Hippocampus and parahippocampal gyrus

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1. Cingulate gyrus

Shown in Images 5 and 7 (but better seen in Image 2 surrounding the corpus callosum and separated from it by the callosal fissure), the cingulate gyrus is part of the limbic lobe together with the hippocampal and parahippocampal gyri. It has a role mediating aspects of endocrine, olfactory, skeletal and visceral function. It may also be involved in some aspects of memory.

Show list

1.
Cingulate gyrus
2.
Corpus callosum
3.
Lateral ventricle
4.
Dorsomedial nucleus (medial dorsal nucleus) of thalamus
5.
Ventral lateral nucleus of thalamus
6.
Third ventricle
7.
Interpeduncular fossa
8.
Hippocampus
9.
Putamen
10.
Globus pallidus
11.
Parahippocampal gyrus
12.
Pons
13.
Internal capsule
14.
Cerebral peduncle (crus cerebri)
15.
Corona radiata
16.
Substantia nigra
17.
Insula
18.
Fornix (plural fornices)
19.
Inferior horn of lateral ventricle

In this view we see:

Coronal section of the brain posterior to the coronal section seen in Image 6.
Towards the bottom of the image the hippocampus in cross-section on both sides of the brain.
The parahippocampal gyrus in the medial temporal lobe below the hippocampal formation.

The hippocampus and parahippocampal gyrus, together with the amygdala (seen in Images 5 and 6) and cingulate gyrus (seen in Images 2, 5, 6 and 7), make up the limbic system, which is primarily involved in emotion, memory and learning. The cingulate gyrus is the only one of these four parts of the limbic system not located in the medial temporal lobe.

Location

As shown in Image 7, the hippocampus is located medially in the temporal lobe, next to the inferior horn of the lateral ventricle.

Although hippocampus means seahorse, when thinking about its location it may be more helpful to think of it as a very small snake with its head immediately behind the amygdala, and its body curving back and up to meet the fornix on the same side of the brain.

Structure

The hippocampus (previously known as the hippocampal formation) consists of the dentate gyrus, subiculum and hippocampus proper – known as Ammon's horn or cornu ammonis (CA).

The hippocampus proper can be divided into three distinct fields: CA3, CA2 and CA1. CA3 is distinctive because axons from the dentate gyrus synapse onto its dendrites. Unlike the cerebral cortex, which consists of six-layered neocortex, the hippocampus is composed of three-layered archicortex (archi means beginning), which has more ancient origins than neocortex.

Function

Memory – the hippocampus works with the prefrontal cortex, which is important for encoding and storing memories. While the hippocampus is critical in processing declarative memory, the prefrontal cortex is the ultimate storage site for these memories. Declarative memories are memories of events, facts or details that we can consciously recall. The hippocampus is not necessary for short-term memory, which lasts only a minute and is a function of the prefrontal cortex. The hippocampus is also not necessary for working memory, which is similar to short-term memory but has the additional function of manipulating information.

We have learned much about the working of memory in the human brain through the tragic case of Henry Molaison.

Spatial skills and navigation – there is accumulating evidence that the hippocampus has a role in spatial skills, most notably that neurons called place cells enable us to map our environment. These neurons may play a part in the success of techniques such as the memory palace where particular memories are ascribed to specific places in the mind.

Practical application of memory research – the long-term study of Henry Molaison and others has taught us that the hippocampus is necessary for transferral of short-term memories to long-term memory. There is now an understanding of the operations involved, including the four processes necessary for building long-term memories:

Encoding – new information is linked to existing information.
Storage – neural sites where memory is retained over time.
Consolidation – making temporarily stored information more stable.
Retrieval – essential for tests and exams and most effective when using retrieval cues.