About the limbic system

First let's see a review. In the human cerebral cortex, the neocortex has developed and occupies 90% of the cortex. Since the neocortex covered the outside, the remaining cortices, the paleocortex and the archicortex, were driven to the bottom and back of the cerebral hemisphere.。

Since the neocortex of the cerebrum has grown, the nerve cell mass that was the motor center in animals below birds has also been driven to the bottom of the cerebrum. This is the basal ganglia. As I mentioned before, the pyramidal tract, a ?locomotor center?unique to mammals, emerged, so the remaining basal ganglia became subordinate. The basal ganglia support the center of the pyramidal tract for smooth and supple movements.

What is in the paleocortex? One of them inside is the rhinencephalon related to smell .

The limbic system is also inside it. It is a mass of old cortex existing along the callosum.

The reason I express it as "the old cortex" here is it is mixed with the paleocortex and the archicortex. We sill haven't figured out the detail of the cells.

First, let's see the rhinencephalon.

The figure and description below are taken from the page of Takatsu Chiropractic clinic at Takatsu / Mizonokuchi(http://www.takatsu-chiro.com/yougoshu/cranial-n-1.htm).

There is olfactory cells in the upper part of nose that have ability of smelling.The bundles of axons from these cells are the olfactory nerves. I will explain about axons later.

How can you smell?

When you breathe in, the molecules of the smell mixed in the breath dissolve in the mucus covering the olfactory epithelium.When odorous molecules stimulate the olfactory hair, the olfactory cells are activated and electrical stimulation is generated. The electrical stimulus is transmitted through the olfactory nerve, passes through the olfactory bulb and olfactory tract, reaches the olfactory area of ??the cerebral cortex, and we perceive smell.

Substances we can smell must be volatile fine particles that can dissolve in mucus and stimulate the olfactory hair.

Adaptation of smell

Olfaction is a sensitive sensation with a low threshold, but adaptation to odors (decrease of sensitivity) occurs rapidly. When you keep smelling the same scent, you come not to feel it. When you are exposed to a strong odor, you smell it less in about 1 minute. However,you can percieve other odors and may be more sensitive for those smell.

The nasal cavity is in three layers, upper, middle, lower, and the olfactory epithelium with olfactory cells is located above the nasal cavity. Expiration and inhalation usually go through the middle and lower parts of the nasal cavity. Little outside air enter the upper part.The odorous substances are reached by diffusion. When the odor is lost, if the outside air enters the upper nasal cavity by strongly smelling, the olfactory cells are stimulated,and you can recognize again.



The olfactory cells capture the chemical substances that cause odor, transmit the stimulus to the olfactory bulb, which is a part of the limbic system, and further to the olfactory area of ??the cerebrum, where odor is recognized.

There are 20 to 50 million human olfactory cells.One cell can react with one chemical substance, and they are classified into about 400.

Odor variations are produced by the combination of these stimuli.

Also, because the distribution of the number of olfactory cells varies from person to person, some people are sensitive to certain odors and others are insensitive.

One of the characteristics of olfaction is that it has a unique route that is transmitted to the brain, compared to visual and auditory senses.

The senses other than the olfactory sense are transmitted to and processed in the brain via the thalamus of the diencephalon,?but only?the?olfactory sense does not pass through the thalamus.It?is directly transmitted to the olfactory cortex at the frontal lobe of the cerebral cortex (orbitofrontal region) and the?limbic system of the?temporal lobe?.?It is not clear why only olfaction is special.

There seems to be no olfactory receptor.?In other words, other sensory nerves have receptors in their respective sensory organs, from which sensory nerves head toward the center.

It is said that the olfactory nerve does not have to pass through the thalamus because part of the brain called the olfactory brain extend to the periphery and became the nerve.

?What is the corpus callosum?


It is a thick bundle of commissural fibers that connect the left and right cerebral hemispheres, and is a pathway for exchanging information between the left and right cerebral cortex . Humans contain approximately 250-350 million nerve fibers.

For example, the anterior corpus callosum (genu of corpus callosum) is composed of the nerve fibers connecting left and right prefrontal cortex, the middle part is composed of the nerve fibers connecting left and right motor areas, and the rear part is composed of the nerve fibers connecting left and right visual cortex. The connection of left and right hemispheres varies in each part, for example there is no communication between the hemispheres in the sensory area of ??the limbs.

The method of estimating the running pattern of nerve fiber bundles such as white matter from MRI images?is called?tractography.?In this method, you can see nerve fibers are transported through the corpus callosum between the right and left cerebral hemispheres in this figure.









About the?limbic system?.

Limbic system is an old cortex surrounding?the?caudate nucleus?deep in the cerebrum?,?basal ganglia composed of putamen ?along the corpus.

The main role is

relating to memory (short-term memory and long-term memory) and emotion (pleasantness and discomfort such as motivation, anger, joy, and sadness).

The limbic system consists of

Orbital gyrus, septum, cingulate gyrus, parahippocampal gyrus, entorhinal cortex, hippocampal formation, amygdala,?mamillary body

You can see the cingulate gyrus and parahippocampal gyrus in the figure below.

