Solitary Tract and Nucleus
The nucleus of the solitary tract, also known as the nucleus tractus solitarius (pl. solitarii) is a pair of cell bodies found in the brainstem. This structure, along with its tract (the solitary tract or tractus solitarius), has far reaching impacts on many homeostatic systems within the body. It has been described by many as the primary visceral sensory relay station within the brain. It receives and responds to stimuli from the respiratory, cardiovascular, and gastrointestinal systems. This article will address the anatomical structure and location of the solitary tract and its nucleus, in addition to its clinical significance.
Gross AnatomyThe nucleus is a Y-shaped structure extending craniocaudally from the inferior border of the facial colliculus (impression of CN VII) to the pyramidal decussation (crossing of the corticospinal fibers to the contralateral side) thus extending throughout the length of medulla. Adjacent to the solitary nucleus is the spinal nucleus of the trigeminal nerve (CN V); while medially it is adjacent to (from rostral to caudal) the walls of the fourth ventricle, the area postrema and the subnucleus commisuralis. The approximate location of the nucleus of the solitary tract can be appreciated on a posteroanterior view of a coronal section through the brainstem (revealing the floor of the fourth ventricle). Other neighbouring structures of the nucleus of the solitary tract include:
- the dorsal motor nucleus of the vagus nerve (CN X), which is located ventral to the nucleus
- the hypoglossal nucleus (CN XII), found medial to the nucleus and rostral to the area postrema
- the fasciculi of nuclei gracilis and cuneatus separate them from the solitary tract nucleus dorsally
- the parasolitary nucleus (nucleus parasolitarius; gamma-aminobutyric acid production) is ventrolateral to the nucleus solitarius
- the vestibular nuclei are dorsal to the nucleus; while the anterior and posterior cochlear nuclei are found ventral and dorsal (respectively) to the same
- also, the parvocellular reticular formation (which is deep to the cuneate tubercle) is dorsolaterally related to the solitary nucleus
The nucleus solitarius has been described as a viscerotopic structure. This means that there is a particular part of the solitary nucleus and tract that is associated with each of the systems it regulates. Research supports the notion that the lateral and caudal aspects of the nucleus receive gustatory (taste) afferents from the epiglottis and larynx indirectly from the vagus nerve (CN X) via its superior laryngeal branch. Sensory input from the subdiaphragmatic parts of the gastrointestinal system (via CN X) is transmitted to the caudal part of the solitary nucleus. Similarly, the entire rostrocaudal aspect of the nucleus (particularly the region rostral to the obex [inferior apex of the fourth ventricle]) receives visceral innervation from the tonsils, tongue, palate, pharynx, and the posterior third of the tongue via the lingual branch of the glossopharyngeal (CN IX) nerve. Chorda tympani and greater petrosal nerves (branches of the facial nerve; CN VII) carry visceral sensation to the rostral part of the solitary nucleus from the roof of the mouth and the anterior two-thirds of the tongue. There is also a medial to lateral arrangement of input coming from the distal intestines, stomach, chemo- and baroreceptors, oesophagus and lungs in the nucleus of the solitary tract.
The solitary tract (tractus solitarius) is formed from fibers of the inferior ganglion of the vagus nerve, geniculate ganglion of the facial nerve, and glossopharyngeal nerve. The tract travels along the lateral aspect of the dorsal nucleus of the vagus nerve before ending the solitary nucleus.
Associated Cranial Nerves
In order to appreciate the function of the nucleus of tractus solitarius and the tract itself, a quick revision of the sensory components of the associated cranial nerves (relevant to this topic) will be conducted. The nucleus tractus solitarius and associated tract receives special and general visceral afferents via the facial (CN VII), glossopharyngeal (CN IX) and vagus (CN X) nerves.
The facial nerve (CN VII) relays general sensory impulses from a small area of skin behind the ear, the lateral surface of the tympanic membrane and the walls of the external acoustic meatus. Special sensory impulses from the anterior two-thirds of the tongue and soft palate are also carried by CN VII. Fibers from the skin behind the ear join the main trunk of the facial nerve before it passes through the stylomastoid foramen. The chorda tympani carry both special sensory (gustatory input from the anterior two-thirds of the tongue) and parasympathetic fibers. The nerve courses deep to the mucous membrane of the medial surface of the tympanic membrane before entering the infratemporal fossa through the petrotympanic fissure of the tympanic part of the temporal bone. Like the chorda tympani, the greater (superior) petrosal nerve also carries special sensory (from the soft palate) and parasympathetic fibers. The greater petrosal nerve is joined by sympathetic fibers of the deep petrosal nerve by way to foramen lacerum. The combined fibers, now called the Vidian nerve of the pterygoid canal continues in the pterygoid canal and reaches the pterygopalatine ganglion in the pterygopalatine fossa. The greater petrosal, chorda tympani and the sensory nerve to the back of the ear all arise from the geniculate ganglion. This ganglion – a product of the nervus intermedius and motor branch of CN VII – houses the cell bodies for the primary sensory fibers of CN VII. The nervus intermedius is ultimately responsible for taking both special and general afferents back to the nucleus tractus solitarius and spinal trigeminal tract, respectively.
