Reading: Haines, Ch 30, 28, 18; also see comparable sections of Guyton & Hall
limbic system: A group of structures located on the medial aspect
of each cerebral hemisphere and diencephalon. Its cerebral structures
encircle the upper apart of the brain stem and include parts of the
rhinencephalon (the septal nuclei, cingulate gyrus, parahippocampal gyrus, and
C-shaped hippocampus), and part of the amygdala. In the diencephalon, the
main limbic structures are the hypothalamus and the anterior nucleus of the
thalamus. The fornix ("arch") and other fiber tracts link these
limbic system regions together.
The limbic system is our emotional, or affective (feelings)
brain. Two parts seem especially important in emotions-the amygdala and
the anterior part of the cingulate gyrus. The observation that odors often
evoke emotional reactions and memories reflects the origin of much of this
system in the "smell brain" (rhinencephalon).
Extensive connections between the limbic system and lower and
higher brain regions allow the system to integrate and respond to a wide variety
of environmental stimuli. Since the hypothalamus is the neural clearinghouse
for both autonomic function and emotional response, it is not surprising that
some people under acute or unrelenting emotional stress fall prey to visceral
illnesses, such as high blood pressure and irritable bowel syndrome.
The limbic system also interacts with higher cerebral
cortical areas, such as the prefrontal lobes, so there is an intimate
relationship between our feelings (mediated by the emotional brain) and our
thoughts (mediated by the cognitive brain). As a result, we (a) react
emotionally to things we consciously understand to be happening, and (b) are
consciously aware of the emotional richness of our lives. Communication
between the cerebral cortex and limbic system explains why emotions sometimes
override logic and, conversely, why reason can stop us from expressing our
emotions in inappropriate situations.
The hippocampus and amygdala also play an important role in
converting new information into long-term memories (Marieb 428-429).
hippocampus: The elongated, medial portion of the temporal
cortex that folds upward and inward to form the ventral surface of the inferior
horn of the lateral ventricle. One end of the hippocampus abuts the
amygdaloid nuclei, and it also fuses along one of its borders with the
parahippocampal gyrus, which is the cortex of the ventromedial surface of the
temporal lobe.
Has numerous but mainly indirect connections with many
portions of the cerebral cortex as well as with the basic structures of the
limbic system - the amygdala, the hypothalamus, the septum, and the mammillary
bodies. Almost any type of sensory experience causes activation of at least
some part of the hippocampus, and the hippocampus in turn distributes many
outgoing signals to the anterior thalamus, hypothalamus, and other parts of the
limbic system, especially through the fornix, its major output pathway.
Thus the hippocampus is an additional channel through which incoming sensory
signals can lead to appropriate behavioral reactions but for different purposes
(Guyton 757-758).
dentate gyrus: Part of the hippocampal formation (along with the subiculum and hippocampus). Forms the medial edge of the hippocampal formation. Composed of 3 layers (Haines 446-449).
perforant pathway: The major input to the hippocampal formation from cells of the entorhinal cortex via a diffuse projection. Most fibers of the perforant pathway terminate in the molecular layer of the dentate gyrus, although a few terminate in the subiculum and hippocampus (Haines 247).
fornix: A fiber tract that links limbic system regions together (Marieb 428). A major efferent path of the hippocampal formation. It is composes of a flattened caudal part, the crus, a compact dorsal portion, the body, and a part that arches around the anterior part of the thalamus and passes through the hypothalamus to terminate in the mammillary body (Haines 216).
cingulate gyrus: Part of the circuit of Papez. Included in the limbic system.
mammillary bodies: Paired pea-like nuclei that bulge anteriorly from the hypothalamus, are relay stations in the olfactory pathways (Marieb 421)
anterior thalamic nucleus: one of the anatomical structures of the limbic system, helps to mediate many of the behavioral functions elicited from the hypothalamus
septal nucleus: a small cell group located internal to the subcallosal area, just rostral to the anterior commissure and in the medial wall of the hemisphere, they have been implicated in a myriad of functions in animal models on the basis of the pattern of their inputs and outputs. They have complex interconnectins with hippocampal, amygdaloid, and limbic structures. There is little information regarding their function in humans but RAGE behavior has been seen in a small group of patients with midline infarcts in this area. Also may have a role in a less potent reward center.