Quoted from Color illustration Structure and Function of human body VII nervous system (1)















You also can see hippocampus, septal nucleus, amygdala and mamillary body in the figure blow.

The fornix is ??a communication channel that transfers stimulation from the hippocampus to the septal nucleus.

Quoted from Color illustration Structure and Function of human body VII (1)

It's tough and complicated,so I made an animation. Even though this is hard to understand, I made a video below it. Let's check it.


In the video, there is a mistake I explained as aqueduct of midbrain.The lateral ventricle is corret.?Thanks.



There are short-term and long-term memory.                    

Short-term memory  Memory information used for diagnosing Alzheimer's disease such as remembering what we ate today


In the short-term memory, the information that we think is important is moved to the association area and becomes long-term memory .  

"About 10 years ago, I walked here like this ."

Information on spatial sense goes to the??parietal association area

"The scene and sound at that time was like this."

Information of color and shape, goesto the temporal association area




The information of old memory is recalled and imaged in the frontal cortex.

To remember, you will first need information from outside.All information such as sight, hearing, somatosensory (excluding olfaction), taste and pain gathers in the occipital lobe, temporal lobe via the thalamus.

Then some of these sensory information gathers in the hippocampus.

In the animation below, the blue arrow indicates that the signal enters the limbic system from the cerebral cortex, where the neocortex and sensory association areas are located.

The red line is the Papez circuit, which represents the neural network that circles like hippocampus-fornix-mamillary body-prethalamic nucleus-zonate gyrus-zonate fascicle-parahippocampus-hippocampus.

Also, nerves extend from various association areas and limbic cortex to the parahippocampal gyrus, From the parahippocampal gyrus -----> into almost all cerebral cortex.


In the Papez circuit, information goes round and round,

When the individual's cerebral cortex determines that information is needed, the necessary memory is stored in the association area.

Otherwise, the information return to?Hippocampus again and repeat the circuit.

If it doesn't need to be stored, the information seems to disappear.


Next, let's talk about the amygdala.

The center of emotion and instinctive behavior

Emotion 1) When you see a certain thing, emotion intuitively determine whether it is profitable or unprofitable.

2) Based on that evaluation, the autonomic nervous system and endocrine system react, stiffen the muscles of the body, and raise the heart rate and blood pressure.

3) These are called emotional expressions . At the same time, it causes feelings of fear and anger.

In other words, subjective emotion and the emotional expression are combined and defined as emotion.

The subjective emotional experience involves the?orbitofrontal cortex and cingulate cortex.

Patients who have had their ?anterior cingulate cortex? removed surgically can see where they have pain and increase their blood pressure and heart rate, but they do not have the strong discomfort associated with pain.

In experiments, animals that destroy theorbitofrontal cortex have reduced aggression and lacked emotional response.

Electrical stimulation of the human amygdala causes fear and anger.?In addition,?patients with bilateral amygdala injuries can recognize the faces of others, but cannot express surprises or fears.?Animals whose bilateral temporal lobes, including the amygdala, have been destroyed do not respond to what they were originally afraid of and approach their enemies without hesitation.

The hypothalamus is involved in emotional expression. The hypothalamus is the center of the autonomic nervous system and endocrine system.

Signals are sent from the amygdala to the hypothalamus, which causes changes in heart rate, blood pressure, breathing, digestive movements, sweating, pupils, and so on.

In the animation below, the red line is output from the amygdala to the hypothalamus, dorsomedial thalamus, brainstem reticular formation, and cranial nerve nuclei, causing emotional expression.

The blue line contains all types of sensory inputs from such as sensory association areas of the cerebral cortex and the olfactory bulb The amygdala also interconnects with the hippocampus, the frontal orbit and the cingulate cortex. ..



下のアニメで、扁桃体には、大脳皮質から種々の感覚入力が入りますIn the animation below, the amygdala receives various sensory inputs from the cerebral cortex.

It seems that the sensory information related to emotions is evaluated by comparing with the memory information from the hippocampus.

Information is sent from the amygdala to the hypothalamus, reticular brainstem, and cranial nerve nucleus.

On the other hand, the output to the cerebral cortex gives rise to subjective feelings.





Also, the limbic system seems?to be interconnected with a structure called the?nucleus accumbens?.

The nucleus accumbens is a region of the cerebrum known as the pleasure center and is associated with sexual stimulation and "high" sensations caused by some illegal drugs.

It is known that a rat with a metal electrode inserted into the nucleus accumbens continues to press the lever that causes electrical stimulation to this site and eventually die due to fatigue without ingesting food or water.we will talk about the nucleus accumben at another opportunity.

Let't talk about left-right difference of limbic system

As for the hippocampus, the neural circuits of the hippocampus appear to be different.?This also seems to affect the way we get memory.?( From the left-right hippocampus difference at the molecular level by Ito Isao)  


My impressions after creating this page

I found it very difficult to see the limbic system anatomically.

It is complicated to understand memories and emotions, and the connection between nerves.

I should make a special page regarding memory.?The amygdala, which integrates instinct and emotion, is also hard to study.

Where is the intention of independence??It seems that it is in the frontal cortex, but it is complex.?

We are not sure yet whether memory is stored at synapses.?we need further study.