Limbic system
The limbic system, also known as the paleomammalian cortex, is a set of brain structures involved in emotional processing and motivation in humans and many other animals. In humans it is located on both sides of the thalamus, immediately beneath the medial temporal lobe of the cerebrum primarily in the forebrain.
Its various components support a variety of functions including emotion, behavior, long-term memory, and olfaction.
The limbic system is involved in lower order emotional processing of input from sensory systems and consists of the amygdala, mammillary bodies, stria medullaris, central gray and dorsal and ventral nuclei of Gudden. This processed information is often relayed to a collection of structures from the telencephalon, diencephalon, and mesencephalon, including the prefrontal cortex, cingulate gyrus, limbic thalamus, hippocampus including the parahippocampal gyrus and subiculum, nucleus accumbens, anterior hypothalamus, ventral tegmental area, midbrain raphe nuclei, habenular commissure, entorhinal cortex, and olfactory bulbs.
Structure
The limbic lobe was originally defined by the French anatomist Paul Broca in 1878, as a series of cortical structures surrounding the boundary between the cerebral hemispheres and the brainstem. The name "limbic" comes from the Latin word for the border, limbus. Further studies began to associate these areas with emotional and motivational processes and linked them to subcortical components that were then grouped into the limbic system.In recent years, multiple additional limbic fiber connectivity has been revealed using diffusion-weighted MRI. The equivalent fiber connectivity of all these pathways has been documented by dissection studies in primates. Some of these fiber tracts include the amygdalofugal tract, amygdalothalamic tract, stria terminalis, dorsal thalamo-hypothalamic tract, cerebellohypothalamic tracts, and the parieto-occipito-hypothalamic tract.
Currently, it is not considered an isolated entity responsible for the neurological regulation of emotion, but rather one of the many parts of the brain that regulate visceral autonomic processes. Therefore, the set of anatomical structures considered part of the limbic system is controversial. The following structures are, or have been considered, part of the limbic system:
- Cortical areas:
- *Limbic lobe
- *Orbitofrontal cortex: a region in the frontal lobe involved in the process of decision-making
- *Piriform cortex: part of the olfactory system
- *Entorhinal cortex: related to memory and associative components
- *Fornix: a white matter structure connecting the hippocampus with other brain structures, particularly the mammillary bodies and septal nuclei
- Subcortical areas:
- *Septal nuclei: a set of structures that lie in front of the lamina terminalis, considered a pleasure zone
- *Hippocampus and associated structures: play a central role in the consolidation of new memories
- *Amygdala: located deep within the temporal lobes and related with a number of emotional processes
- *Nucleus accumbens: involved in reward, pleasure, and addiction
- Diencephalic structures:
- *Hypothalamus: a center for the limbic system, connected with the frontal lobes, septal nuclei, and the brain stem reticular formation via the medial forebrain bundle, with the hippocampus via the fornix, and with the thalamus via the mammillothalamic fasciculus; regulates many autonomic processes
- *Mammillary bodies: part of the hypothalamus that receives signals from the hippocampus via the fornix and projects them to the thalamus
- *Anterior nuclei of thalamus: receive input from the mammillary bodies and involved in memory processing
Function
The limbic system also interacts with the basal ganglia. The basal ganglia are a set of subcortical structures that direct intentional movements. The basal ganglia are located near the thalamus and hypothalamus. They receive input from the cerebral cortex, which sends outputs to the motor centers in the brain stem. A part of the basal ganglia called the striatum controls posture and movement. Recent studies indicate that if there is an inadequate supply of dopamine in the striatum, this can lead to the symptoms of Parkinson's disease.
The limbic system is also tightly connected to the prefrontal cortex. Some scientists contend that this connection is related to the pleasure obtained from solving problems. To cure severe emotional disorders, this connection was sometimes surgically severed, a procedure of psychosurgery, called a prefrontal lobotomy. Patients having undergone this procedure often became passive and lacked all motivation.
