The Brain is a very complex structure in our body. The brain is made up of several different parts and each of those parts has different functions. Researchers are investigating the processes and the functions of the different structures of the brain. There is plenty of information that they have found to help us understand a little of the complexity of the human brain. One structure that researchers have pay attention to is the hippocampus. Researchers had the opportunity to study the case of Henry Gustav Molaison also known as H. M. an amnesic patient in 1953 when part of his hippocampus and other structures close to the hippocampus were removed through surgery. (Kalat) This case was the beginning of new investigations to find the functions of the hippocampus and how it is related with memory. The hippocampus is involve in different functions, is part of the limbic system, it has an important role in memory, acetylcholine is a neurotransmitters that is involved in the functioning of the hippocampus, and amnesia is a disease related with damage in the hippocampus. These are some of the function of the hippocampus in the human brain.
The hippocampus is part of the limbic system, the limbic system is important in motivations and emotions such us eating, drinking, sexual activity, anxiety, and aggression. The word hippocampus comes from the Latin word meaning “sea horse”. (Kalat) The hippocampus’s shape resembles a sea horse. The hippocampus is between the thalamus and cerebral cortex and it is located in the inferior of the temporal lobe. The hippocampus is made up of CA1, CA2, and CA3 pyramidal cell fields and the hippocampus consists of two C-shaped interlocking principal cellular layers: the granular cell layer of the dentate gyrus and the pyramidal cell layer of the hippocampus proper. (Leonard) The major roles in the hippocampus are cover in the next paragraphs.
The hippocampus serves a critical role in declarative memory – our capacity to recall everyday facts and events. (Howard) The hippocampus is critical for declarative memory and researchers have been testing this hypothesis. The special case that we mentioned in our introduction about H.M. occurred about 50 years ago, and researchers were interested in studying his case. H.M. suffered several epileptic seizures per day, his neurosurgeon removed his hippocampus and other nearby structures, the result was that he was impaired in his declarative memory. Removal of the hippocampus had several problems in H.M.’s brain functioning, furthermore, the deficit in new learning ability is accompanied by a temporally graded retrograde amnesia, such that H.M. can recall information obtained remotely in life but he is impaired in recalling events that occurred recently before the onset of amnesia. (Howard) Researchers have been examining different concepts to support their hypothesis.
The hippocampus is envisioned as critically involved in the rapid encoding of events as associations among stimulus elements and context, in the encoding of episodes as sequences of events, and in linking episodes by common features into relational networks that support flexible inferential memory expression. The details of memory representations are likely contained within widespread areas of the cortex, such that the properties of declarative memory described here involve a combination of hippocampus and cortical processing.
Another approach to support that the hippocampus is critical in declarative memory is in the experimental field with animals. Researchers use selective lesions in the brain where they damage the hippocampus and then use different tests to observe the results. Researchers will continue to find out more support for the critical roles of the hippocampus.
Another hypothesis related with the hippocampus is that spatial memory is dependent in the hippocampus. Spatial memory is responsible for responding to cues in our environment. One famous test to find this evidence is the Morris water maze task – in which a rat must swim through murky water to find a rest platform that is just under the surface and rats with damage in the hippocampus does not find the platform. In a behavioral experiment lesions were performed in the hippocampus to test spatial information vs. non spatial information. The researcher used three different tests, the radial maze, the Morris water maze and the rewarded alternation task. The results of the three tests indicate that rats that have the hippocampus removed selectively with ibotenate injections are severely impaired in acquisition of spatial information. (Leonard) Researchers also observed the behavior of birds in the Jay family; these birds rely in their spatial memory to store food in the summer for the winter when the food is scarce. These birds store seeds in thousands of locations and they are able to remember those locations. The hippocampus of these birds is larger than the hippocampus of other birds in other families. (Kalat) These are some examples of how important is the hippocampus in memory. There are other things involved in the functioning of the hippocampus like neurotransmitters.
Neurotransmitters are substances that transmit nerve impulses across the gap between neurons. One neurotransmitter involved in the functioning of the hippocampus in declarative memory is acetylcholine. Acetylcholine is considered essential for proper functioning of the hippocampus – dependent declarative memory system, and it represents a major neuropharmacological target for the treatment of memory deficits, such as those in Alzheimer’s disease. (Steffen) Researchers observed that low levels of acetylcholine, during slow- wave sleep is necessary for the replay of new memories in the hippocampus and for the consolidation of declarative memory.
But there are also impairments related with hippocampus damage. One example is the case of H.M. which was diagnosed with amnesia as a result of the removal of the hippocampus. Amnesia is the loss of memory. He loss the ability in forming long – term memories after his surgery. His main problem was massive anterograde amnesia which is the loss of memories for events that happened after brain damage, and he also suffered a moderate retrograde amnesia which is the loss of memory for events that occurred shortly before brain damage. (Kalat) Amnesia is related with damage of the hippocampus. Researchers have different hypothesis of what causes amnesia, because the surgery performed to H.M. removed the hippocampus and nearby structures. There have been performing experiments with animals to find the causes of amnesia if it’s related with only the hippocampus or if it involves other brain structures.
The different structures of the brain have very specific functions. They can be related with different systems in our body or they may be involved in essential tasks for our surviving. The hippocampus is an important structure that is related in different emotions and motivations with the limbic system, it is critical in the function in declarative memory and spatial memory, acetylcholine is a neurotransmitter that is related with declarative memory, and damage in the hippocampus can lead to amnesia. The brain and its structures are an open field for research, because of its complexity and plasticity. There is still a lot of information that we need to be able to understand the human brain.
Howard, Eichenbaum. Hippocampus: Cognitive Processes and Neural Representation that Underlie Declarative Memory. Neuron, Volume 44, Issue 1. 29 Sept. 2004. Pg. 109-120.
Jarrad, Leonard E. What does the hippocampus really do? Elsevier: Behavioral Brain Research 71 (1995) Pg. 1-10.
Kalat, James W. Biological Psycology. Canada: Thomson 2007.
Steffen, Gais and Born, Jan. Low acetylcholine during slow-wave sleep is critical for declarative memory consolidation. PNAS, Vol. 101 no. 7 Pg. 2140-2144.