What is the parahippocampal gyrus?
The parahippocampal gyrus is in close proximity to the hippocampus.
It is part of the archicortex, which in turn is part of the cerebrum. Phylogenetically, the archicortex is younger than the neocortex but older than the paleocortex. The medicine allocates the hippocampus to the limbic system, which also includes the parahippocampal gyrus. The hippocampus is involved in this system, especially in memory processes.
Anatomically speaking, the parahippocampal gyrus is not completely defined by the surrounding brain mass. It merges on the one side into the uncus and on the other side borders on the gyrus occipitotemporalis medialis (gyrus lingualis or gyrus infracalcarinus). Beneath the parahippocampal gyrus and the medial gyrus occipitotemporalis lies the lateral gyrus occipitotemporalis (subcuneus).
Anatomy & Construction
In the anterior part of the parahippocampal gyrus is part of the entorhinal cortex.
This is also known as association cortex and consists of three sections: the frontal, parietal and limbic association cortex. The latter is the part that lies in the parahippocampal gyrus. It corresponds to the Brodmann areas 28 and 34. The limbic association cortex can be further divided into a ventral and a dorsal area.
The posterior part of the parahippocampal gyrus belongs to the parahippocampal cortex, to which the anatomy also assigns areas of the lateral occipitotemporal gyrus. Within the cerebral convolution there is also the "parahippocampal place area", which is relevant for visual cognition.
The bark of the parahippocampal gyrus is composed of six layers of cells. Overall, the tissue counts to the gray matter, since it consists mainly of nerve cell bodies. The actual information processing takes place in the neural networks. In contrast to the gray matter, the white matter of the brain consists predominantly of myelinated nerve fibers. Nerve fibers are the thread-like extensions of the neurons and transport the electrical signals of the nerve cells.
Function & Tasks
The parahippocampal gyrus forms part of the limbic system, which is composed of various anatomical structures. These are linked and dedicated to tasks such as emotions, memory, learning and some vegetative control processes. However, these functions are not limited to the limbic system. For example, there is no central memory for memory in the brain. Instead, memory processes such as memorizing and retrieving memories are spread over different brain areas.
The default mode network plays a central role in memory processes. It represents a functional web of different brain structures. Findings from the research suggest that the parahippocampal gyrus may play a key role in the default mode network by mediating between the network and the medial temporal lobe (Ward et al., 2014). The parahippocampal gyrus also produces associations. The so-called association cortex is the entorhinal cortex, which occupies a central position among others in Alzheimer's dementia. In addition, the parahippocampal gyrus may have associations with social situations.
Furthermore, the parahippocampal gyrus is involved in visual recognition, with the "parahippocampal place area" playing an important role. The activity of this area is related to the viewing of landscapes and spaces. However, the parahippocampal gyrus is not responsible for the primary sensory perception (seeing in the true sense) but exerts a higher cognitive function. Cognition comes into play only after the perception of the senses and refers to the identification or assignment of the seen.
Decreased activity in the parahippocampal gyrus and hippocampus is associated with schizophrenia. Schizophrenia is a mental disorder characterized by delusions and hallucinations.
Other possible symptoms include association disintegration, ego disorders, conspicuous expressions (eg, neologisms), emotional arousal, and delirium. These symptoms represent so-called positive symptoms. The counterpart to them are negative symptoms such as emotional flattening, reduced affect, social withdrawal, cognitive and linguistic reduction, apathy and reduced activity and initiative. Since schizophrenia is a very complex disorder, it can manifest differently from person to person. For the treatment of schizophrenia in addition to the drug therapy also an accompanying psychotherapy, psychoeducation or a special training into consideration.
The "parahippocampal place area", located within the parahippocampal gyrus, is important for the visual recognition of landscapes and spaces. Lesions in this area therefore typically lead to problems in recognizing these views. The affected person is still able to see and identify individual objects, but they can no longer associate the overall picture. Such lesions can be caused, for example, by a tumor, bleeding, inflammation or stroke.
Also in connection with temporal lobe epilepsy, abnormalities of the parahippocampal gyrus are possible. The disease may be associated with hippocampal sclerosis, also known as mesial temporal sclerosis, which manifests as failure of nerve cells in the affected area. The medicine distinguishes four different types of hippocampus sclerosis, of which type 1B is most common and is considered severe hippocampal sclerosis. Doctors often treat medications with temporal lobe epilepsy, but in some cases, other treatments such as brain surgery are also possible.