The Limbic System

The basal ganglia | Main Anatomy Index | The cerebral cortex

Robinson, S. (1998) The Hippocampal Formation and the Limbic System [Lecture notes]. University of NSW, 12 and 13 October, 1998.

• The limbic lobe fit many of the criteria for an anatomical substrate for drive-related and emotional behaviour.
• The limbic cortex is connected in one direction with widespread neocortical areas and in another direction with the hypothalamus.

• Although there is no universal agreement on the total list of structures which compromise the limbic system, it can be considered to be consisting of:

1. The hippocampal formation;
2. The cingulate gyrus;
3. The amygdala;
4. The septal area;
5. The mamillary bodies;
6. The anterior nuclear group of the thalamus;
7. The inferior temporal lobe;
8. The prefrontal cortex;
9. And the tracts that link these areas (e.g., fornix, mammillothalamic tract and stria terminalis).

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"the flight of fancy which led Arantius, in 1587, to introduce the term 'hippocampus'.... recorded in what is perhaps the worst anatomical description extant. It has left its readers in doubt whether the elevations of cerebral substance were being compared with fish or beast, and no one could be sure which end was the head." -- Lewis FT.

Click here for a diagram of the hippocampal formation.

• This is in the temporal lobe as the floor of the inferior horn of the lateral ventricle.
• The hippocampal formation is a curved and recurved sheet of cortex folded into the medial surface of the temporal lobe.

• Transverse sections reveal that it has 3 distinct zones: the dentate gyrus, the hippocampus proper and the subiculum.
• In such sections, the hippocampal formation has the appearance to 2 interlocking Cs.
• Its shape has also been described to resemble a rams horn and is thus also called the cornu ammonis.

• Embryologically, the hippocampal formation is an extension of the medial edge of the temporal lobe.
• During development, it becomes invaginated by the hippocampal sulcus and the tip (dentate gyrus) rotates around the adjacent hippocampus.

• The entire hippocampal formation has a length of about 5 cm from its anterior end at the amygdala to its tapering posterior end near the splenium of the corpus callosum.

• A thin layer of fibres, the alveus, covers the ventricular surface of the hippocampus.
• This gives the surface a shiny, white appearance.
• These fibres coalesce to form the fimbria and subsequently the crura of the fornix (main efferent pathway of the hippocampal formation).

• This is the transitional zone between the 6-layered entorhinal cortex and the 3-layered hippocampus.

• This is between the subiculum and dentate gyrus.

• This is between the fimbria and the dentate gyrus.

• This is between the fimbria and overlying forebrain.
• Through it runs the anterior and posterior choroidal vessels.

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• The hippocampus and the dentate gyrus are 3 layered.
• They both have a superficial molecular layer and a deep polymorphic layer.

• The intermediate stratum is the granule cell layer in the dentate gyrus.
• It is the pyramidal cell layer in the hippocampus.

• This is a synaptic layer that is continuous over the molecular layers of the dentate gyrus, hippocampus and entorhinal cortex.

• This is a mix of dendrites, axons and interneurons.
• It is similar to layer 6 of the neocortex.

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1. The most prominent source is the entorhinal cortex. The entorhinal cortex itself receives almost all types of sensory information.
2. In addition, some septal and hypothalamic fibres reach the hippocampal formation via the fornix.
3. A few fibres also arrive from the contralateral hippocampal formation passing from one crus to another via the hippocampal commissure.

• Granule cells in the dentate gyrus receive input from the entorhinal cortex via the perforant pathway.
• The granule cells project via mossy fibres to the hippocampal pyramidal cells.

• Many fibres are sent directly back to the entorhinal cortex.
• The most anatomically prominent output pathway is the fornix, however.
• The pyramidal cells send an axon into the fornix as well as a Schaffer collateral that projects to CA1 part of the hippocampal formation.

• These fibres arch forward under the corpus callosum.

• At the level of the interventricular foramen, some fibres split off anterior to the anterior commissure as the precommissural fornix.
• Most of these end in the septal and preoptic areas but some continue on to reach orbital and anterior cingulate cortex.

• The remaining fibres of the postcommissural fornix do one of 2 things:

1. Some turn sharply posteriorly to end in the anterior thalamic nuclei;
2. The rest travel through the hypothalamus in the column of the fornix and end mainly in the mammillary bodies.

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• The most prominent role ascribed to the hippocampal formation has to do with learning and memory.
• The synaptic connections within the hippocampus are readily modifiable by a single experience (long-term potentiation).
• This may be the circuitry that converts short-term to long-term memory.

