HIPPOCAMPUS
Hippocampal Formation. The hippocampus, the posterior part of the dentate gyrus and the indusium griseum are sometimes grouped together as the hippocampal formation. In humans, the attenuated gray and white structures of this formation are produced by the enormous enlargement of the corpus callosum, which encroaches upon the parahippocampal and dentate gyri and the hippocampi, thus expanding them. The hippocampus is a part of the marginal cortex of the parahippocampal gyrus that has been invaginated, or rolled, into the floor of the inferior horn of the lateral ventricle by the exuberant growth of the nearby temporal cortex. The curved hippocampal eminence is composed mostly of gray matter, and its anterior end is expanded and grooved like a paw, the pes hippocampi. Axons conveying efferent impulses from the pyramidal cells of the hippocampus form a white layer on its surface, the alveus, and then converge toward its medial edge to form a white strip, the fimbria. The hippocampus is an important part of the olfactory apparatus in lower animals; in humans, few or no secondary olfactory fibers end in it. However, it possesses substantial connections with the hypothalamus, which regulates many visceral activities that influence emotional behavior and with temporal lobe areas reputedly associated with memory.
The dentate gyrus (dentate
fascia) is a crenated fringe of cortex occupying the narrow furrow between the
fimbria of the hippocampus and the parahippocampal gyrus. Anteriorly, this
fringe fades away on the surface of the uncus, and posteriorly, it becomes
continuous with the indusium griseum through the gyrus fasciolaris.
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| Plate 2-9 |
The hippocampus contains pyramidal
cells in regions CA1 and CA3 that project via the efferent fornix to the septal
nuclei and hypothalamus. The subiculum receives input from the hippocampal
pyramidal cells and also projects via the fornix to the mammillary nuclei and
anterior nucleus of the thalamus. It is connected reciprocally with the
amygdala and sends axons to cortical association areas of the temporal lobe.
The dentate gyrus contains granule
cells that project to the pyramidal cells of the hippocampus and subiculum and
receive hippocampal input. The afferent connections to the hippocampal
formation include the cerebral association cortices, prefrontal cortex,
cingulate cortex, the insular cortex, amygdaloid nuclei, and olfactory bulb via
projections to the entorhinal cortex. Afferent cholinergic axons from septal
nuclei traverse the fornix to provide the dentate gyrus and hippocampal CA
regions.
There exist several clinical
conditions where damage unique to the hippocampal formation occurs. CA1 neurons
are particularly susceptible to ischemic conditions as seen in
cardiorespiratory arrest. Also, patients with temporal lobe epilepsy can suffer
CA1 neuronal loss. The most common clinical scenario affecting the hippocampal
formation is Alzheimer disease (AD). AD is pathologically associated with neuronal
cell loss, neurofibrillary tangles, neuritic amyloid plaques, and granule
vacuolar degeneration of the hippocampal region. AD is discussed in more detail
in Plates 2-24 to 2-26.
