Olfactory pathways in the brain. Olfactory nerve: symptoms and signs

This structure contains three types of cells: mitral, tufted, and interneurons (granule cells, periglomerular cells) (Fig. 37.6). The long branching dendrites of the mitral and fascicular cells form the postsynaptic components of these glomeruli (glomeruli). Olfactory afferent fibers (running from the olfactory mucosa to the olfactory bulb) branch near the olfactory glomeruli and terminate in synapses on the dendrites of the same cells. At the same time, olfactory axons converge significantly on the dendrites of mitral cells: each of them contains up to 1000 synapses of afferent fibers. Granule cells (granular cells) and periglomerular cells are inhibitory interneurons. They form reciprocal dendrodendritic synapses with mitral cells. When the latter are activated, the interneurons in contact with it depolarize. As a consequence, an inhibitory neurotransmitter is released at their synapses on mitral cells. The olfactory bulb receives inputs not only through the ipsilateral olfactory nerves, but also through the contralateral olfactory tract running in the anterior commissure (commissure).

The axons of the mitral and fascicular cells leave the olfactory bulb and enter the olfactory tract (Fig. 37.6; Fig. 37.7). Starting from this site, olfactory connections become much more complicated. The olfactory tract passes through the anterior olfactory nucleus. The neurons of this nucleus receive synaptic connections from neurons in the olfactory bulb and project through the anterior commissure to the contralateral olfactory bulb. Approaching the anterior perforated substance at the base of the brain, the olfactory tract is divided into the lateral and medial olfactory strips. Lateral axons terminate in synapses in the primary olfactory region, including the prepiriform (prepiriform) region of the cortex (and in animals, the piriform (piriform) lobe). The medial olfactory strip gives projections to the amygdala and cortex of the basal forebrain (Fig. 37.7).

It should be noted that the olfactory pathway is the only sensory system without a mandatory synaptic switch in the thalamus. Probably, its absence reflects the phylogenetic antiquity and the relative primitiveness of the olfactory system. However, olfactory information still enters the posteromedial nucleus of the thalamus and from there is directed to the prefrontal cortex and orbitofrontal cortex.

In a standard neurological examination, an olfaction test is usually not performed. However, the perception of odors can be tested by asking the subject to smell and identify the odorous substance. At the same time, one nostril is examined, the other must be closed. In this case, strong incentives such as

Olfactory nerve (olfactory nerves) (lat. nerviolfactorii) - the first of, responsible for olfactory sensitivity.

Anatomy

Olfactory nerves are nerves of special sensitivity - olfactory. They originate from olfactory neurosensory cells that form first olfactory pathway and lying in the olfactory region of the nasal mucosa. In the form of 15-20 thin nerve trunks (olfactory threads), consisting of unmyelinated nerve fibers, they, without forming a common trunk of the olfactory nerve, penetrate through the horizontal plate of the ethmoid bone (lat. lamina cribrosa os ethmoidale) into the cranial cavity, where they enter the olfactory bulb (lat. bulbus olfactorius) (here lies body of the second neuron), passing into the olfactory tract (lat. tractus olfactorius), which is the axons of cells lying in (lat. bulbus olfactorius). The olfactory tract passes into the olfactory triangle (lat.). The latter consists mainly of nerve cells and is divided into two olfactory strips that enter into the anterior perforated substance (lat. ), lat. area subcallosa and a transparent partition (lat. septum pellucidum), where are bodies of third neurons. Then the cell fibers of these formations in various ways reach the cortical end, which lies in the region of the hook (lat. uncus) and parahippocampal lat. gyrus parahypocampalis temporal lobe of the cerebral hemispheres.

Olfactory nerves - nerves of special sensitivity.

The olfactory system begins with the olfactory part of the nasal mucosa (the region of the upper nasal passage and the upper part of the nasal septum). It contains the bodies of the first neurons. These cells are bipolar.

