It is the set of brain parts that are responsible for regulating emotions.
The limbic system is a convenient way to describe various nuclei and functionally and anatomically interconnected cortical structures found in the telencephalon and diencephalon.
These nuclei fulfill several functions. However, most have to do with controlling the parts necessary for self-preservation and species preservation. They regulate autonomic and endocrine function, particularly in response to emotional stimuli.
They establish the level of arousal and are involved in motivating and reinforcing behaviors. Also, many of these areas are critical for certain types of memory.
Some of these regions are closely connected to the olfactory system, as this system is critical to the survival of many species.
The areas usually included in the limbic system fall into two categories. Some of these are subcortical structures, while many are parts of the cerebral cortex.
Cortical regions that are involved in the limbic system include the hippocampus, as well as areas of the neocortex, which include the insular cortex, the orbital frontal cortex, the subcallosal gyrus, and the cingulate gyrus, and the parahippocampal gyrus.
This cortex has been called the “limbic lobe” because it forms a border surrounding the corpus callosum, following the lateral ventricle. The subcortical portions of the limbic system include:
- The olfactory bulb.
- The hypothalamus.
- The amygdala.
- Septal nuclei.
- Some thalamic cores, including the anterior body and possibly the dorsomedial heart.
One of how the limbic system has been conceptualized is as the “feeling and reacting brain” that stands between the “thinking brain” and the output mechanisms of the nervous system.
In this construction, the limbic system is generally under the control of the “thinking brain,” but it can react on its own.
The limbic system has its input and processing side (the limbic cortex, amygdala, and hippocampus) and an output side (the septal nuclei and hypothalamus).
The hypothalamus, the primary exit node for the limbic system, has many meaningful connections. It is connected with the frontal lobes, the septal nuclei, and the brainstem’s reticular formation through the bundle of the forebrain.
It also receives inputs from the hippocampus through the fornix and the amygdala through two pathways (ventral tonsilofugal pathway and terminal striae). The hypothalamus has sexual function, endocrine function, behavioral function, and autonomic control centers.
To perform its essential functions, the hypothalamus requires several types of inputs. There are inputs from most of the body and smell, viscera, and retina. It also has internal sensors for temperature, osmolarity, glucose, and sodium concentration.
Additionally, there are receptors for various internal signals, particularly hormones. These include steroid hormones and other hormones, as well as internal cues (such as hormones involved in appetite control, such as leptin and orexin).
The hypothalamus has a significant influence on many functions, including autonomic, endocrine functions, and behaviors. Autonomic functions are controlled by projections to the brainstem and spinal cord.
There are areas located in the hypothalamus that will activate the sympathetic nervous system and some that will increase parasympathetic activity.
Endocrine functions are controlled by direct axonal connections to the posterior pituitary gland (vasopressin and oxytocin control) or by releasing factors in the hypothalamic-pituitary portal system (to influence the function of the anterior pituitary).
There are also projections of the reticular formation involved in certain behaviors, particularly emotional reactions.
Some functions are intrinsic to the hypothalamus. These functions require direct input to the hypothalamus and where the response is generated directly through hypothalamic outputs. Such things as temperature and osmolarity regulation are included.
There are many functions in which the hypothalamus controls the internal milieu and produces a regulatory response. These include the regulation of endocrine functions and appetite.
For example, the ventromedial nucleus of the hypothalamus is considered an area of satiety, while the lateral hypothalamic area is a feeding center.
Additionally, the hypothalamus shapes many complex behaviors, including sexual responses.
The preoptic area is one of the areas of most significant sexual dimorphism (i.e., the difference in structure between the sexes) and, together with the septal nuclei, is an area of projections of gonadotropin-releasing hormones to the mid-eminence region of the hypothalamus.
These sexual responses involve autonomic, endocrine, and behavioral responses.
Finally, the suprachiasmatic nucleus receives direct retinal input. This nucleus is responsible for dragging circadian rhythms into the day-night cycle.
The amygdala is a crucial structure in the anterior temporal lobe within the uncus.
The amygdala makes reciprocal connections with many brain regions, including the thalamus, hypothalamus, septal nuclei, frontal orbital cortex, cingulate gyrus, hippocampus, parahippocampal gyrus, and brain stem.
The olfactory bulb is the only area that enters the amygdala and does not receive reciprocal projections from the amygdala.
The amygdala is a critical center for coordinating behavioral, autonomic, and endocrine responses to environmental stimuli, especially those with emotional content.
It is essential for coordinated responses to stress and integrates many behavioral reactions involved in the survival of the individual or the species, particularly to stress and anxiety.
Lesions of the amygdala reduce responses to stress, especially conditioned emotional responses.
Stimulation of the amygdala produces behavioral arousal and can have targeted anger reactions.
Various stimuli produce responses mediated by the amygdala. The convergence of inputs is essential as it allows the generation of learned emotional responses to different situations.
The amygdala responds to various emotional stimuli, but primarily those related to fear and anxiety.
The hippocampus is an ancient area of the cerebral cortex with three layers. This is located in the medial aspect of the temporal lobe, forming the medial wall of the lateral ventricle in this area. The hippocampus has several parts.
The dentate gyrus contains densely packed granule cells. There is a curved area of the crust called Cornu Ammonis (CA) divided into four regions called CA fields. These are designated CA1 through CA4.
These contain prominent pyramidal cells. The AC fields mix in the adjacent subiculum, which, in turn, is connected to the entorhinal cortex at the parahippocampal gyrus of the temporal lobe.
There are several sources of afferents from the hippocampus. These are mainly from the septum and hypothalamus through the fornix and the adjacent entorhinal cortex. This cortical region receives information from diffuse areas of the neocortex, especially from the limbic cortex and the amygdala.
