Hippocampus and Memory

Hippocampus
The hippocampus (red), a limbic system structure, is responsible for long-term memory. Credit: Sciepro/Getty Images

The hippocampus is the part of the brain that is involved in forming, organizing, and storing memories. It is a limbic system structure that is particularly important in forming new memories and connecting emotions and senses, such as smell and sound, to memories. The hippocampus is a horseshoe shaped structure, with an arching band of nerve fibers (fornix) connecting the hippocampal structures in the left and right brain hemispheres.

The hippocampus is found in the brain's temporal lobes and acts as a memory indexer by sending memories out to the appropriate part of the cerebral hemisphere for long-term storage and retrieving them when necessary.

Anatomy

The hippocampus is the main structure of the hippocampal formation, which is composed of two gyri (brain folds) and the subiculum. The two gyri, the dentate gyrus and Ammon's horn (cornu ammonis), form interlocking connections with one another. The dentate gyrus is folded and nestled within the hippocampal sulcus (brain indentation). Neurogenesis (new neuron formation) in the adult brain occurs in the dentate gyrus, which receives input from other brain areas and aids in new memory formation, learning, and spacial memory. Ammon's horn is another name for the hippocampus major or hippocampus proper. It is divided into three fields (CA1, CA2, and CA3) that process, send, and receive input from other brain regions.

Ammon's horn is continuous with the subiculum, which acts as the main output source of the hippocampal formation. The subiculum connects with the parahippocampal gyrus, a region of the cerebral cortex that surrounds the hippocampus. The parahippocampal gyrus is involved in memory storage and recall.

Function

The hippocampus is involved in several functions of the body including:

  • Consolidation of New Memories
  • Emotional Responses
  • Navigation
  • Spatial Orientation

The hippocampus is important for converting short-term memories into long-term memories. This function is necessary for learning, which relies on memory retention and proper consolidation of new memories. The hyppocampus plays a role in spatial memory as well, which involves taking in information about one's surroundings and remembering locations. This ability is necessary in order to navigate one's environment. The hippocampus also works in concert with the amygdala to consolidate our emotions and long-term memories. This process is critical for evaluating information in order to respond appropriately to situations.

Location

Directionally, the hippocampus is located within the temporal lobes, adjacent to the amygdala.

Disorders

As the hippocampus is linked to cognitive ability and memory retention, people who experience damage to this area of the brain have difficulty recalling events. The hippocampus has been the focus of attention for the medical community as it relates to memory disorders such as Post Traumatic Stress Disorder, epilepsy, and Alzheimer's disease.

Alzheimer's disease, for example, damages the hippocampus by causing tissue loss. Studies have shown that Alzheimer's patients who maintain their cognitive ability have a larger hippocampus than those with dementia. Chronic seizures, as experienced by individuals with epilepsy, also damage the hippocampus causing amnesia and other memory related problems. Prolonged emotional stress negatively impacts the hippocampus as stress causes the body to release cortisol, which can damage neurons of the hippocampus.

Alcohol is also thought to negatively impact the hippocampus when consumed in excess. Alcohol influences certain neurons in the hippocampus, inhibiting some brain receptors and activating others. These neurons manufacture steroids which interfere with learning and memory formation resulting in alcohol-related black outs.

Heavy long term drinking has also been shown to lead to tissue loss in the hippocampus. MRI scans of the brain indicate that alcoholics tend to have a smaller hippocampus than those who are not heavy drinkers.

Divisions of the Brain

  • Forebrain - encompasses the cerebral cortex and brain lobes.
  • Midbrain - connects the forebrain to the hindbrain.
  • Hindbrain - regulates autonomic functions and coordinates movement.

References:

  • Alcoholism: Clinical & Experimental Research. (2006, October 25). Heavy, Chronic Drinking Can Cause Significant Hippocampal Tissue Loss. ScienceDaily. Retrieved August 29, 2017 from www.sciencedaily.com/releases/2006/10/061025085513.htm
  • Washington University School of Medicine. (2011, July 10). The biology behind alcohol-induced blackouts. ScienceDaily. Retrieved August 28, 2017 from www.sciencedaily.com/releases/2011/07/110707092439.htm