Hippocampus (A complete guide)

The hippocampus is a complex structure in the brain that plays a major role in learning and memory. 

What is the hippocampus?

Humans and other mammals have two hippocampi, one on each side of the brain.

The hippocampus is part of the limbic system, and plays important roles in the consolidation of information from short-term memory to long-term memory, and in spatial memory that enables navigation.

It is an S shaped structure which is a layer of densely packed neurons.

The hippocampus is a plastic and vulnerable structure that can get damaged by a variety of stimuli.

Emotions and memories are controlled by the hippocampus, along with other structures, deep within the brain.

These constitute the full limbic system and each part is duplicated, one on each side of the brain, as follows:  

·      thalamus acts as a gatekeeper for messages passed between the spinal cord and the cerebral hemispheres

·      hypothalamus controls emotions, and also regulates the body’s temperature and controls crucial urges such as eating or sleeping

·      hippocampus sends memories to be stored in appropriate sections of the cerebrum and then recalls them when necessary.

What happens when things go wrong in the hippocampus?

The hippocampus is thought to be a crucial structure in memory consolidation (i.e., the formation of new memories).

The hippocampus is also one of the brain regions most affected by Alzheimer’s disease, a type of dementia, is a debilitating illness that is characterized by severe memory loss.

Alzheimer’s disease involves neurodegeneration, or the deterioration and death of neurons.

While this neurodegeneration is widespread, neurons in the hippocampus are particularly susceptible.

When brain structures called hippocampi are smaller, that may point to a higher risk of getting Alzheimer’s disease and other types of dementia, new research suggests.

People who did better on certain memory tests tended to have a larger hippocampus.

These two seahorse-shaped structures deep inside the brain are important for navigation and for storing and retrieving memories.

In people with dementia, the hippocampi are often the first regions of the brain to suffer damage.

Earlier studies have found a link between the volume (size) of hippocampi and the risk of dementia, but the results have been conflicting.

Also, few studies have included people with mild cognitive impairment (MCI), a condition that often precedes dementia.


Early studies have found a link between the volume of hippocampi and the risk of dementia.

How does the hippocampus work?

The hippocampus is a complex structure and to fully understand it, and other parts of the brain, you do literally have to be a brain doctor.

This blog will not go into the deep science of the structure and functioning of the hippocampus; however we will try and offer useful information which is scientific but not too technical.

The hippocampus is divided into three regions CA1, CA2, and CA3 which form what is called the trilaminar loop.

This is the processing center of long-term memory. Long-term potentiation (LTP), which is a form of neural plasticity, occurs in the hippocampus, and LTP is a vital brain mechanism involved in memory storage.

The complex processes of memory encoding in the hippocampus and retrieval of experiences from the frontal lobe involve two prominent pathways: polysynaptic and direct.

In the polysynaptic pathway, the hippocampus receives afferent inputs via axons of the entorhinal cortex, which terminate in the dentate gyrus.

From there, neurons of the dentate gyrus send mossy fibres to the neurons of pyramidal cells in CA3.

These pyramidal cell neurons are further divided into two branches: one branch reaches the opposite side of the hippocampus via corpus callosum; the other branch connects to CA1 through Schaffer collateral pathways.

From there, the projections leave the hippocampus to return to the inferior temporal cortex, temporal pole, and prefrontal cortex.

The polysynaptic pathway is important for semantic memory (facts and concepts), and the direct pathway is important for the episodic (recollection of events) and spatial memory (recognition).


The hippocampus plays important roles in the consolidation of information from short-term memory to long-term memory

What is the role of the hippocampus in spatial navigation?

One of the major functions of the hippocampus is to form a cognitive map, which is a type of mental representation related to acquisition, coding, storing, recalling and decoding of information about relative locations within a specific environment.

Place cells, a type of pyramidal cell, are mainly involved in hippocampus-mediated spatial navigation. 

What is the role of the hippocampus in behavior?

The hippocampus plays a vital role in flexible and goal-directed behavior.

An intact hippocampal activity is required for forming and reconstructing relational memory associated with flexible cognition and social behavior (required for remembering arbitrary associations between objects or events).

Many studies have revealed that any damage to the hippocampus can impair flexible use of information and produce inconsistent behavior.

The role of the hippocampus in behavioral inhibition is also well-established in research.

The link between hippocampus and inhibition has been derived mainly from two basic observations: firstly, that damage to the hippocampus makes animals hyperactive; and secondly, that damage to the hippocampus reduces the learning ability of animals to inhibit responses that they have learned previously.

The hippocampus acts as an evaluation center associated with behavioral inhibition, obsessional thinking, scanning and spatial map formation.

However, the hippocampus does not actively participate in controlling behavior, once an experience is undertaken.

Increased levels of corticosterone brought on by stress eventually reduce the firing rate of the hippocampus. Recently, a new hippocampal function has been proposed.

It has been found that low-frequency firing/activity in the hippocampus can influence the functional integration between spatially separated regions in the cerebral cortex, leading to increased sensory responses, such as vision, hearing and touch.  

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Frequently asked questions (FAQs) about the hippocampus: 

1.    What is the hippocampus and what does it do?

The hippocampus is a brain structure embedded in the temporal lobe of each cerebral cortex.