The entorhinal cortex projects to the dentate gyrus of the hippocampus through the perforating pathway and synapses in granule cells.
These granule cells connect to pyramidal neurons in the CA3 region, which, in turn, are projected by Sheaffer’s guarantees to pyramidal CA1 cells.
It is these last cells that give rise mainly to the fornix. The physiology of these pathways has been studied extensively, particularly in terms of long-term physiological changes associated with memory.
Hippocampal neurons have been extensively studied in terms of long-term potentiation.
The exits of the hippocampus pass mainly in two ways. The first of these exits are through the fornix.
These fibers project to the mammary bodies through the postcommissural fornix, to the septal nuclei, to the preoptic nucleus of the hypothalamus, to the ventral striatum, and portions of the frontal lobe through the commissural fornix.
There are many projections from the hippocampus to the entorhinal cortex.
Note that the hippocampus has reciprocal connections with the cortex and exits along the fornix.
Historically, a vital circuit was thought to begin with the hippocampus projecting into the mammary bodies, relay to the anterior thalamic nucleus, then the cingulate gyrus, entorhinal cortex, and return to the hippocampus.
This received the name “circuit of paper.” However, the circular nature of this connection does not appear to have a functional meaning.
The hippocampus has several functions. Helps control corticosteroid production. It also has a significant contribution to understanding spatial relationships within the environment.
Additionally, the hippocampus is critically involved in many declarative memory functions.
There are several types of memory. Explicit or declarative memory refers to the memory of facts and events. Any memory that can be fully explained in words is of this type. However, implicit or non-declarative memory is also crucial.
Skills and associative learning, such as conditioned and emotional responses, are typical examples of non-declarative or implicit memory.
Explicit memory depends on the medial temporal lobe and the relationship between the hippocampus and the entorhinal region of the parahippocampal gyrus.
There are several areas involved in explicit memory. The hippocampus plays a critical role in short-term memory, necessary for establishing long-term memory patterns.
Hippocampal injuries do not affect old, established memories.
These injuries affect declarative new learning. Ultimately, memory storage is transferred to other areas of the cerebral cortex, and the location of the encoding of these memories may be a function of the type of memory.
Established memories involve areas of association in the frontal lobe and parietal-temporal-occipital association.
The hippocampus is not only active in encoding memories but also in retrieving them. Activation of the hippocampus can be seen in this case from learning about new environments and retrieving directions.
The prefrontal cortex is anterior to the premotor cortex. The orbital frontal cortex is the portion above the orbits. This part of the cortex is exceptionally well developed in humans and is critical for judgment, perception, motivation, and mood.
It is also essential for conditioned emotional reactions. The prefrontal cortex receives information from the other areas of the limbic cortex, the amygdala, and the septal nuclei. It has reciprocal connections with each of these areas and with the dorsomedial nucleus of the thalamus.
Damage to the prefrontal area produces difficulties with abstract reasoning, judgment moods, and puzzle-solving. Frontal lobe damage in spirit depends on the specific part of the prefrontal cortex damage.
The patient’s behavior is often described as disrespectful. In addition, this part of the bark can also be strongly affected by alcohol.
The function of the prefrontal cortex is abnormal in mood disorders. Depression is most often associated with increased activity in portions of the frontal lobe, especially in the medial regions, including the subgenual part of the anterior cingulate cortex, and decreased activity in the posterior cingulate gyrus.
The smell makes strong connections with the anterior portions of the temporal lobe and the amygdala. The olfactory cortex is structurally more straightforward than other portions of the cerebral cortex and is called the allocortex (see section XI).
It includes the prepyriform and peritonsillar cortex that comprises the anterior part of the parahippocampal gyrus that covers the uncus. In some species, of course, the smell is more important than in others.
The olfactory filaments cross the cribriform plate and synapse with the mitral cells in the olfactory bulbs.
The axons of these cells from the olfactory tract extend to the anterior temporal structures bilaterally, as well as the basal forebrain.
Olfactory signals are transmitted to various other brain regions after their initial termination in the olfactory cortex. The olfactory cortex affects the frontal lobe through connections with the dorsomedial nucleus of the thalamus.
The projections of the olfactory cortex to the amygdala can influence emotional and endocrine reactions, mainly through connections with the hypothalamus.
Several exciting syndromes clarify aspects of limbic functions. Kluver-Bucy syndrome occurs with bilateral lesions of the temporal lobes. It blocks emotional responses in animals, which become pretty tame.
They are not afraid of things their species should react to, such as an ape or a rope. Animals become hypersexual and engage in compulsive exploring behavior, especially with their mouths.
As described above, there are pathways through the forebrain involved in reinforcing behaviors and in “reward.” The electrical stimulation of these sites is highly enhanced for behavior.
Many of these pathways involve dopamine and are commonly affected by drug addiction. Habituation in these pathways with the chronic administration of addictive drugs is one of the most important goals of addiction research.
The ventral striatum consists primarily of the nucleus accumbens, an essential target of dopaminergic projections from the ventral tegmental area.
Several addictive compounds affect dopamine transmission in the nucleus accumbens (mesolimbic) and frontal cortical systems (mesocortical). Furthermore, these pathways appear to be functionally unbalanced in patients with schizophrenia.
Patients with schizophrenia have diminished effects of dopamine through the mesocortical systems to the prefrontal cortex.
This could lead to symptoms such as social withdrawal and decreased emotional responsiveness.
At the same time, there is a relative increase in the effects of dopamine through the mesolimbic system to the ventral striatum, resulting in positive symptoms of delusions and hallucinations.