An overview of the limbic system is helpful in understanding the potential role of faulty fluid mechanics in the brain when it comes to mental, mood, memory and movement disorders associated with neurodegenerative diseases such as: Alzheimer’s disease, dementia, schizophrenia, bipolar disorder, Parkinson’s disease, Parkinson’s Plus variants, Huntington’s disease and multiple sclerosis, as well as others. Its location, makes it vulnerable to the consequences of faulty fluid mechanics, which among other things, can be caused by structural problems in the cranial vault and spinal canal.

The Triune Brain

The limbic system is part of a theoretical concept called the triune brain, a term coined by neuroscientist Dr. Paul MacLean in 1960. He used the triune brain to describe the evolutionary hierarchical anatomical and functional layout of the brain. The other two parts of the triune brain are the reptilian brain (also known as the R-complex, paleomammalian or basal brain), and the neocortex (also known as the neomammalian brain).

In terms of Freudian psychology, the reptilian brain and the limbic system are the equivalent of the lower levels of consciousness called the id and the ego. The neocortex is the superego or highest level of consciousness. The lower levels of the reptilian brain and limbic system control a highly self-centered system concerned only with self-gratification, self-survival and avoidance of pain. The higher limbic system of the old mammalian brain, however, is also important to the formation of long-term memories, motivation, emotion, behavior and movement. It is at the center of an individual’s ego or core personality. Pleasure and pain are important memorable and motivating causes for remembering things necessary to survival and propagation.

Limbic is derived from the French word for border. The limbic system is part of the inner borders of the lobes of the brain. The thalamus of the diencephalon separates the left and right hemispheres of the brain and limbic system. The left and right thalamus form the walls of the third ventricle.

The picture below shows the main structures that make up the limbic system.

The different structures will be explained further below. As can be seen in the picture, the structures make up and surround the core of the brain. The core is in the area of the corpus callosum and thalamus in the picture.

The lateral ventricles are located below the corpus callosum. The third ventricle is between the left and right thalamus. The ventricles are chambers in the core of the brain that are filled with cerebrospinal fluid (CSF).

What sets the brain and brainstem apart from lower levels in the spinal cord is their command and control nerve centers. The highest levels of command and control are in the outer grey mantle layer of the brain, called the cortex. The next level down is clusters of nerve centers that are located deep within the lobes of the brain below the cortex collectively called subcortical nuclei. The reptilian is the lowest level of command and control nuclei centers that are located in the brainstem and cerebellum. Functionally, the reptilian brain plays an important role in finding, feeding, fighting and reproduction, as well as seeking satisfaction and avoiding pain. It also plays a role in aggressive, defensive and ritualistic behaviors and movements such as mating plummage displays and dances.

Some scientists also include the thalamus and hypothalamus in the reptilian brain. Others include them in the limbic system. The thalamus and hypothalamus are part of the diencephalon which sits on top of the brainstem beneath the lobes of the brain. The diencephalon, and its role in neurodegenerative diseases, is discussed elsewhere on this site.  MacClean included parts of the basal ganglia as part of the reptilian brain and some scientists refer to it as the basal brain.The basal ganglia are covered on the Parkinson’s page and will be covered further as a separate topic on this site. What distinguishes the reptilian brain from the limbic and other higher parts of the brain is that it is associated mostly with automatic movements and behavior.

Limbic Nuclei

The limbic system, as mentioned above, is also known as the paleomammalian (ancient mammal) brain. It was the first higher level to evolve around the more primitive reptilian brain. It is found in all mammals from the smallest rodent to the largest whale. It includes nuclei, communication links and lobes. Most of the nuclei and communication links of the limbic system are located around the third ventricle.They include structures seen in the picture above such as the amygdala, hippocampus, fornix, mammillary bodies, anterior thalamic nuclei and septal nuclei. In addition to nuclei, the limbic system also includes inner parts of the lobes of the brain. The lobes will be discussed further below.

The amygdala nuclei are almond-shaped structures located on either side of the thalamus near the front portion of the inferior horn of the lateral ventricle. They are driven by pleasure and play a role in addiction. The amygdala play an important role in episodic (stories) and autobiographical (stories of self) memory. They also provide strong reinforcing stimulus from pleasure and relay the information to the hippocampus. In addition, they appear to assist in converting short-term into long-term memories. They do so by signaling the cortex of motivationally significant stimuli, such as those related to reward and fear, as well as social functions and mating. The amygdala are connected to the hippocampus.