As noted above, the olfactory analyzer is a three-neuron circuit:

1. The bodies of the first neurons are represented by bipolar cells located in the nasal mucosa. Their dendrites terminate on the surface of the nasal mucosa and form the olfactory receptor apparatus. The axons of these cells in the form of olfactory threads end on the bodies of the second neurons, morphologically located in the olfactory bulbs.
2. The axons of the second neurons form the olfactory tracts, which terminate on the bodies of the third neurons in the anterior perforated substance (lat. substantia perforata anterior), lat. area subcallosa and a transparent partition (lat. septum pellucidum)
3. The bodies of third neurons are also called primary olfactory centers. It is important to note that the primary olfactory centers are connected to the cortical territories of both their own and the opposite side; the transition of part of the fibers to the other side occurs through the anterior commissure (lat. comissura anterior). In addition, it provides a link to the limbic system. The axons of the third neurons are sent to the anterior sections of the parahippocampal gyrus, where the Brodmann cytoarchitectonic field 28 is located. In this area of ​​\u200b\u200bthe cortex, projection fields and an associative zone are presented.

An appetizing odor causes salivation at the same time, while an unpleasant odor leads to nausea and vomiting. These reactions are associated with . Smells can be pleasant or unpleasant. The main fibers that provide communication between the olfactory system and the autonomous areas of the brain are the fibers of the medial bundles of the forebrain and the brain strips of the thalamus.

The medial forebrain bundle consists of fibers that ascend from the basal olfactory region, the perimyndala, and the septal nuclei. On its way through some of the fibers, it ends at the nuclei of the hypotuberous region. Most of the fibers are sent to and make contact with the vegetative zones, with salivary and dorsal lat nuclei. n.intermedius (Wrisberg's nerve), glossopharyngeal (lat. n. glossopharyngeus) and wandering (lat. n.vagus) nerves.

The brain strips of the thalamus give synapses to the nuclei of the leash. From these nuclei to the interpeduncular nucleus (Ganser's node) and to the nuclei of the tire goes leash-leg path, and from them the fibers are sent to the autonomic centers of the reticular formation of the brain stem.

The fibers that connect the olfactory system with the optic thalamus, hypothalamus, and limbic system are likely to provide emotional accompaniment of olfactory stimuli. The area of ​​the septum, in addition to other brain areas, is connected through associative fibers with the cingulate gyrus (lat. gyrus cinguli).

Clinic of defeat

Anosmia and hyposmia

Anosmia (lack of smell) or hyposmia (decrease in smell) on both sides is more often observed in diseases of the nasal mucosa. Hyposmia or anosmia on one side is usually a sign of a serious illness.

Possible causes of anosmia:

1. Underdevelopment of the olfactory pathways.
2. Diseases of the olfactory nasal mucosa (rhinitis, nasal tumors, etc.).
3. Rupture of the olfactory filaments in a fracture of the lamina cribrosa of the ethmoid bone due to a craniocerebral injury.
4. Destruction of the olfactory bulbs and tracts in the focus of contusion by the type of counterblow, observed when falling on the back of the head
5. Inflammation of the sinuses of the ethmoid bone (lat. os ethmoidale, inflammation of the adjacent pia mater and surrounding areas.
6. Median tumors or other volumetric formations of the anterior cranial fossa.

It should be noted that the interruption of the integrity of the pathways from the primary olfactory centers does not lead to anosmia, since they are bilateral.

Hyperosmia

Hyperosmia - an increased sense of smell is noted in some forms of hysteria and sometimes in cocaine.

Parosmia

A perverted sense of smell is observed in some cases of schizophrenia, damage to the hook of the parahippocampal gyrus, and in hysteria. Parosmia can be attributed to obtaining gasoline and other technical liquids that are pleasant from the smell in patients with iron deficiency.

Olfactory hallucinations

Olfactory hallucinations are observed in some psychoses. They can be an aura of an epileptic seizure, which are caused by the presence of a pathological focus in the temporal lobe.

Also

The olfactory nerve can serve as an entry gate for brain and meningeal infections. The patient may not be aware of the loss of smell. Instead, in connection with the disappearance of the sense of smell, he may complain of a violation of taste sensations, since the perception of smells is very important for the formation of the taste of food (there is a connection between the olfactory system and lat. nucleus tractus solitary).

Research methodology

The state of smell is characterized by the ability to perceive odors of varying intensity by each half of the nose separately and to identify (recognize) various odors. With calm breathing and closed eyes, the wing of the nose is pressed with a finger on one side and the odorous substance is gradually approached to the other nostril. It is better to use familiar non-irritating odors (volatile oils): laundry soap, rose water (or cologne), bitter almond water (or valerian drops), camphor. The use of irritating substances, such as ammonia or vinegar, should be avoided, as this simultaneously causes irritation of the endings of the trigeminal nerve (lat. n.trigeminus). It is noted whether odors are correctly identified. In this case, it is necessary to keep in mind whether the nasal passages are free or there are catarrhal phenomena from them. Although the subject may be unable to name the test substance, the mere awareness of the presence of the odor rules out anosmia.