It is an important part of the limbic system, which is a group of brain areas that regulate emotion, motivation, learning and memory. 

2.    What happens if the hippocampus is damaged?

If the hippocampus is damaged on either one or both sides of the brain by an accident or diseases such as Alzheimer’s disease, the person can experience memory loss and loss of the ability to form new long-term memories. 

3.    What is the hippocampus? 

The hippocampus is a small, curved structure in the brain that plays an important role in the formation of new memories and is also crucial in learning and emotions.

Since the brain is symmetrical, we have two sides of the hippocampus.

4.    Why is the hippocampus called hippocampus? 

The name hippocampus comes from the Greek word for seahorse.

When the hippocampus is removed from the brain, its shape resembles a seahorse. 

5.    How is the hippocampus affected by stress?

High levels of cortisol that result from stress can cause chronic damage to many brain areas, including the hippocampus.

Elevated cortisol can impair the ability of the hippocampus to recall old memories and encode new memories. 

6.    Does alcohol damage the hippocampus?

It has been shown that alcoholics have a reduced hippocampal volume compared to healthy controls.

This suggests that heavy alcohol drinking can reduce the size of the hippocampus, and thus impair its function.

People who drink large amounts of alcohol may be susceptible to damage to the hippocampus and thus can experience significant impairments in learning and memory. 

7.    At what age is the hippocampus fully developed? 

People are not born with a fully developed hippocampus.

Over the course of the first two and a half years of life, the hippocampus reaches full development.

This makes sense if you think about the fact that you do not have many declarative memories from your first two years of life. 

8.    What happens when your hippocampus shrinks?

When your hippocampus shrinks, or atrophies, you may experience dementia, which is cognitive deficits and memory loss that can be associated with Alzheimer’s disease.

Shrinking of the hippocampus has also been associated with obesity, back pain and depression. 

9.    What diseases affect the hippocampus?

There are many diseases that affect hippocampus size and function.

These diseases include Alzheimer’s disease, temporal lobe epilepsy (TLE), cognitive ageing, post-traumatic stress disorder (PTSD), transient global amnesia, schizophrenia and both depression and anxiety disorders. 

10.                  How is the hippocampus important for memory? 

The hippocampus is extremely important for the consolidation of information that gets transferred from short-term memory to stored long-term memory.

The hippocampus is also an important brain structure that controls spatial memory and thus navigation in the environment. 

11.                  Can hippocampal atrophy be reversed?

Treatment of stress-related conditions that are known to cause loss of hippocampal neurons can partially reverse this atrophy, which leads to hippocampal growth rather than further loss. 

12.                  What behaviors would be affected if there was damage to the hippocampus? 

If there is damage to the hippocampus or its connections to other brain areas, people can develop amnesia, which is the loss of ability to learn and then recall new information.

People who have amnesia cannot form new long-term memories, which involves forgetting information soon after they hear or see it. 

13.                  Can you grow your hippocampus?

Research now shows that we do have the capacity to grow new neurons, and particularly in our hippocampus, in as short a time as weeks.

The best way to generate new neurons in the hippocampus is to exercise.

In fact scientists are now proclaiming that physical exercise is THE best exercise you can do for your brain.

14.                  How does exercise affect the hippocampus?

In a study done at the University of British Columbia, researchers found that regular aerobic exercise, the kind that gets your heart and your sweat glands pumping, appears to boost the size of the hippocampus, the brain area involved in verbal memory and learning.

If you would like more information on the function and effect of the hippocampus try these books:

The Hippocampus Book (Oxford Neuroscience Series)

The Hippocampus Book promises to facilitate developments in the field in a major way by bringing together, for the first time, contributions by leading international scientists knowledgeable about hippocampal anatomy, physiology, and function.

This authoritative volume offers the most comprehensive, up-to-date account of what the hippocampus does, how it does it, and what happens when things go wrong.

At the same time, it illustrates how research focusing on this single brain structure has revealed principles of wider generality for the whole brain in relation to anatomical connectivity, synaptic plasticity, cognition and behavior, and computational algorithms. 

The Human Hippocampus: Functional Anatomy, Vascularization and Serial Sections with MRI

This new edition, like previous ones, offers a precise description of the anatomy of the human hippocampus based upon neurosurgical progress and the wealth of medical imaging methods available.

The first part describes the fine structures of the hippocampus and is illustrated with new original figures.

A survey is then provided of current concepts explaining the functions of the hippocampus, and the external and internal hippocampal vascularization is precisely described.

The last and main part of the book presents serial sections in coronal, sagittal, and axial planes; each section is accompanied by a drawing to explain the MR 3T images.

From Here to There: The Art and Science of Finding and Losing Our Way

How is it that we can walk unfamiliar streets while maintaining a sense of direction? Come up with shortcuts on the fly, in places we’ve never traveled?

The answer is the complex mental map in our brains. This feature of our cognition is easily taken for granted, but it’s also critical to our species’ evolutionary success.

In From Here to There Michael Bond tells stories of the lost and found―Polynesian sailors, orienteering champions, early aviators―and surveys the science of human navigation.

References:

Hippocampus functions – news-medical.net – August 2019

Size of These Brain Parts Tied to Dementia Risk – WebMD – October 2015

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