Solitary tract

Of the Solitary tract is a central nerve pathway surrounded by the nucleus tractus solitarii. The pathway plays a role above all for the sense of taste and smell, the sensory cells of which transmit signals to the central nervous system via the solitary tract. In the case of lesions in the duct, reflexes such as the gag reflex fail.

What is the solitary tract?

In medicine, a tract is a stretch of tissue or a group of fibers with the same course. The medical literature also uses the literal translation "tract" as a synonym. In neurology, the term refers to nerve tracts, especially nerve tracts in the central nervous system. In this context, each tract corresponds to a duct.

The tractus solitarius is also a conduction path localized in the central nervous system. "Solitarius" means "single" in German. The medical literature uses synonymous terms instead of the designation Fasciculus solitarius and Funiculus solitarius such as Fasciculus rotundus. The literal translation of the Latin "fasciculus" is "small bundle" and indicates the compact anatomy of the duct. The tract lies within the dorsal medulla oblongata, i.e. in the elongated medulla of the brain stem.

Anatomy & structure

The solitary tract runs longitudinally through the laterally posterior section of the medulla oblongata. The conduction pathway is surrounded at this point by the core complex of the associated nerve nuclei Nuclei tractus solitarii. From here the fibers descend to the upper cervical segments in the spinal cord. Different fiber qualities can be found in the pathway of the tract.

The viscera-afferent fibers originate, for example, from cranial nerves such as the facial, glossopharyngeal and vagus nerves. The solitary tract mainly leads to primary afferent fibers that descend to the core areas on the same side. To a small extent there are also mutually ascending fibers in the solitary tract that cross in the caudal section. The nucleus tractus solitarii is arranged directly around the tractus solitarius and corresponds to a heavily myelinated area of ​​nerve fibers.

Function & tasks

The tractus solitarius plays a role as a pathway of the central nervous system for both the sense of taste and smell. In addition, the train conducts various signals from the skin's sensory cells. In this context, the primarily viscerosensory fibers of the conduction path primarily convey signals from the chemo, stretching and pressure receptors. Chemoreceptors are sensory cells that recognize chemical-based substances dissolved in air or liquid.

These receptors play a central role in the sense of smell and taste. Stretch receptors, in turn, correspond to mechanoreceptors and are therefore sensory cells of the skin's senses or deep sensitivity. In addition to the skin and mucous membrane, they are found in the vessels of the human body. They react to a stretching of the surrounding tissue with depolarization and create an action potential through stretching stimuli. Pressure receptors are related to stretch receptors and, with their localization in the vessels, play an important role in the cardiovascular system.

The tractus solitarius conducts the signals of all the aforementioned types of receptors from the head, chest and stomach area. The receptors form the afferent (ascending) limb of many respiratory, cardiovascular, and intestinal reflexes. The conduction path of the solitary trunk is thus essentially involved in vital reflex responses. The gag reflex and the nausea are such reflexes. These automatic reflex responses, which can hardly be influenced at will, take place in response to certain smell or taste stimuli.

The special viscerosensory fibers of the tractus solitarius correspond to primary afferents (ascending pathways) of the sense of taste. These afferents are called taste fibers and convey taste information in the direction of the central nervous system. Specifically through the conduction and distribution of the facial, glossopharyngeal and vagus cranial nerves, the solitary tract conveys important information from the stretching and chemoreceptors from the gastrointestinal tract, including the tongue.

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Diseases

Like any other nerve structure, the solitary tract can be damaged. In some cases, such damage is preceded by a stroke. The solitary tract lesion in this context can be the symptom of Wallenberg's syndrome. This is a neurological clinical picture after occlusion of the inferior cerebellar artery or the vertebral artery.

The consequence of such an occlusion is an infarction of certain parts of the medulla oblongata in the brain stem. This form of stroke is a rather rare variant with a comparatively low prevalence. The symptoms in this case can be varied and depend heavily on the affected structures of the brain stem. If the solitary tract, including the core area of ​​the solitary tract, is affected by the infarction, important reflexes fail. The tractus solitarius conducts the signals from sensory cells of the sense of taste and smell. This line is impaired after a heart attack in the area described.

The signals from the sense of smell and taste play a crucial role in the gagging and vomiting reflexes. Therefore, the Wallenberg syndrome can manifest itself in the sense of a brainstem infarction with involvement of the nucleus tractus solitarii in a complete failure of the gag reflex. The primary cause of this phenomenon is a lack of oxygen in the relevant area. A causal therapy is not available for patients with this type of stroke. Treatment is purely symptomatic.

Stroke relapse prophylaxis is prescribed in the long term. Above all, the patient's risk factors need to be reduced. Not only insufficient oxygen supply, but also inflammation of the conduction pathway can lead to a failure of the gagging and vomiting reflex. Such inflammation can be bacterial inflammation. In individual cases, autoimmune inflammations are also possible. Mechanical damage to the solitary tract or nucleus tratus solitarii is also conceivable, but rather rare.