The hippocampus consists of two horns that curve back from the amygdala. The two horns arch forward over the third ventricle where they become the fornix. The fornix forms part of the floor of the lateral ventricles and sits above the roof of the third ventricle. The fornix relays information sent from the amygdala and hippocampus to the mammillary bodies and the septal nuclei located in front of the hypothalamus and third ventricle. The hippocampus is important to long-term memories and is at the center of current Alzheimer’s disease research. It is also important for spatial memory. The amygdala provides extra stimulus that further helps the hippocampus to remember many details surrounding environmental situations.

The amygdala and hippocampus are part of the inner (medial) portion of the temporal lobe, which lies between the ears. Among other things the temporal lobe is important to the recognition of sounds and for speech, which is related to sound. Its connection to sound and speech may explain the prominent role of the hippocampus in memory. For example, our thoughts are recorded in terms of “silent speech” spoken to ourselves. We record events and thoughts in terms of our native language. We see the color red and it stimulates centers related to sound and symbols we use to identify it. The hippocampus also records and updates cognitive maps for spatial navigation. We use memories of spatial maps to recall certain signs to guide us, such as trees, stumps and rocks that line a path we frequently follow or to drive to a local store.

The mammillary bodies transmit their signals to the anterior nuclei of the thalamus, which relays it to the inner limbic portions of the inner lobes of the brain. The mammillary bodies, likewise, appear to be important for the formation of memory. It further seems they link odor recognition to the formation of long-term memories. Memories of odors are important to most mammals for survival. The septal nuclei provide critical interconnections within the limbic system that are related to pleasure. Damage to the hippocampus can affect the ability to form long-term memories, while preserving older ones. The mammillary bodies are involved with the processing of recognition memory.

Limbic Lobes

The lobes of the brain are divided into sections, called gyri. The gyri are separated by large splits in the surface of the brain called fissures. The wrinkled surface of the gyri are further separated by spaces called sulci. Gyri is plural for gyrus. The gyri of the different lobes of the brain that are related to the limbic system are seen in the middle of the picture below surrounding the dark center. The dark space in the middle is the lateral ventricle. The lateral ventricles are chambers in the middle of the brain that are filled with cerebrospinal fluid (CSF).

The limbic gyri include the: cingulate gyrus, parahippocampal gyrus and entorhingal gyrus. There are other related gyri not shown as well, such as the piraform gyri.  As can be seen, the gyri of the limbic system form a ring around the lateral ventricles. The corpus callosum sits below the limbic gyri and forms the roof of the lateral ventricle. As an aside, major blood vessels pass between the limbic gyri and the corpus callosum. This location makes these blood vessels vulnerable to faulty fluid mechanics occurring in the lateral ventricles that cause tension, compession and shear stresses on surrounding structures.

The parahippocampal gyrus is important for spatial memory. The cingulate gyrus plays a role in autonomic functions such as regulating heart rate and blood pressure. It also plays a role in cognition and processing related to attention. The entorhinal gyrus is important to memory and one of the first areas to degenerate in Alzheimer’s disease. The piraform gyrus lies next to the entorhinal gyrus. Both are related to the nose and the recognition of, and memories related to, odors. The parts of the lobes of the brain related to the nose are some of the oldest in mammalian evolution.

The limbic system also works closely with the thalamus and hypothalamus of the diencephalon. The thalamus is the primary router and processor of the entire central nervous system. The anterior thalamic nuclei play a key role in the limbic system by routing information, between the gyri of the higher brain and nuclei of the lower brain, related to the memories, emotions and movement. The hypothalamus regulates all vital functions, as well as the pituitary gland and autonomic nervous system. The pituitary gland is the master gland of the hormone secreting endocrine system. The autonomic nervous system regulates all the internal automatic organs and systems in the body that are mostly beyond conscious control.

The hypothalamus is also highly interconnected with the nucleus accumbens. The nucleus accumbens is the pleasure center that plays a prominent role in sexual arousal, as well as pleasure associated psychotropic prescription and recreational drugs such as alchohol and tobacco, that are highly addictive and hard to quit. The nucleus accumbens is heavily influenced by dopaminergic signals from the limbic system.

Faulty Cranial Hydrodynamics and the Limbic System

Because of their proximity to the lateral and third ventricles, the structures of the limbic system are subjected to destructive forces from faulty fluid mechanics (cranial hydrodynamics) inside the cranial vault and brain. Faulty cranial hydrodynamics (fluid mechanics inside the cranium) can cause tension stresses, shear stresses and compression loads that can lead to ventriculomegaly and brain atrophy among other things.

These system’s association with ventriculomegaly suggests that faulty cranial hydrodynamics may play a role in mental, memory and movement disorders associated with neurodegenerative diseases such as Alzheimer’s disease, dementia, Parkinson’s disease, Huntington’s disease, schizophrenia, bipolar disorders, multiple sclerosis and others.