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Where are the hippocampus and limbic system?

Medial parts of the temporal lobe


3 C-shaped structures and pathways of the limbic system

1) Limbic association cortex: cingulate and parahippocampal gyri

2) Hippocampus and fornix

3) Amygdaloid complex and stria terminalis


Limbic cortical areas

Medial orbital gyri of frontal lobe

Cingulate gyrus of frontal and parietal lobes

Parahippocampal gyrus of temporal lobe

All receive input from higher order sensory and association areas


How is the medial prefrontal cortex related to the limbic system?

Can influence various cortical and subcortical parts of the limbic system:

Cingulate gyrus


Dorsal thalamus

Amygdaloid complex

Some brainstem nuclei


What structures are in the medial part of the temporal lobe?



(both underneath the parahippocampal gyrus!)


Parahippocampal gyrus

Continuation of cingulate gyrus on inferior surface of the brain

Cortex of parahippocampal gyrus includes subiculum and entorhinal cortex (both functionally related to hippocampus)

Rostrally, parahippocampal gyrus expands to form uncus (amygdala and anterior part of hippocampus underlie the uncus)


Diverse functions of the limbic system




Control of visceral function


Papez circuit

Outdated, but a good model

Mammillary body --> anterior thalamic nucleus -->cingulate gyrus --> cingulum --> hippocampus --> hippocampus and subiculum --> fornix --> mammillary body


Amygdala anatomy

Collection of several distinct nuclei, so called amygdaloid complex

In temporal lobe just beneath cortex of uncus (or parahippocampal gyrus!), immediately rostral to hippocampal formation


3 major divisions of amygdaloid complex

Don't need to know

Basolateral nuclei: largest group; connections with cerebral cortex, particularly sensory association areas, limbic association cortex (cingulate cortex, orbital and medial prefrontal cortex), and thalamus (MD nucleus); general function = emotion/experience

Central nuclear group: connections with brainstem and hypothalamic regions that include viscerosensory areas such as the nucleus of the solitary tract and autonomic nuclei such as the dorsal motor nucleus of the vagus; general function = visceral control

Medial group: connections with olfactory bulb, olfactory cortex and hypothalamus; general function = olfaction


Efferent pathways of the amygdala

Stria terminalis: from medial group to parts of hypothalamus and septal areas

Ventral amygdalofugal system: diffuse but connects frontal cortex to brainstem

Interconnections among amygdaloid nuclei: connect cortical regions that provide highly processed sensory info (sensory association areas) with hypothalamic and brainstem regions that serve as effector systems


Function of amygdala

Sensory events connected to emotional significance

Emotion-related aspects of learning

Learning about fearful stimuli

Evaluating the emotional significance of events in the environment


Overview of function of hippocampal formation

Important role in memory

Bilateral damage causes loss of ability to consolidate short-term memory to long-term memory

Spatial memory


3 regions of hippocampal formation

1) Dentate gyrus

2) Hippocapmus (Ammon's Horn or Cornu Ammonis)

3) Subiculum


Cells of the dentate gyrus vs. cells of the hippocampus

Granule cells major cell of dentate gyrus

Pyramidal cells major cell of hippocampus


3 layers of cells of the dentate gyrus

1) Molecular layer = dendritic layer

2) Granule cell layer = cell body layer

3) Polymorph layer/hilus = axonal layer

In general, dendrites of granule cell layer extend into relatively neuron-free region above granule cell layer and layer of polymorph cells below or within granule cell layer


3 layers of hippocampus

1) Stratum radiatum = dendritic layer

2) Pyramidal cell layer = cell body layer

3) Stratum oriens = axonal layer

Interneurons in each layer but most prominent within or immediately below granule cell/pyramidal cell layers; many of the interneurons are basket cells that use GABA and inhibit input to cell bodies of principal cells

In general, dendrites of pyramidal cell layer extend into relatively neuron-free region above pyramidal cell layer and layer of polymorph cells below or within pyramidal cell layer


Basket cells

One type of GABA interneuron

Inhibit cell bodies of principal cells of dentate gyrus (granule cells) and hippocampus (pyramidal cells)

Neuronal cell bodies and axon terminals in dentate gyrus


Trisynaptic circuit through hippocampal formation (most classical hippocampal pathway)

Info from all over the cortex --> entorhinal cortex --> across hippocampal fissure (perforated path) --> dentate gyrus (and also small input to hippocampus) --> granule cells send axons to CA3 of hippocampus ONLY --> pyramidal cells send projection to CA1 field (but some from CA3 leave hippocampus via fornix) --> most sent to subiculum --> entorhinal cortex --> cortical association areas

Summary: entorhinal cortex through perforant path to dentate gyrus --> mossy fibers to CA3 --> Schaffer collaterals to CA1 --> subiculum --> entorhinal cortex


Links in hippocampal pathways

Perforant path: major projection from entorhinal cortex to dentate gyrus and hippocampus; crosses or "perforates" hippocampal fissure to reach dentate

Mossy fibers: axons of granule cells of dentate gyrus that have large, characteristic terminals that contact CA3 neurons

Schaffer collaterals: axon collaterals from CA3 neurons to CA1


Where does the hippocampus send info via fornix?

Hippocampus sends info to mammillary bodies and septal region through the fornix


What NT is used in the basic hippocampal circuitry fron entorhinal cortex --> dentate gyrus --> CA3 --> CA1?

Glutamate (excitatory) used in this circuitry!!

Mossy fibers which project from dentate gyrus to CA3 also contain high levels of zinc and other neuropeptides


Where is acetylcholine used?

ACh is major NT of neurons from septal nuclei to hippocampal formation; part of group of cholinergic neurons in basal forebrain; remember basal nucleus of Meynert project to all areas of cerebral cortex

Choliergic neurons in septal nuclei project to hippocampal formation through fornix


Why is cholinergic innervation of the hippocampus special?

1) Cholinergic neurons that innervate hippocampus (from septal nuclei, through fornix) are severely depleted in Alzheimer's disease

2) Nerve growth factor (NGF) has been found to influence survival of cholinergic neurons in CNS


What NT is used by intrinsic neurons within the hippocampus and dentate gyrus?

GABA used by many intrinsic neurons (one example is basket cells)

(There are also GABA neurons in septum that project to hippocampus)


Which neuropeptides are found in interneurons in hippocampus?

Somatostatin, CCK, VIP

Co-localized with other neuroactive substances such as GABA


What diseases show selective vulnerability of neurons in hippocampal formation?

Ischemia: CA1 field of hippocampal formation; excitotoxicity involved (get loss of short term memory)

Temporal lobe epilepsy: CA1 field and hilus of dentate gyrus, and CA3 field; hippocampus becomes shrunken (hippocampal sclerosis); granule cells of dentate gyrus and CA2 field are spared

Alzheimer's disease: entorhinal cortex and CA1 field; however other brain regions involved, such as cerebral cortex and basal forebrain


Neurogenesis in the adult brain

Occurs in dentate gyrus and subventricular zone

New granule cells generated from dividing progenitor cells: new neurons generated near base of granule cell layer, differentiate, extend axons and are incorporated into existing circuitry of hippocampus

Environmental factors and stimulation can influence rate of neurogenesis


Do axons of the CNS regenerate?

Not over long distance, and is limited

However, axonal reorganization or "sprouting" does occur!


Axonal sprouting/reorganization

Undamaged neurons form new synaptic connections in response to either loss of normal targets or deafferentation of nearby neurons

Axon collaterals of remaining neurons occupy synaptic sites that have been vacated by damaged neurons

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