amygdala: a complex of nuclei located immediately beneath the cortex of the medial anterior pole of each temporal lobe (internal to the uncus). It has abundant bidirectional connections with the hypothalanus as well as with other adjacent areas of the limbic system. Seems to be a behavioral awareness area that operates at a semiconscious level. It also seems to project into the limbic system one’s current status in relation to both surroundings and thoughts. It is believed to help pattern the person’s behavioral response so that it is appropriate for each occasion. (Guyton p.758 &759)
parahippocampal gyrus: a five layered entorhinal cortex (paleocortex) of the ventromedial surface of the temporal lobe, surrounds a group of deep structures associated with overall behavior and with emotions. This ring of limbic cortex functions as a two-way communication and association linkage between the neocortex and the lower limbic structures.
Kluver-Bucy syndrome: when the anterior portions of both temporal lobes are destroyed in a monkey, this removes not only the temporal cortex but also the amygdalas that lie inside these parts of the temporal lobes. This causes a combination of changes in behavior called the Kluver-Bucy Syndrome which includes:
- excessive tendency to examine objects orally
- loss of fear
- decreased aggressiveness
- tameness
- changes in dietary habits, evento the extent thata herbivorous animal frequently becomes carnivorous
- sometimes psychic blindness
- often excessive sex drive
The characteristic picture is of an animal that is not afraid of anything, has extreme curiosity about everything, forgets rapidly, has a tendency to place everything in its mouth and sometimes even tries to eat solid objects, and often has a sex drive so strong that it attempts to copulate with immature animals, animals of the wrong sex, and animals of a different species. Although similar lesions in humans are rare, afflicted people respond in a manner not too differently.
Papez circuit: an early hypothesis that became one of two pivotal observations made that formed the basis for the concept of a limbic system. The Papez circuit was largely based on the morphology of the brain. The circuit suggested that emotion, mediated through the hypothalamus, is controlled and modulated by fibers from the fornix. Specifically, the cortical control of emotional activity is presumed to originate from cingulate and hippocampal regions. This was the first time a specific anatomic substrate was proposed for a phenomenon as complex as emotion.
stria terminalis: one of the two major efferent bundles of the amygdala, a small fiber bundle that arises primarily from cells of the corticomedial group of the amygdaloid complex. In the temporal horn, the stria terminalis is located just medial to the tail of the caudate nucleus. As it arches rostrally, it assumes a postiion in the shallow groove between the caudate nucleus and the dorsal thalamus. At the level of the interventricular foramen, the fibers of the stria terminalis fan out to enter and terminate in the hypothalamus, septal area, and the neostriatum.
describe the significance of the hippocampus:
Has numerous but mainly indirect connections with many portions of the cerebral
cortex as well as with the basic structures of the limbic system - the amygdala,
the hypothalamus, the septum, and the mammillary bodies. Almost any type
of sensory experience causes activation of at least some part of the
hippocampus, and the hippocampus in turn distributes many outgoing signals to
the anterior thalamus, hypothalamus, and other parts of the limbic system,
especially through the fornix, its major output pathway. Thus the
hippocampus is an additional channel through which incoming sensory signals can
lead to appropriate behavioral reactions but for different purposes.
In those people that have bilateral hippocampi removed, they
can remember experiences from before the removal but they can essentially learn
no new information (Guyton 757-758).
recognize specific regions of the limbic system: ????
describe the relationship between the limbic system and the hypothalamus: The hypothalamus has 2-way communicating pathways with all levels of the limbic system. It is one of the most important of the output control pathways of the limbic system. It controls most of the vegetative and endocrine functions of the body as well as many aspects of emotional behavior (Guyton 753-755).
describe the mechanisms of reward and punishment: Electrical stimulation of certain limbic areas please or satisfy, while others cause terror, pain, fear, defense, escape reactions and all the other elements of punishment. The degree of stimulation of these two oppositely corresponding systems greatly affects behavior.
