, the brainstem along with the cerebellum is mainly responsible for vital functions (e.g., breathing), reactive responses and coordinating motor movement and balance.

After the hindbrain and midbrain, comes the limbic system (mammalian brain). The limbic system is named after the Latin word for "border" (limbus) because of its cortical (outer) borders surrounding the brainstem. The limbic system is a group of cortical and subcortical structures that relate to protecting the brain, maintaining structure, epicenter of emotion, cognition and complex behaviors (Catani et al., 2013).

In this chapter we will discuss some individual regions of the mammalian brain and then how these regions interact with one another (networks) to form cognition and higher learning.

Limbic system diagram
Source: https://commons.wikimedia.org/wiki/File:Limbisches_System.jpg

Amygdala

The Amygdala is composed of a collection of nuclei shaped as an almond, hence the name amygdala. The region is highly associated with fear, aggression and emotional learning, linking emotional significance to memories and sensory inputs (LeDoux, 2007). Through its connections with the hypothalamus and prefrontal cortex, it helps coordinate behavioral and physiological responses to emotional stimuli (AbuHasan et al., 2023; Torrico & Abdijadid, 2023). People with fear or anxiety induced disorders, depression and other psychiatric disorders experience functional and structural changes to the Amygdala because of one of these conditions is associated with fear and anxiety to some extent.

Hippocampus

The Hippocampus (Sea Horse in Greek) is a region thoroughly investigated by the science community and it is shaped like a sea horse. The region is essential for the formation and retrieval for long-term memories (facts and events) and for spatial navigation. It communicates extensively with the amygdala, hypothalamus and neocortex (the most exterior part of the brain) and sends information via the fornix to other limbic structures (Knierim, 2015). Damage to the region can lead to forms of amnesia.

Parahippocampal Gyrus

The Parahippocampal Gyrus surrounds the hippocampus and acts as a cortical relay that supports memory encoding and retrieval. It is involved in scene recognition and contextual association—functions crucial for recalling places and experiences. It also connects the hippocampus with widespread cortical areas (Van Hoesen et al., 2000).

Mammillary Bodies

The mammillary bodies are small paired structures located on the inferior aspect of the hypothalamus. They form a key relay in the limbic circuit by receiving input from the hippocampus through the fornix and projecting to the anterior thalamic nuclei via the mammillothalamic tract. They are vital for recollective memory, and damage here is a hallmark of Wernicke–Korsakoff syndrome (vitamin B1 deficiency leading to acute or chronic memory concerns, Sullivan & Pfefferbaum, 2009).

Hypothalamus

The hypothalamus sits below the thalamus and bridges the nervous and endocrine systems (network of glands and organs producing hormones). It regulates homeostatic processes such as temperature, hunger, thirst, and circadian rhythms, and interacts with limbic circuits to coordinate emotional and autonomic responses. The hypothalamus determines these effects by bringing together a range of sensory inputs. Through the pituitary gland (more on glands on later chapters), it converts neural activity into hormonal output (Saper & Lowell, 2014).

Anterior Thalamic Nuclei

The anterior thalamic nuclei receive input from the mammillary bodies and hippocampus and project to the cingulate cortex. This relay forms part of the Papez circuit, which supports episodic memory and emotional experience (responsible for connecting emotions to memories). Lesions in this region can lead to memory and learning deficits (Nelson, 2021). Major brain networks such as the Papez circuit deserve an article of their own.

Papez circuit diagram
Source: Dharani, 2014, p. 66

Septal Nuclei

The septal nuclei are a collection of gray matter structures located beneath the corpus callosum and anterior to the hypothalamus (Butler et al., 2014). They receive input from the hippocampus via the fornix and send reciprocal connections to the hypothalamus and midbrain. Functionally, they are associated with reward, motivation, and the modulation of emotional and autonomic responses. Electrical stimulation of the septal area has been linked to sensations of pleasure and calm.

Septal nuclei diagram
Source: Butler et al., 2014

Nucleus Accumbens

The Nucleus Accumbens lies at the junction of the head of the caudate and the putamen (parts of the basal ganglia, more about it in future publications), forming part of the ventral striatum. It serves as a major interface between the limbic system and the motor system, processing reward, pleasure, reinforcement learning, and motivation. Dopaminergic input from the ventral tegmental area (VTA) and glutamatergic input from the prefrontal cortex and amygdala converge here to modulate goal-directed behavior. (Salgado & Kaplitt, 2015).

Fornix

The fornix is a C shaped, white-matter tract that arches from the hippocampus to the mammillary bodies and septal area. It carries signals crucial for memory consolidation and forms the main output pathway of the hippocampal formation. Damage to the fornix disrupts limbic connectivity and can cause memory loss (Thomas et al., 2011).

Cingulate Gyrus

The cingulate gyrus curves over the corpus callosum and acts as a central hub integrating emotional, cognitive, and autonomic information. Its anterior part regulates emotions and attention, while the posterior portion is involved in spatial memory and self-reflection. It links the thalamus, hippocampus, and prefrontal cortex in emotional and behavioral regulation (Assari & Zare, 2025; Hirono et al., 1998).

The limbic system's orchestration of emotion, memory, and survival instincts extends far beyond anatomy, serving as a cornerstone for therapeutic strategies that recalibrate dysregulated circuits to promote resilience and emotional equilibrium. Therapeutic, spiritual and healthy behaviors activate networks of these brain regions to foster neuroplasticity, modulate autonomic responses, and cultivate profound states of awe, peace, and interconnectedness—transforming raw survival wiring into pathways for growth and harmony.

Limbic System Structures Overview

Structure Primary Functions Key Interactions / Connections
Amygdala Processes fear, aggression, and emotional learning; links emotions to memories. Connects with hypothalamus (autonomic/emotional responses) and prefrontal cortex (emotional regulation).
Hippocampus Forms and retrieves long-term declarative memories; supports spatial navigation. Communicates with amygdala, hypothalamus, and neocortex; sends signals via the fornix.
Parahippocampal Gyrus Aids in memory encoding, contextual association, and scene recognition. Connects hippocampus with cortical regions for memory and perception integration.
Mammillary Bodies Relay for recollective memory and part of the Papez circuit. Receive input from hippocampus (via fornix); project to anterior thalamic nuclei.
Hypothalamus Regulates homeostasis, hormones, and autonomic functions; bridges brain and endocrine system. Communicates with limbic structures and pituitary gland for emotional–physiological responses.
Anterior Thalamic Nuclei Supports episodic memory and emotional experience within the Papez circuit. Receives from mammillary bodies and hippocampus; projects to cingulate cortex.
Septal Nuclei Involved in reward, motivation, and emotional modulation. Receives hippocampal input (via fornix); connects with hypothalamus and midbrain.
Nucleus Accumbens Processes reward, pleasure, and reinforcement learning. Receives dopaminergic input from VTA and input from prefrontal cortex and amygdala.
Fornix Major output tract of the hippocampus; supports memory consolidation. Links hippocampus with mammillary bodies and septal areas.
Cingulate Gyrus Integrates emotion, cognition, and attention; regulates behavior and self-awareness. Connects with thalamus, hippocampus, and prefrontal cortex.

References

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