The limbic system interacts heavily with the cerebral cortex. These interactions are closely linked to olfaction, emotions, drives, autonomic regulation, memory, and pathologically to encephalopathy, epilepsy, psychotic symptoms, cognitive defects. The functional relevance of the limbic system has proven to serve many different functions such as affects/emotions, memory, sensory processing, time perception, attention, consciousness, instincts, autonomic/vegetative control, and actions/motor behavior. Some of the disorders associated with the limbic system and its interacting components are epilepsy and schizophrenia.
Hippocampus
The hippocampus is involved with various processes relating to cognition and is one of the best understood and heavily involved limbic interacting structures.Spatial memory
The first and most widely researched area concerns memory, particularly spatial memory. Spatial memory was found to have many sub-regions in the hippocampus, such as the dentate gyrus in the dorsal hippocampus, the left hippocampus, and the parahippocampal region. The dorsal hippocampus was found to be an important component for the generation of new neurons, called adult-born granules, in adolescence and adulthood. These new neurons contribute to pattern separation in spatial memory, increasing the firing in cell networks, and overall causing stronger memory formations. This is thought to integrate spatial and episodic memories with the limbic system via a feedback loop that provides emotional context of a particular sensory input.While the dorsal hippocampus is involved in spatial memory formation, the left hippocampus is a participant in the recall of these spatial memories. Eichenbaum and his team found, when studying the hippocampal lesions in rats, that the left hippocampus is "critical for effectively combining the 'what', 'when', and 'where' qualities of each experience to compose the retrieved memory". This makes the left hippocampus a key component in the retrieval of spatial memory. However, Spreng found that the left hippocampus is a general concentrated region for binding together bits and pieces of memory composed not only by the hippocampus, but also by other areas of the brain to be recalled at a later time. Eichenbaum's research in 2007 also demonstrates that the parahippocampal area of the hippocampus is another specialized region for the retrieval of memories just like the left hippocampus.
Learning
The hippocampus, over the decades, has also been found to have a huge impact in learning. Curlik and Shors examined the effects of neurogenesis in the hippocampus and its effects on learning. This researcher and his team employed many different types of mental and physical training on their subjects, and found that the hippocampus is highly responsive to these latter tasks. Thus, they discovered an upsurge of new neurons and neural circuits in the hippocampus as a result of the training, causing an overall improvement in the learning of the task. This neurogenesis contributes to the creation of adult-born granules cells, cells also described by Eichenbaum in his own research on neurogenesis and its contributions to learning. The creation of these cells exhibited "enhanced excitability" in the dentate gyrus of the dorsal hippocampus, impacting the hippocampus and its contribution to the learning process.Hippocampus damage
Damage related to the hippocampal region of the brain has reported vast effects on overall cognitive functioning, particularly memory such as spatial memory. As previously mentioned, spatial memory is a cognitive function greatly intertwined with the hippocampus. While damage to the hippocampus may be a result of a brain injury or other injuries of that sort, researchers particularly investigated the effects that high emotional arousal and certain types of drugs had on the recall ability in this specific memory type. In particular, in a study performed by Parkard, rats were given the task of correctly making their way through a maze. In the first condition, rats were stressed by shock or restraint which caused a high emotional arousal. When completing the maze task, these rats had an impaired effect on their hippocampal-dependent memory when compared to the control group. Then, in a second condition, a group of rats were injected with anxiogenic drugs. Like the former these results reported similar outcomes, in that hippocampal-memory was also impaired. Studies such as these reinforce the impact that the hippocampus has on memory processing, in particular the recall function of spatial memory. Furthermore, impairment to the hippocampus can occur from prolonged exposure to stress hormones such as glucocorticoids, which target the hippocampus and cause disruption in explicit memory.In an attempt to curtail life-threatening epileptic seizures, 27-year-old Henry Gustav Molaison underwent bilateral removal of almost all of his hippocampus in 1953. Over the course of fifty years he participated in thousands of tests and research projects that provided specific information on exactly what he had lost. Semantic and episodic events faded within minutes, having never reached his long-term memory, yet emotions, unconnected from the details of causation, were often retained. Dr. Suzanne Corkin, who worked with him for 46 years until his death, described the contribution of this tragic "experiment" in her 2013 book.