The sense of smell is one of the first sensations that a baby has. It begins with the knowledge of the world around and oneself. The taste that a person feels while eating is also a merit of smell, and not of the tongue, as it seemed before. Even the classics claimed that our sense of smell is able to help in a difficult situation. As J. R. R. Tolkien wrote: “When you are lost, always go where it smells best.”

Anatomy

The olfactory nerve belongs to the group of cranial, as well as nerves of special sensitivity. It originates on the upper mucosa and processes of neurosensory cells form the first neuron of the olfactory tract there.

Fifteen to twenty unmyelinated fibers enter the cranial cavity through the horizontal plate of the ethmoid bone. There they combine to form the olfactory bulb, which is the second neuron of the pathway. Long nerve processes emerge from the bulb, which go to the olfactory triangle. Then they are divided into two parts and immersed in the anterior perforated plate and transparent septum. There are the third neurons of the path.

After the third neuron, the tract goes to the cerebral cortex, namely to the area of ​​the hook, to The olfactory nerve ends in this area. Its anatomy is quite simple, which allows doctors to identify violations in different areas and eliminate them.

Functions

The very name of the structure indicates what it is intended for. The functions of the olfactory nerve are to capture the smell and decipher it. They cause appetite and salivation if the aroma is pleasant, or, on the contrary, provoke nausea and vomiting when the amber leaves much to be desired.

In order to achieve this effect, the olfactory nerve passes through and travels to the brainstem. There, the fibers connect with the nuclei of the intermediate, glossopharyngeal and vagus nerves. In this area are also the nuclei of the olfactory nerve.

It is a known fact that certain smells evoke certain emotions in us. So, in order to provide such a reaction, the fibers of the olfactory nerve communicate with the subcortical visual analyzer, the hypothalamus and the limbic system.

Anosmia

"Anosmia" translates as "lack of smell". If such a condition is observed on both sides, then this testifies in favor of damage to the nasal mucosa (rhinitis, sinusitis, polyps) and, as a rule, does not threaten any serious consequences. But with a one-sided loss of smell, it is necessary to think about the fact that the olfactory nerve can be affected.

The causes of the disease can be an underdeveloped olfactory tract or fractures of the bones of the skull, for example, the cribriform plate. The course of the olfactory nerve is generally closely related to the bone structures of the skull. Fragments of bone after a fracture of the nose, upper jaw, and orbit can also damage the fibers. Damage to the olfactory bulbs is also possible due to bruising of the substance of the brain, when falling on the back of the head.

Inflammatory diseases such as ethmoiditis, in advanced cases, melt and damage the olfactory nerve.

Hyposmia and hyperosmia

Hyposmia is a decrease in the sense of smell. It can occur due to the same reasons as anosmia:

• thickening of the nasal mucosa;
• inflammatory diseases;
• neoplasms;
• injuries.

Sometimes this is the only sign of an aneurysm of cerebral vessels or tumors of the anterior cranial fossa.

Hyperosmia (increased or heightened sense of smell) is noted in emotionally labile people, as well as in some forms of hysteria. Hypersensitivity to odors is seen in people who inhale drugs such as cocaine. Sometimes hyperosmia is due to the fact that the innervation of the olfactory nerve extends to a large area of ​​the nasal mucosa. Such people, most often, become workers in the perfume industry.

Parosmia: olfactory hallucinations

Parosmia is a perverted sense of smell that normally occurs during pregnancy. Pathological parosmia is sometimes observed in schizophrenia, damage to the subcortical centers of smell (parahippocampal gyrus and hook), and hysteria. Patients with iron deficiency anemia have similar symptoms: pleasure from the smell of gasoline, paint, wet asphalt, chalk.

Damage to the olfactory nerve in the temporal lobe causes a specific aura before epileptic seizures and causes hallucinations in psychoses.

Research methodology

In order to determine the state of smell in a patient, a neuropathologist conducts special tests for the recognition of various odors. Indicator aromas should not be too harsh, so as not to disturb the purity of the experiment. The patient is asked to calm down, close his eyes and press his nostril with his finger. After that, a smelling substance is gradually brought to the second nostril. It is recommended to use odors familiar to humans, but at the same time avoid ammonia, vinegar, since when they are inhaled, in addition to the olfactory, the trigeminal nerve is also irritated.