• After bilateral removal of the medial parts of the temporal lobe, humans have a striking memory deficit, anterograde amnesia for declarative memories.
• Such a patient could, e.g., learn in repeated attempts to assemble a jigsaw puzzle more and more skilfully, at the same rate as a normal individual, despite never remembering having seen the puzzle before.
• Intelligence is relatively unaffected.

• The pyramidal cells in CA1 are particularly sensitive to anoxia so these symptoms may arise after revival from drowning.

• Damage to the mamillary bodies (often as a result of chronic alcoholism) results in a similar memory deficit.
• The afflicted patients have relatively intact intelligence but an inability to form new memories.
• They do, however, typically make up answers (confabulate) as they go along, concealing to some extent their memory loss.

• Thus, Korsakoff's psychosis is also known as amnestic confabulatory syndrome.

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• This is a collection of nuclei lying beneath the uncus of the temporal lobe.
• It lies at the anterior end of the hippocampal formation and the anterior tip of the inferior horn of the lateral ventricle.

• It merges with the periamygdaloid cortex, which forms part of the surface of the uncus.

• The amygdala also abuts the tail of the caudate nucleus as it ends in the temporal lobe.

• The amygdala does have some connections with the striatum but its overall pattern of connections is typical of the limbic system.

Click here for a diagram on the stria terminalis.

Click here for a schematic of the amygdalo-septal circuit.

• The dorsomedial amygdala blends with the cortex of the uncus and receive afferents from the olfactory bulb.
• This part of the amygdala relays olfactory input to the ventrolateral part of the amygdala.
• Other afferents to the amygdala come from the frontal and temporal cortex.

• The amygdala projects to the septal area via the stria terminalis, which runs adjacent and medial to the tail of the caudate nucleus.

• It also sends fibres to the anterior hypothalamus and centres in the brainstem (e.g., DMnX, solitary nucleus, and raphe nuclei) via the medial forebrain bundle.

• The amygdala has a high order modulating influence on autonomic function based on learning and past experiences.
• For example, generalised fear increases the heart rate, sweating, and respiration.
• This instinctive modulation of autonomic function is different from the reflexive modulation of the hypothalamus.

• Electrical stimulation of the amygdala in humans elicits emotions ranging from pleasure to aggression though fear (along with the normal autonomic manifestations) is the most common.

• Bilateral destruction of the amygdala causes a great decrease in aggression.
• This also often causes an eating disorder, either hyperphagia or hypophagia.

• This has extensive reciprocal connections with the hippocampus (via fornix).
• The septal area projects to the habenula nuclei via the stria medullaris thalami.
• It also projects to the anterior hypothalamus and modulates hypothalamic function.

• This area (and the nucleus accumbens) is associated with pleasure.
• Rats will perform 5000 bar-presses an hour to obtain self-stimulation of this region from implanted electrodes.

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Click here for a schematic of Papez's circuit.

• This circuit approximates the hippocampo-mammillo-cortical circuit.
• This and the amygdalo-septal circuit "begins" in the frontal cortex, and overlap in the temporal cortex and the hippocampus and septal areas.

• The entorhinal cortex receives afferents from the olfactory tract and diverse areas of the temporal lobe.
• It relays highly processed sensory information to granule cells in the dentate gyrus.

• The hippocampus sends projections back to the entorhinal and inferior temporal cortex.
• However, most of the efferents travel in the fornix and terminate in the mammillary bodies, septal area and contralateral hippocampus.

• The mamillary bodies project to the anterior nuclei of the thalamus via the mammillothalamic fasciculus.

• The anterior thalamic nuclei project to the cingulate gyrus via the internal capsule.

• The cingulum lies within the cingulate gyrus and serves as a cortical association pathway for adjacent regions of neocortex in the frontal, parietal and occipital lobes.

• The hippocampus and entorhinal cortex are both affected at an early stage of Alzheimer's disease (olfactory toxin?).

• Complete removal of both temporal lobes in monkeys leads to:

1. The animals are fearless and placid, showing an absence of emotional reactions.
2. The male animals become hypersexual and are indiscriminate in their choice of sexual partners.
3. They show in inordinate degree of attention to all sensory stimuli. They respond to every object within sight or reach by sniffing it or examining it orally (consequently leading to hyperphagia).
4. Although they incessantly examine all objects, they recognise nothing, i.e., they have visual agnosia (due to the loss of the visual association cortex).

• Smaller lesions of the temporal poles produce the same symptoms minus the visual agnosia.
• A similar set of symptoms can be seen in humans.

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