The nucleus tractus solitarii is the neural nucleus of taste in humans and is located in the rhomboid fossa in the brain stem. Its nerve fibers connect the brain to the taste buds of the tongue as well as to the vagus nerve. Damage to the nucleus tractus solitarii – for example, from space-occupying lesions, traumatic damage, or circulatory problems – can cause taste disorders.
What is the nucleus tractus solitarii?
The nucleus tractus solitarii (NTS) or nucleus solitarius is a neural processing center in the brain. The nucleus interconnects nerve fibers from the tongue and in this way contributes to gustatory perception. Only at a higher processing level does this become the conscious perception of a particular taste; this step takes place in the cortex, where the signals from the taste nucleus ultimately also reach. The nucleus tractus solitarii belongs to the cranial nerve nuclei, since it forms a junction where cranial nerves end or begin. It belongs to the group of general and specifically viscerosensitive nuclei; unlike other central brain anatomical structures, it carries both types of fibers.
Anatomy and structure
The nucleus tractus solitarii is located in the medulla oblongata (medulla oblongata), which connects the spinal cord to other parts of the central nervous system. The spinal cord and medulla oblongata are not sharply demarcated from each other, but merge smoothly. Within the medulla oblongata, the nucleus tractus solitarii begins at the rhomboid fossa, which forms the floor of the fourth ventricle of the brain. From there, the NTS extends to the pyramidal tract junction (decussatio motoria or decussatio pyramidum), where nerve tracts originating from the motor cortex cross. Three different nerves pass through the nucleus tractus solitarii: the glossopharyngeal nerve (9th cranial nerve), the facial nerve or nervus facialis (7th cranial nerve), and the vagus nerve (10th cranial nerve or nerve X). According to these areas, physiology also divides the nucleus tractus solitarii into three rough regions, which are often just called caudal, medial and rostral NTS according to their location. The only exception to this is the rostral portion, which also goes by the names nucleus gustatorius, nucleus ovalis, or pars gustatoria.
Function and tasks
In the processing of gustatory information, the nucleus tractus solitarii plays an important role. The sense of taste belongs to the chemical senses: Receptors on the tongue respond to substances that come into contact with them. The sensory cells then generate an electrical impulse that travels along the nerve fiber as an action potential. These signals travel via various nerves to the brain, where all three converge in the nucleus tractus solitarii. The glossopharyngeal nerve has the task of collecting information from the posterior region of the tongue. To enable it to receive all nerve signals, it divides into three main branches and several smaller branches. Via the facial nerve, the nucleus tractus solitarii also receives information from the anterior region of the tongue. Sensory fibers in the nerve are responsible for this task. However, the functions of the facial nerve are even more extensive and also play a role in hearing, temperature, pain and pressure sensation in the face. The facial nerve also connects the lacrimal gland and oral salivary gland to the central nervous system. The vagus nerve is also not limited to innervating the taste buds. Instead, it combines various viscerosensory, somatosensory as well as visceromotor signals from extensive regions of the body. The vagus nerve covers areas of the head, neck, abdomen, and chest, forming numerous, progressively finer branches. The nucleus tractus solitarii does not represent the only cranial nerve nucleus relevant to the vagus nerve; its fibers also lead to the nucleus spinalis nervi trigemini, nucleus dorsalis nervi vagi, and nucleus ambiguus.
Diseases
The nucleus tractus solitarii may contribute to the development of various taste or gustatory disorders. However, causes of such perceptual disorders are in principle possible anywhere in gustatory processing. Damage to the nucleus tractus solitarii may be due, for example, to circulatory disturbances (stroke, increased incranial pressure, etc. ), space-occupying tumors, craniocerebral trauma, congenital malformations, and neurodegenerative diseases.If the lesion is not located directly at the nucleus tractus solitarii but at all, some or one of the nerves, the nucleus receives no, incomplete or incorrect information and accordingly operates on the basis of faulty signals; the NTS itself, however, may be unharmed. The taste disorder that manifests and whether other symptoms occur depends on the affected areas of the brain in each individual case. For example, if damage to the facial nerve is the cause of the taste disorder, facial paralysis often manifests as well. Medicine distinguishes between quantitative and qualitative taste disorders. Individuals suffering from total ageusia can no longer perceive any taste; a special form occurs in the context of post-traumatic anosmia-ageusia syndrome, which also affects the sense of smell and is due to craniocerebral trauma. Hypogeusia limits gustatory perception but does not eliminate it completely. Partial hypogeusia is when affected individuals still perceive certain tastes with normal intensity; this is the case, for example, when only certain sections of nerve fibers are damaged but information processing is otherwise correct. In contrast, people with hypergeusia suffer from a pathological increase in the sense of taste. A qualitative taste disorder is phantogeusia, which is characterized by gustatory perceptions for which there is no actual stimulus. In contrast, an interchange of tastes occurs in parageusia. Qualitative taste disturbances need not be permanent but may be transient.
