The tractus solitarius is a central nerve guidance pathway with a surrounded nucleus tractus solitarii. The conduction pathway plays a role primarily in the senses of taste and smell, whose sensory cells transmit signals to the central nervous system via the tractus solitarius. Reflexes such as the gag reflex fail in lesions of the conduction pathway.
What is the tractus solitarius?
In medicine, a tract is a tissue tract or group of fibers with the same course. As a synonym, medical literature also uses the literal translation “tract.” In neurology, the term refers to nerve tracts, so especially nerve tracts in the central nervous system. In this context, each tract corresponds to a conduction pathway. The tractus solitarius is also a conduction pathway with localization in the central nervous system. “Solitarius” means “solitary” in German. The medical literature also uses the synonymous names fasciculus solitarius and funiculus solitarius as well as fasciculus rotundus instead of the name. The literal translation of the Latin “fasciculus” is “small bundle,” suggesting the compact anatomy of the conduction tract. The tract lies within the dorsal medulla oblongata, that is, in the medulla oblongata of the brainstem.
Anatomy and structure
The tractus solitarius passes longitudinally through the laterally posterior portion of the medulla oblongata. Surrounding the tract at this point is the nucleus 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 are found in the conduction pathway of the tract. For example, visceroafferent fibers originate from cranial nerves such as the facial, glossopharyngeal, and vagus nerves. The tractus solitarius carries primarily primary afferent fibers descending to the equilateral nuclear areas. To a lesser extent, mutually ascending fibers are also found in the tractus solitarius, crossing in the caudal segment. The nucleus tractus solitarii is located directly around the tractus solitarius and corresponds to a highly myelinated area of nerve fibers.
Function and tasks
The tractus solitarius plays a role as a conduction pathway of the central nervous system for both the senses of taste and smell. In addition, the tract conducts various signals from the skin sensory cells. In this context, the primarily viscerosensory fibers of the pathway mediate signals primarily from the chemoreceptors, stretch receptors, and pressure receptors. Chemoreceptors are sensory cells that detect chemically 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 thus sensory cells of the skin sense or deep sensibility. In addition to the skin and mucous membrane, they are located in the vessels of the human body. They respond to stretching of the surrounding tissue with depolarization and form an action potential as a result of stretching stimuli. Pressure receptors are related to stretch receptors and, with localization in the vessels, play an important role in the cardiovascular system. The tractus solitarius conducts the signals of all mentioned receptor types from the head, chest and abdominal regions. The receptors form the afferent (ascending) limb of many respiratory, cardiovascular, and intestinal reflexes. Thus, the truncus solitarius pathway is significantly involved in vital reflex responses. The gag reflex and the emetic reflex are such reflexes. These automatic reflex responses, which can hardly be influenced voluntarily, occur in response to specific odor 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 toward the central nervous system. Specifically, by directing and distributing the facial, glossopharyngeal, and vagus cranial nerves, the tractus solitarius conveys important information from the stretch and chemoreceptors from the gastrointestinal tract, including the tongue.
Diseases
Like any other nerve structure, the tractus solitarius can be damaged. In some cases, such damage is preceded by a stroke. In this context, the lesion of the tractus solitarius may be a symptom of Wallenberg syndrome.This is a neurological clinical picture following occlusion of the inferior posterior cerebellar artery or 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 multifaceted and depend strongly on the affected structures of the brainstem. If the tractus solitarius including the nucleus tractus solitarii is affected by the infarction, important reflexes fail. The tractus solitarius conducts signals from sensory cells of the sense of taste and smell. After infarction of the described area, this conduction is impaired. The signals from the senses of smell and taste play a crucial role in the gag and vomit reflex. Therefore, Wallenberg syndrome in the sense of brainstem infarction involving the nucleus tractus solitarii may manifest in a complete failure of the gag and vomit reflex. The primary cause of this phenomenon is an oxygen deficiency of the corresponding area. Causative therapy is not available for patients with this form of stroke. Treatment is purely symptomatic. Stroke recurrence prophylaxis is prescribed for the long term. Above all, the patients’ risk factors must be reduced. Not only oxygen deficiency, but also inflammation of the conduction pathway can cause failure of the gag and vomiting reflex. Such inflammations can be bacterial inflammations. In individual cases, autoimmunological inflammations are also a possibility. Mechanical damage to the tractus solitarius or nucleus tratus solitarii is also conceivable but rather rare.
