Chapter 5: Limbic System: Hippocampus

Anthony Wright, Ph.D., room of Neurobiology and also Anatomy, McGovern clinical School modification 10 Oct 2020

5.1 introduction to the Limbic mechanism Limbic is a Latin hatchet which means border. Prefer the acquainted word “limbo”, it way an intermediate or transitional state, i beg your pardon is a border. In this case, the border is between the neocortex and also the subcortical structures (diencephalon). The limbic system contains the hippocampal formation, amygdala, septal nuclei, cingulate cortex, entorhinal cortex, perirhinal cortex, and also parahippocampal cortex. This last three cortical areas comprise different portions of the temporal lobe. (Some professionals would additionally include components of the hypothalamus, thalamus, midbrain reticular formation, and olfactory locations in the limbic system.)

5.2 Hippocampus

The term hippocampal formation generally refers to the dentate gyrus, the hippocampus proper (i.e., cornu ammonis), and the subicular cortex. A hippocampal formation is located in the temporal lobe of each cerebral cortex, medial come the worse horn that the lateral ventricle.

Hippocampus means seahorse in Greek. Each hippocampus looks favor a seahorse due to the method it is folded throughout development.

Figure 5.1 (above) Cross-section the hippocampus and its similarity to the seahorse.

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Figure 5.2 (left) development of the hippocampus in cross-section.

The hippocampus is additionally called cornu ammonis. Ammon to be an Egyptian god, close to whose temple ammonia or the salt of Ammon was prepared. The hippocampus is additionally called Ammon’s Horn due to the fact that the two hippocampi bend approximately in the form of the horns that a ram.

Figure 5.3 Schematic drawing showing the major and surrounding structures of the limbic system.

5.3 as whole Structure the the Hippocampus, Fornix, and also Anterior Commissure

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Figure 5.4 framework of the hippocampus, uncus, crua, fornix, anterior commissure, precommissural fornix, postcommissural fornix, and columns.

The fornix is a “C” shaped street (in sagittal section). The fornix starts as the bundle the fibers referred to as the alveus. The alveus is white matter consisting the myelinated afferents and efferents. Together the yarn of the alveus travel posteriorly, they aggregate medially to form the fimbria the the fornix. Fimbria means fringe and also in this instance it is the fringe the the hippocampus. The fimbria looks choose a special rubber band. The fimbria of each hippocampus thickens as it move posteriorly and also eventually splits turn off from the hippocampus developing the crua or “legs” (singular—crus) of every hippocampus. The two crua come together and kind the hippocampal commissure. The hippocampal commissure offers one that two major paths by which the hippocampi communicate with every other.

After the hippocampal commissure the single fiber bundle is correctly referred to together the fornix. The fornix continues in an arc come the anterior commissure.

The anterior commissure is crucial as a landmark since this is wherein the fornix splits right into three parts and goes to different structures:

The split just before the anterior commissure is called the precommissural fornix and this branch goes to the septal nuclei, the ventral striatum, and also the cingulate cortex. part fibers indigenous the fornix also pass with the anterior commissure come the contralateral hippocampus. This is the 2nd of the two significant paths through which the hippocampi communicate with each other. The break-up after the anterior commissure is referred to as the postcommissural fornix and this branch goes come the mammillary bodies of the hypothalamus and the anterior nuclei that the thalamus.

5.4 output Pathways the the Hippocampus

Figure 5.5 Outputs or efferents indigenous the hippocampus pass directly from the subiculum to the entorhinal cortex and also amygdala or through the fornix come a range of anterior structures. Click on any of the brand to see its location determined by a green haze.

It is necessary to remember that afferents and also efferents of the hippocampus space bundled with each other in the same paths. Thus, by learning the output paths, for example, you will also know the intake paths, or angry versa.

Two major pathways into and also out the the hippocampus space the fornix and also entorhinal cortex (via the cingulate cortex). The precommissural branch of the fornix connects to the septal nuclei, preoptic nuclei, ventral striatum, orbital cortex and also anterior cingulate cortex. The postcommissural branch the the fornix connects to the anterior nucleus of the thalamus and the mammillary bodies of the hypothalamus. The mammillary body are destroyed in Korsakoff’s syndrome together the an outcome of alcoholism and thiamine deficiency. Patients through Korsakoff’s syndrome have profound an obstacle forming brand-new memories. Due to the fact that the mammillothalamic tract also goes to the anterior thalamic nucleus, the hippocampus can impact the thalamus indirectly and directly.

The anterior thalamic nuclei in turn attach to the cingulate cortex. The cingulate cortex projects ago to the entorhinal cortex that parahippocampal gyrus, completing a “great” loop called the Papez circuit. The Papez circuit like many other areas of the limbic mechanism is associated in learning and also memory, emotion, and also social behavior, and was originally thought (by James Papez) come the anatomical substrate of emotional experience. The amygdala, together with neocortical areas, room now recognized to it is in centrally associated in emotionally experience. Its relationships to the initial Papez circuit are displayed in the following figure and also the amygdala and emotion space discussed much more thoroughly in the following section.