Reward Centers - Major reward centers have been found to be located along the course of the medial forebrain bundle, especially in the lateral and ventromedial nucleus of the hypothalamus. Less potent reward centers are found in the septum, the amygdala, certain areas of the thalamus and basal ganglia, and extending downward into the basal tegmentum of the mesencephalon.
Punishment Centers - the most potent punishment and escape tendency have been found in the central gray area surrounding the aquaduct of Sylvius in the mesencephalon and extending upward into the periventricular zones of the hypothalamus and thalamus. Less potent punishment areas are found in some locations in the amygdala and the hippocampus.****It is interesting that stimulation in the punishment centers can frequently inhibit the reward and pleasure centers completely, demonstrating that punishment and fear can take precedence over pleasure and reward.
compare and contrast habituation and reinforcement, explain why these mechanisms are important in memory system of the brain: Almost everything we do is related in some way to reward or punishment. If we are doing something that is rewarding, we continue to do it; if it is punishing, we cease to do it. Therefore, the reward and punishment centers constitute one of the most important of all the controllers of our bodily activities, our drives, our aversions, our motivations.
Animal experiments have shown that sensory experience causing neither reward nor punishment is hardly remembered at all. Habituation is a state in which the animal has been exposed to a sensory stimulus over and over again until almost complete extinction of the cortical response is reached. This happens because the stimulus is not one that elicits a sense of either reward or punishment, and the animal becomes accustomed to the stimulus and ignores it.
If the stimulus causes either reward or punishment rather than indifference, the cortical response becomes progressively more and more intense during repeated stimulation instead of fading away, and the response is said to be reinforced. An animal builds up strong memory traces for sensations that are either rewarding or punishing but, on the other hand, develops complete habituation to indifferent sensory stimuli.
****It is evident that the reward and punishment centers of the limbic system have much to do with selecting the information that we learn, usually throwing away more than 99% of it, and selecting less than 1% for retention.
summarize the postulated roles of the amygdala: The amygdala is a complex of nuclei located immediately beneath the cortex of the medial anterior pole of each temporal lobe. It has abundant connections with the hypothalamus as well as with other adjacent areas of the limbic system.
The amygdala receives neuronal signals from all portions of the limbic cortex as well as from the neocortex of the temporal, parietal, and occipital lobes, especially from the auditory and visual association areas. Because of these multiple connections, the amygdala has been called the "window" through which the limbic system sees the place of the person in the world. In turn, the amygdala transmits signals 1)back to the same cortical areas 2)into the hippocampus 3)into the septum 4)into the thalmus, 5)and especially into the hypothalamus.
In general, stimulation in the amygdala can cause almost all the same effects as those elicited by stimulation of the hypothalamus, plus other effects.
- increases or decreases in arterial pressure, heart rate, gastrointestinal motility and secretion
- defecation and micturition
- pupillary dilation or, rarely, constriction
- piloerection
- secretion of the various anterior pituitary hormones, expecially the gonadotropins and adrenocorticotropic homone.
- Some types of involuntary movement such as tonic movements, like raising the head or bending the body, circling movements, occasionally clonic, rhythmical movements, and different types of movements associated with olfaction and eating such as licking, chewing, and swallowing
- Stimulation of certain nuclei can, rarely, cause a pattern of rage, escape, punishment, and fear similar to the rage pattern elicited from the hypothalamus, and stimulation of other nuclei can give reactions of reward and pleasure
- Excitation of still other portions of the amygdala can cause sexual activities that include erection, copulatory movements, ejaculation, ovulation, uterine activity, and premature labor.
****see section above for an overall view of amygdala function
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