The Human Limbic System: The Brain's Emotional Core
Residing deep within the brain's center, lay a group of closely associated structures most noted for governance of emotion, sensation and motivation. These structures are collectively referred to as the “limbic system,” a label first coined in the 1950's by American doctor and neuroscientist Paul MacLean, who is also well-known for development of the Triune Brain Model. The limbic system, responsible for autonomic nervous system functioning, is necessary for control and regulation of vital survival mechanisms of hunger, thirst, heart rate, temperature and more. So it is with good reason that these structures are set deep inside the midbrain, as they are well protected in this region.
Paul Broca, a neurologist, first established the term “limbic” in 1878 when he began to refer to the parahippocampal and cingulate gyri as the “great limbic lobe.” He attributed the name limbic to describe the arching border area of the cortex, as it is comprised of both structures. At the time he did not elaborate on the purpose or functioning of these structures, but only noting their shape and presence. Broca also understood this distinctive area of the brain exists in all mammals.
The Great Limbic Lobe
Triune Brain Theory
According to MacLean, the brain can be viewed through an evolutionary lens depicting three distinct phases: the paleomammalian complex, the reptilian complex and the neomammalian complex. The paleomammalian complex consists of the limbic system, while the reptilian complex encompasses the basal ganglia and is the oldest of the three. The youngest portion, the neomammalian complex, is the neocortex.
The reptilian complex (basal ganglia) is associated with vital functions: breathing, heart rate and body temperature regulation. It is termed the reptilian brain complex due to the presence of the brainstem and cerebellum, which encompasses the entire brain of all reptiles. MacLean also suggests that this area of the brain is necessary for instinctive behaviors such as territoriality and aggression. The basal ganglia is important for motor control and procedural learning. Parkinson's disease, a condition affecting one million Americans as of 2015, is caused by reduced neuronal output from the basal ganglia. This leads to the production of excessive movement in people diagnosed with Parkinson's Disease.
The neomammalian complex (neocortex) is thought to have appeared first in primates and later in humans. This section of the brain, MacLean purports, is responsible for making abstract thought, creativity, learning and language possible. It is the upper-most part of the human brain which is divided into left and right cerebral hemispheres.
The paleomammalian complex (limbic system) is where emotion is produced. It is necessary for memory formation and attribution of memories to human experiences. First appearing in mammals, the paleomammalian complex is essential for making judgments relating their importance and priority. The hypothalamus, hippocampus, and amygdala are the most important structures of the limbic system.
Necessary for hormonal balance and excretion, the hypothalamus can be found next to the pituitary gland, connected by a stalk of nuclei and hangs beneath the brain encased in a cradle of bone. Survival of mammals requires that blood composition and body temperature fall within a specific range. The hypothalamus is responsible for regulating and maintaining these restrictive ranges in responding to environmental changes. This process is referred to as homeostasis.
Additionally, the hypothalamus is extremely important for regulating hunger, thirst, sex, blood pressure, mood, fluid-electrolyte balance and biological rhythm. Many pathways are contained in the hypothalamus, and are beneficial for communicating with the forebrain and other midbrain structures. Lesions and stimulation of the hypothalamus may cause violent rage and aggression, as well as changes in vital functions. The response may vary and is determined by the location of the hypothalamus damage or stimulation. For instance, stimulation to the lateral hypothalamus produces a silent biting attack in cats.
Particularly associated with long-term and spatial memory, the hippocampus is found in the middle of the temporal lobe. It communicates directly with the amygdala and the entorhinal cortex, both of which have pathways to and from other areas of the brain, such as the neocortex, the largest portion of the cerebral cortex. Damage to the hippocampus can lead to anterograde amnesia, a condition causing the individual to lose the ability to form new memories, so that anything that happens after the date of injury will be immediately forgotten. Events that occurred prior to injury are still retrievable.
- The amydala plays a major role in fear and anxiety. People with bilateral damage show an atypical trust for strangers and cannot discern harmful situations.
- The site of action for anti-anxiety medications is the amygdala.
- The amygdala has receptor sites for opiates and benzodiazepines.
- Stimulation of the amygdala produces fear and anxiety.
The amygdala consists of a pair of almond-shaped organs residing medially in the temporal lobe. The principle structure of the limbic system, the amygdala is associated with fear and aggression, and receives input from all sensory areas. Lesions and damage to this area causes a flattened emotional affect, characterized by a considerable decrease in emotional response particularly in situations where fear and aggression would be considered reasonable or logical. Research findings indicate that rats receiving an amygdalaectomy on both sides will fearlessly approach a cat. A lynx, normally a wild cat will exhibit the temperament of a domestic house cat. Amygdala damage produces an inability to recognize a familiar face in humans.
Common Diseases Involving the Limbic System
Epilepsy is characterized by involuntary movement and sensation disturbances, as well as altered levels of consciousness, caused by excessive, uncoordinated neuronal activity. Frequently described as a “brain storm,” epileptic seizures are classified as partial or generalized, depending upon whether the affected individual experiences unconsciousness or not. If the person has a loss of consciousness, it is typically called a grand mal, a generalized seizure. Most seizures are of the temporal lobe type, and are the predominate kind of seizure. It is thought to be caused by hippocampal sclerosis, a condition in which cells of the hippocampus atrophy and die.
Schizophrenia refers to a serious, chronic mental condition which causes an abnormal interpretation of reality. Symptoms include hallucinations, delusions, lack of emotional expression, uncoordinated movements and speech. The specific cause of schizophrenia is unknown, but several structural brain abnormalities have been discovered in patients with the condition. Research reports state that the hippocampus plays a role in the psychotic symptoms of schizophrenia.
Post-Traumatic Stress Disorder (PTSD) primarily involves the amygdala and hippocampus. It is manifest by flashbacks, panic attacks, constant remembering and re-experiencing details of a traumatic event. PTSD occurs due to actual exposure to a life-threatening situation, such as a natural disaster, sexual assault, war and witnessing suicide or homicide. When fear is experienced, the amygdala switches on the fear reaction (fight or flight) and a memory trace is created. Under normal conditions the hippocampus will shut down the "fear reaction" when the threat is over, but in the case of PTSD, it fails to do so. With PTSD, the fear response system is faulty and as a result, it is unable to properly regulate itself.
Science has advanced considerably since the old days of lobotomies, when it comes to comprehending how the brain is involved in regulating emotions. We now know that the concept of a lone system controlling all of human emotion is simplistic and actually requires action of many areas of the brain. We also know that the particular type of stimulus experienced determines the areas stimulated.
Dr. Robert Sapolsky, Neuroscientist - Stanford University School of Medicine
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