Rhinencephalon: what it is, parts and its functions in the brain

The functioning of the human brain is based on four main blocks: the neocortex, in charge of motor activity, planning or attention; the basal ganglia, responsible for movement control; the reticular formation, in charge of vital functions such as sleep, wakefulness or cardiovascular control; and the limbic system, formerly called the rhinencephalon, emotional management center and the olfactory system.

Let's see what exactly the rhinencephalon is, and what brain functions are attributed to it.

• Related article: "Parts of the human brain (and functions)"

What is the rhinencephalon?

The rhinencephalon is now known as the part of the brain related to smell and included within the limbic system or “emotional brain”.

With the increase in the differentiation of the other senses in vertebrates, smell has been losing prominence. However, the fundamental connections of the olfactory mechanism with both visceral and somatic activities are as important in humans as in other mammals, although olfactory sensitivity is minor.

The rhinencephalon, being interconnected with the hypothalamus (center in charge of homeostasis or organic balance) and with several areas of emotional control, helps us to concentrate all the information that comes from the outside and that we translate into processes through which we perceive, learn, act and remember.

We must not forget that the sense of smell is up to 10 thousand times more sensitive than any other of our senses, and that the olfactory response is immediate and extends directly to the brain. It is also estimated that we have 100 million receptors for the olfactory sensation.

It is believed that **human beings can capture up to 7 primary odors: camphor, musk, flowers, mint, ether, pungent and rotten, odors that would correspond to the seven types of receptors existing in the olfactory mucosa. However, research data from recent years indicates that there are at least a hundred primary sensations of smell.

Location in the brain

In humans, we can locate the rhinencephalon in the lower lateral part of the brain, just between the eyes, and above it is the frontal lobe.

In the upper portion of the nasal cavity there is a small area called the olfactory epithelium. This zone contains between 10 and 100 million olfactory receptors, and each one of them is in actually a neuron that has olfactory cilia sensitive to the chemical stimuli of substances odorants.

These neurons emit processes that come together in groups (called axons) and that form two nerves known as olfactory nerves. These are directed towards the brain and end their journey in an area of ​​gray matter called the olfactory bulb. The information will travel a path from the bulb, through the olfactory tract, until reaching the primary olfactory area of ​​the cerebral cortex.

It is in the cortex where we finally consciously perceive a certain odor, and from this brain area the nervous pathways that communicate with other systems such as the limbic or hypothalamus, areas responsible for many of the emotional responses associated with smells or memories olfactory.

Parts of this nervous system structure

The bodies of these neurons emit processes called axons that come together in groups and are grouped together to form two nerves called olfactory nerves. These approach the brain and end in an accumulation of gray matter called the olfactory bulb.

The captured information passes through the bulb and reaches other structures in the brain. limbic system, a network of interconnected structures located near the medial part of the brain. The main structures of this system, of which the rhinencephalon is a part, are the following:

1. hypothalamus

This brain structure, located in the diencephalon, has a fundamental role in the regulation of vital functions such as body temperature, hunger and thirst, sexual impulses or the regulation of mood.

All this is achieved by acting as a bridge between the cerberus and the endocrine system, releasing torrents of hormones and playing a coordinating role in the autonomic nervous system.

2. Hippocampus

It is a small curved and elongated structure, located in the inner part of the temporal lobe, very close to the thalamus and the tonsils. It is involved in the regulation of mental processes such as memory, memory retrieval, spatial location or orientation.

Although at first it was thought that this structure was directly involved in smell as a central component of the rhinencephalon, today its involvement in the memory of odors is more discussed.

3. Amygdala

The tonsils are located next to the hippocampus and, therefore, we have one in each cerebral hemisphere. Its function is related to the learned emotional response that elicit certain contexts and situations, and therefore also with emotional learning.

This structure is, then, one of the main control centers of emotions such as fear or anger. Hence it is an important structure for understanding how odors can alter our mood, awakening emotions or evoking memories.

4. orbitofrontal cortex

At the limits of the limbic system we can find the orbitofrontal cortex, the escape valve for emotional orders to areas of the frontal lobe Responsible for strategy generation and planning.

Therefore, it has a very prominent role in appeasing the most primitive and "irrational" impulses that come directly from the limbic system. and act as a filter for some signals, allowing only those that serve to achieve the objectives defined in the medium or long-term goals to pass through. term.

5. olfactory bulb

It is a small vesicular structure composed of a pair of bumps located on the olfactory epithelium and below the frontal lobes. It is believed that it is responsible for detecting, differentiating and amplifying odors and our sensitivity to them. This structure also stands out for being an area where there is adult neurogenesis, that is, the generation of new neurons throughout life.

In animals it has an influence on sexual behavior, defensive and aggressive behaviors, as well as on the care of young.

functions

our rhinencephalon converts chemical signals into perception and electrical impulses that travel through our brain in order to fulfill the following vital functions:

• Survival: detection of harmful substances, polluting gases and rotting food.
• Collaboration with the sense of taste in the perception of food flavors.
• Identification of a wide variety of odors (between 5,000 and 10,000).
• Reinforcement of memory through association of smells and memories.
• Distinction of new odors among environmental odors in the background.
• Detection of environmental information.
• Creation of a representation of the smell.
• Detection of a pair for mating (has been lost in humans).
• Detection and orientation to a nutrient source (mainly in non-human animals).
• Evaluation of the state, type and quality of nutrients (in non-human animals).

The olfactory system and memory

Another body structure to take into account when talking about the olfactory system, beyond the importance of the rhinencephalon, is one known to all of us: the nose, the olfactory organ par excellence. If it were not for it, we would be unable to perceive the vast range of odors that our towns and cities emanate day and night. However, odor processing goes beyond this highly visible part of our body.

Olfactory molecules that are dispersed in the environment come into contact with the nasal mucosa and reach the receptors of the olfactory epithelium. Receptive neurons send information to the olfactory bulb by means of electrical impulses. And from there, to the limbic system, in charge of centrifuging our brain, stimulating our system of memories and associated emotions.

When perceiving a specific smell, we look for a relationship with a stored memory and if we don't have it, we make it up. Depending on the type of memory that is evoked, mood can change or remain unchanged. It is what happens to us when we evoke memories of our childhood, of nature, of the perfume of a person we met, etc.

The effect is so powerful that our brain does not distinguish between the real situation and the evoked situation thanks to the olfactory effect. If the perceived smell is not associated with any memory, the experience of that moment will mark whether that instant in the future will be associated with a positive memory, a dangerous situation, a situation sad etc

All this, of course, happens in our brain automatically and unconsciously, without the direct participation of our conscious attention; the rhinencephalon and other associated biological structures take care of this in a discreet way. In many cases, from that moment on, that particular smell or aroma will be the one that motivates our future actions and the one that generates our acceptance or rejection of the circumstances associated with the same.

Bibliographic references:

• Trivino Mosquera, M.; Bembibre Serrano, J.; Arnedo Montoro, M. (2019). Neuropsychology of perception. Madrid: Synthesis.