Hippocampus


 * For other uses, see Hippocampus (disambiguation).



The hippocampus is an essential structure in the brain for forming, retaining, and storing memories. Other complex activities such as experiencing emotions, feeling hunger, pain, arousal, and learning could not take place or would be heavily weakened without the influence of the hippocampus. It is highly sensitive to outside stimuli, and can be greatly affected by mental disorders. Much is yet to be learned about this incredibly intricate area within the brain. As science progresses we will better understand the full scope of its influence on day-to-day activities.

Functions
This portion of the brain belongs to the limbic system, which is a collection of neurons that are essential to human memory and emotions. It controls powerful sensations such as pain, sexual arousal, anger, hunger and thirst, and different forms of pleasure. The limbic system also includes the hypothalamus, thalamus, fornix, amygdala, mammillary bodies, and septal areas.

More specifically, the hippocampus directly influences and controls the organization, formation, and storage of memories. Although this structure of the brain is tiny, its small size has no correlation to its vast importance. Indeed, humans and other creatures would have a vastly diminished ability to uniquely perceive the world, feel emotions, and remember events without the presence of the hippocampus. It is heavily relied upon to associate certain memories with smell, sound, and tactile sensations. For example, a person without a functioning hippocampus might be able to remember the time her or she went hiking up to the mountains, but the person would have difficulty remembering how he or she felt about it, when or where it took place, what the air smelled like, the tired feeling experienced afterwards, etc. This would only take place if the person was even able to form new memories, for the hippocampus plays a major role in creating or storing memories.

Anatomy


Upon looking at this small piece of the brain, one can see that it has a similar shape to that of the body of a seahorse. The "head" of this portion thrusts out from the body, and a long, gradually narrowing tail curls at the end. The hippocampus is directly located in the hippocampal sulcus below the temporal horn of the lateral ventricle. The individual parts that the hippocampus includes: the dentate gyrus, the CA1-CA3 fields, and the subiculum, or, more specifically, the parahippocampal gyrus. The hippocampus also lies next to the cerebral cortex, which is a major component of the limbic system. This essential system in the body includes the cingulate cortex, olfactory cortex, amygdala, and of course, the hippocampus.

Physiology


Physiology organs within the body is defined as the study and understanding of their capabilities, processes, and uses. This also includes researching organs' weaknesses and vulnerabilities, such as specific diseases they can fall prey to. When focusing on the hippocampus, researchers have discovered that it displays two different types of functions. These functions are called modes, which are two specific arrangements of neural and electrical activity that the hippocampus exhibits. A device called an electroencephalogram or EEG can measure these modes. This is achieved by the EEG recording the "electrical impulses" that nerves send out within the brain. In this way, certain parts of the brain can communicate with each other by sending out information.

The two different modes of the hippocampus consist of the Theta mode and the LIA Mode. These different functions have been studied more extensively by researchers in rats' brains, so any further information provided has been discovered by observing the electrical impulses within the craniums of these rodents.

Theta Mode
This mode is exhibited by the hippocampus while the rat is in REM sleep. REM stands for Rapid Eye Movement, which is a stage of sleep when the neurons of the brain behave very similarly to how they function while the subject is awake. Not surprisingly, Theta Mode can also be observed in rats while they are awake and mobile. During this stage, the EEG records the brain waves as being normal. This "normal condition" consists of the brain waves as flowing in a constant, full manner. While the hippocampus is in Theta Mode it has a frequency of six to nine Hz, or Hertz. The majority of the hippocampus is made up of granule and pyramidal cells which display a small amount of activity during this time. This means that the bulk of this portion of the brain is quiet and mostly dormant while in this mode. The remaining cells in the hippocampus are very active and are distinguished by firing up at "high rates, up to as much as 50 spikes per second." These bursts of activity typically last for a couple of seconds.

LIA Mode
This mode stands for Large Irregular Activity mode. This mode of the hippocampus can be observed in rats while they are not in Theta mode. These are moments where the rats are not in REM sleep or particularly active. Specifically, LIA mode takes place while the rodent is in "slow wave sleep" or while it is awake but inactive, such as when it is siting down or lounging about. While in LIA mode, the EEG often records the brains' electrical impulses as sharper and more random. These waves last for shorter amounts than Theta mode waves. Compared to a couple seconds, these waves exhibit "random deflections" for two hundred to three hundred milliseconds. During this time the pyramidal and granule cells display a mild amount of activity, but are still mainly quiet.

Video
Starting at 2:15, a dissection of the hippocampus. rTQBtlSJFxI