The doctor records the test results and interprets them relative to the norm. Even if the patient cannot name the substance, the mere fact of smelling excludes nerve damage.

Brain tumors and the sense of smell

With brain tumors of various localizations, hematomas, impaired outflow of cerebrospinal fluid and other processes that compress the substance of the brain or press it against the bone formations of the skull. In this case, one- or two-sided violation of the sense of smell may develop. The doctor should remember that they intersect, therefore, even if the lesion is localized on the one hand, hyposmia will be bilateral.

The defeat of the olfactory nerve is an integral part of the craniobasal syndrome. It is characterized not only by compression of the medulla, but also by its ischemia. Patients develop pathology of the first six pairs Symptoms can be uneven, there are various combinations.

Treatment

Pathologies of the olfactory nerve in its first section occur most often in the autumn-winter period, when there is a massive incidence of acute respiratory infections and influenza. Prolonged course of the disease can cause a complete loss of smell. Recovery of nerve function takes from ten months to a year. All this time it is necessary to carry out course treatment to stimulate regenerative processes.

In the acute period, the ENT prescribes physiotherapy treatment:

• nose and maxillary sinuses;
• ultraviolet irradiation of the nasal mucosa, with a capacity of 2-3 biodoses;
• magnetic therapy of the wings of the nose and sinuses of the upper jaw;
• infrared radiation with a frequency of 50-80 Hz.

You can combine the first two methods and the last two. This speeds up the recovery of lost functions. After clinical recovery, the following physiotherapy treatment is also carried out for rehabilitation:

• electrophoresis with the use of drugs "No-shpa", "Prozerin", as well as nicotinic acid or lidase;
• ultraphonophoresis of the nose and maxillary sinuses for ten minutes daily;
• irradiation with a red laser spectrum;
• endonasal electrical stimulation.

Each course of therapy is carried out up to ten days with interruptions of fifteen to twenty days until the function of the olfactory nerve is fully restored.

(tractus olfactorius, PNA, BNA, JNA)

part of the olfactory brain in the form of a thin cord located on the lower surface of the frontal lobe of the cerebral hemisphere between the olfactory bulb and the olfactory triangle.

• - the way, the direction of the cargo or postal forwarding ...

  Reference commercial dictionary

• Medical Encyclopedia

• - a movable filamentous structure extending from the olfactory club ...

  Medical Encyclopedia

• - a set of terminal branches of olfactory filaments and dendrites of mitral cells in the olfactory bulb ...

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• - see Olfactory mace...

  Medical Encyclopedia

• - a paired protrusion of the telencephalon of the embryo, which is the rudiment of the olfactory tract ...

  Medical Encyclopedia

• - part of the olfactory brain, which is an extension of the olfactory tract in its posterior section on the border with the anterior perforated substance ...

  Medical Encyclopedia

• - a bundle of nerve fibers connecting the olfactory tract and the olfactory triangle with the nuclei of the hypothalamus, mastoid bodies, the interpeduncular nucleus and the reticular formation of the midbrain ...

  Medical Encyclopedia

• - I 1) in Russia, an improved dirt road connecting important settlements. It had stations and milestones. There were regular transportations of passengers, cargo and mail along the T. ...

  Great Soviet Encyclopedia

• - improved dirt road connecting important settlements; had stations and milestones. Regular transportation of passengers, cargo and mail was carried along the route. Since the 19th century a paved tract is called a highway...

  Big encyclopedic dictionary

• - ; pl. tra/kty, R....

  Spelling Dictionary of the Russian Language

• - lat. big road, torn, driven way, postal road, established. Traktovye, tractovye coachmen...

  Dahl's Explanatory Dictionary

• - -and husband. 1. Large well-trodden road. Postal v. 2. Devices, structures that form the path of something. . T. connection. T. sound transmission...

  Explanatory dictionary of Ozhegov

• - tract, husband. . 1. Big road. Postal route. 2. Direction, route. The gastrointestinal tract is the digestive system. Direct route - direct communication, direct ...

  Explanatory Dictionary of Ushakov

• - tract I m. obsolete. Big paved road...

  Explanatory Dictionary of Efremova

• - oh, oh. Servant for smell...

  Small Academic Dictionary

"Olfactory tract" in books

Gastrointestinal tract

Gastrointestinal tract