Figure 5.6 The original circuit suggest by Papez is shown by thick lines and an ext recent relations are displayed by slim lines. Note the reciprocal connections between the hippocampal formation and also the combination cortex, and the consist of of the amygdala and prefrontal cortex.

The hippocampus has straight connections come the entorhinal cortex (via the subiculum) and the amygdala. These structures attach to numerous other areas of the brain. The entorhinal cortex projects to the cingulate cortex. Therefore, the hippocampus can influence the cingulate cortex v the anterior thalamic nucleus or the entorhinal cortex. The cingulate cortex, in turn, projects to the temporal lobe cortex, orbital cortex, and also olfactory bulb. Thus, every one of these areas can be affected by the hippocampus.


5.5 input Pathways the the Hippocampus

Figure 5.7 push the play switch to see original Papez circuit.

Figure 5.8 input or afferents to the hippocampus. Significant inputs come indigenous the entorhinal cortex, i m sorry in turn communicate inputs from the cingulate, temporal, orbital, and olfactory cortices and also amygdala to the hippocampus.

The input paths are just the reverse of the calculation paths. The entorhinal cortex is a major source of inputs to the hippocampus. In addition, the cingulate cortex, temporal lobe cortex, amygdala, orbital cortex, and olfactory bulb all have actually inputs come the hippocampus via the entorhinal cortex.

The hippocampus receives input via the precommissural branch of the fornix native the nucleus basalis of Meynert, which is a part of the substantia innominata and also which in turn is a part of the septal nuclei. Also the hippocampus receives input via the postcommissural branch the the fornix inputs native the mammillary body of the hypothalamus.


5.6 Structures and Processes in ~ the Hippocampus

Figure 5.9 framework of the hippocampus. The medial section mirroring the right hemisphere. The heat shows place of a cut through the left hemisphere. A blowup the the reduced surface through the hippocampus shows the partnership of the hippocampal development to the entorhinal and also parahippocampal cortices. The box diagram shows output and also input pathways v the hippocampus.

The hippocampus proper and the dentate gyrus processes details that passes with the hippocampus. These two structures, the hippocampus proper and the dentate gyrus, kind two interlocking “Cs.” The ax dentate gyrus comes from the beaded or toothed figure of this structure resulting native the many tiny blood ship from subarachnoid space that penetrate the dentate gyrus. The hippocampus and also dentate gyrus are actually cortex, yet it is 3-layered cortex quite than 6-layered cortex together in the neocortex. Due to the fact that of the smaller number of layers and also their location between the neocortex and also diencephalon, these cortices have been dubbed paleocortex, which way old cortex or archicortex which way ancient cortex. These terms are misleading because they give the false impression that these cortices space antiquated remnants left over together the brain evolved and became more complex. They are mind regions that have actually ongoing to build structurally and functionally transparent phylogeny.

The hippocampus and dentate gyrus, choose the neocortex, have actually a superficial molecular layer and a deep polymorphic layer. But since these structures are "inside-out" cortex, the molecule layer is on the inside and also the polymorphic great is ~ above the outside. The middle layer of the hippocampus suitable is a pyramidal cell layer. The center layer of the dentate gyrus is a granular layer. The molecular layer of the hippocampus suitable faces the dentate gyrus. The area of the hippocampus appropriate that is capped by the dentate gyrus is described as CA3 (CA for cornu ammonis).

The polymorphic class is the alveus and is identical to the white matter of the neocortex. The subiculum is the transition layer indigenous the hippocampus to the parahippocampal gyrus and changes gradually from 3 to 6 layers.

The hippocampus coordinates information indigenous a range of sources. A significant flow of information through the hippocampus is a one-way circuit. Part inputs come the hippocampus (perforant pathway) indigenous the entorhinal cortex pass v to the dentate gyrus. From the dentate gyrus relationships are made to CA3 the the hippocampus proper via mossy fibers and to CA1 via Schaffer collaterals. From these 2 CA areas information climate passes with the subiculum start the alveus, fimbria, and also fornix and also then come other areas of the brain.

Figure 5.10 Slice v the hippocampal formation showing the ar of CA1 and CA3 cells and the Dentate gyrus.

Information operation into and through the hippocampus by three primary pathways: 1. The perforant pathway indigenous the entorhinal cortex come granule cells of the dentate gyrus; 2. The mossy fiber pathway native the granule cabinet of the dentate gyrus to the pyramidal cell of the CA3 an ar of the hippocampus; and 3. The Schaffer collateral pathway native the CA3 region of the hippocampus to the CA1 an ar of the hippocampus.

5.7 instrument of Hippocampal duty

Temporal Lobe Epilepsy

Identification of an epileptic seizure deserve to be made through EEG (Electroencephalogram) recordings. The EEG records from numerous thousands that neurons native scalp electrodes. A few electrodes are put on the scalp. Voltage differences are videotaped relative to a referral electrode some distance native the site.

The pyramidal cell of the center layer of the hippocampus ideal are the significant input of the EEG recording. The pyramidal cells have dedicated structures because that input referred to as dendritic spines, little spines that are attached to the dendritic shafts. The pyramidal cells are glutamate excitatory neurons and also are the major neurons that project to the cerebral cortex and are the major driving pressure of temporal lobe epilepsy.

Figure 5.13 Schematic diagram of the relationship between a pyramidal neuron, a basket cell, and the resulting recurrent inhibition.

Special nature of Hippocampal Pyramidal Cells:

The dendrites room parallel to one one more resulting in summation that extracellular current flow and also hyper-excitability watched in epilepsy. The pyramidal dendrites are perpendicular come the cortical surface resulting in different layers that cortex impinging at various points follow me the dendritic tree. The pyramidal dendrites save dendritic spines the amplify currents (inputs) so that distant synaptic website can more easily generate activity potentials. The pyramidal cells (dendrites) get inputs from basket cell that regulate excitability the the pyramidal cells v recurrent inhibition.

Pyramidal cells would be in a consistent bursting firing setting if there to be no basket cells. The diagram shows a cartoon pyramidal cell, axon, excitatory collateral come basket cell, basket cell, and inhibitory link to pyramidal cell. This basic circuit is what is dubbed recurrent inhibition. This is a general function of worried system, a collateral come a one-of-a-kind cell—the basket cell. The pyramidal cabinet excites the basket cell. Excitation of the basket cell consequently produces negative feedback or inhibition of the pyramidal cell. The is, action of the pyramidal cell acts v the basket cabinet to manage its own activity. The neurotransmitter of the basket cabinet is GABA or gamma-aminobutyric acid, common of many inhibitory neurons.

Epilepsy is a synchronous discharge the pyramidal cells. This synchronous discharge produces stereotyped and involuntary jerking movements, loss of awareness, and in the many extreme instance convulsions and loss that consciousness. Next to stroke, epilepsy is the most usual neurological disease. Maybe as lot as 1% that the populace is impacted at one time or the other.

Focal or partial epilepsy is restricted to a particular mind region. Yet an epileptic attack can start as a focal distance attack and spread to other cortical areas and also become a generalized seizure.

For an epileptic seizure thousands of thousands the neurons should be shooting in synchrony. The means this happens is a break down in postsynaptic inhibition. The importance of inhibition and the role of the basket cell deserve to be prove by disabling the connection between the basket cell and also the pyramidal cell.

Picrotoxin, a GABA antagonist, will disable this junction and results in a cascade of excitation and the synchronous activity of one epileptic seizure.

Also, examination of sections with the hippocampus in patients with temporal lobe epilepsy has shown a ns of cell in the plexus surrounding pyramidal cells. This is whereby the basket cell are generally found. Thus, there is a loss of the inhibitory input the is normally found there. The brake top top the system, that is control of the burst firing of pyramidal cells, has actually been removed as soon as the basket cells have actually been removed.

Long-term Potentiation

Long-term potentiation (or LTP) was discovered by initiatives to determine exactly how the hippocampus could determine memory. Patience H.M. Had actually just freshly demonstrated that the hippocampus was vital in lied down new memories. Researchers uncovered that neural task can change synaptic strength in specific areas that the hippocampus. This amendment synaptic stamin (LTP) might be a storage device for memory. For an ext information ~ above LTP and also how that is affiliated in storage formation, see the thing on Learning and also Memory.

Test your Knowledge

The postcommissural fornix projects to i m sorry structure?

A. Substantia innominata.

B. Mammillary bodies

C. Ventromedial nuclei that the hypothalamus

D. Nucleus basalis of Meynert

E. Anterior cingulate cortex.

The postcommissural fornix tasks to i beg your pardon structure?

A. Substantia innominata. This answer is INCORRECT.

The precommissural branch goes come the septal nuclei and the substantia innominata.

B. Mammillary bodies

C. Ventromedial nuclei the the hypothalamus

D. Cell nucleus basalis the Meynert

E. Anterior cingulate cortex.

The postcommissural fornix tasks to i m sorry structure?

A. Substantia innominata.

B. Mammillary body This prize is CORRECT!

It is the postcommissural branch that the fornix the goes to the mammillary bodies.

C. Ventromedial nuclei the the hypothalamus

D. Cell core basalis the Meynert

E. Anterior cingulate cortex.

The postcommissural fornix jobs to i m sorry structure?

A. Substantia innominata.

B. Mammillary body

C. Ventromedial nuclei of the hypothalamus This answer is INCORRECT.

The postcommissural branch that the stria terminalis goes come the ventromedial nuclei the the hypothalamus.

D. Cell nucleus basalis the Meynert

E. Anterior cingulate cortex.

The postcommissural fornix jobs to i beg your pardon structure?

A. Substantia innominata.

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B. Mammillary bodies

C. Ventromedial nuclei that the hypothalamus

D. Cell nucleus basalis of Meynert This price is INCORRECT.

The precommissural branch goes to the septal nuclei and the cell nucleus basalis the Meynert.