Both special and general visceral afferents are conveyed through the glossopharyngeal nerve (CN IX). The pharyngeal, tonsillar and lingual divisions of CN IX are responsible for transmitting special sensory afferent information. They are joined by the carotid sinus branch of Hering, which carries general sensory information from the carotid body and sinus back the CN IX. Baroreceptors (sensory nerve endings in the carotid sinus walls) and chemoreceptors (in the carotid body) relay information about blood pressure and arterial oxygen saturation (respectively) via unipolar cell bodies in the CN IX ganglia. There are two such ganglia associated with CN IX, a large inferior ganglion caudal to the jugular foramen, and a smaller superior ganglion cranial to the jugular foramen. The information is then relayed to the solitary nucleus.
Similarly, general and special visceral afferent data is carried via the vagus nerve (CN X). It primarily transmits the special sensory component from the epiglottis and general sensory fibers from the thoracoabdominal viscera. It receives baroreceptors and chemoreceptor information from the aortic arch and aortic bodies, respectively. Its supply to the gastrointestinal system extends to the splenic flexure of the colon. The visceral sensory cell bodies of CN X are in the inferior (nodose) ganglion, inferior to the jugular foramen. The information is relayed to the caudal part of the solitary nucleus.
Of note, both CN IX and CN X carry somatic afferent information of touch, pain and temperature. CN IX does this for the posterior third of the tongue, Eustachian tube, and the proximal pharynx. The associated cell bodies are found in both CN IX ganglia. CN X is responsible for relaying the same information from the larynx, oesophagus and distal pharynx. Its associated cell bodies are in the superior (jugular) ganglion of the vagus nerve (within the jugular foramen). Both CN IX and CN X eventually relay this information to the spinal trigeminal tract and nucleus. All twelve cranial nerves are covered in detail in other articles on this platform.
Input Integration and Projection
The solitary nucleus is a major relay center in the medulla oblongata. It has reciprocal connections with the spinal trigeminal nucleus, periaqueductal grey region of the dorsal tegmentum, the stria medullaris, and the raphe nuclei. These dual-way connections also extend to higher centres such as the lateral and paraventricular hypothalamic nuclei, the amygdala and the insular and medial prefrontal parts of the cortex. Of note, it is believed that fibers travelling to the hypothalamus are intended to facilitate conscious perception of satiety and hunger. There are also reticulobulbar, reticulospinal and solitariospinal tracts that assist with sympathetic and parasympathetic reflex responses, as well as somatic motor responses to the muscles of respiration.
Stimulation of the solitary tract is achieved by the release of excitatory neurotransmitters (like glutamate) from the mixture of myelinated (type A) and unmyelinated (type C) fibers. The action potentials of the solitary tract are stimulated via synaptic input from the afferent fibers of the solitary tract and circulating hormones. The circulating hormones can access the solitary nucleus via its fenestrated capillaries and semi-permeable blood brain barrier (i.e. the solitary nucleus is a circumventricular organ).
Chemoreceptors and mechanoreceptors of the gastrointestinal tract relay signals of satiety to the solitary nucleus during feeding. Similarly, their absence would convey the converse. These sensory afferent fibers utilize glutamate as its excitatory neurotransmitter to signal fullness to the solitary nucleus. Ionotropic inhibition of glutamate receptors can result in a delay in the sensation of fullness and consequently, result in an increase in the patient’s meal size.
The gustatory (taste) pathway, mediated by CN VII and CN IX, also utilizes GABA and glutamate neurotransmitters in its relay pathway. The relay pathway of the nucleus tractus solitarius and taste sensation involves rostral projection from the solitary nucleus to the pontine gustatory relay area. From there, fibers are sent to the reticular formation, which passes the information on to the motor neurons of CN V (muscles of mastication), CN VII (facial muscles), CN IX (stylopharyngeus to elevate and dilate the pharynx), CN X (laryngeal, pharyngeal and oesophageal muscles) and CN XII (tongue muscles).
Nucleus tractus solitarius is involved in generating and synchronizing the peristaltic activity of the upper gastrointestinal tract during swallowing. The rhythmic release of inhibitory (GABA) and excitatory (glutamate) neurotransmitters is responsible for the sequential motor pattern seen in peristalsis. The data obtained by the solitary nucleus during swallowing is passed on to the nucleus ambiguous, which is responsible for motor innervation to the intrinsic laryngeal muscles, pharynx and oesophagus. A lesion to the solitary tract or nucleus can result in dysphagia (pain or difficulty swallowing).
Vagal excitation of the solitary nucleus via glutamate neurotransmitters may result subsequent to chemical or mechanical irritation of the proximal gastrointestinal tract. The excitatory activity of the neurotransmitters results in receptive relaxation (active relaxation of gastric smooth muscles to accommodate incoming food) of the stomach. Inhibition of gastric motor activity by catecholamines or GABAergic receptors will reduce the vago-vagal response. This would result in an increase in intragastric pressure and early satiety.
The dorsomedial part of the solitary nucleus is the primary convergence site for cardiovascular sensory fibers. Excitation of these neurons will result in a baroreceptor-like response that decreases the blood pressure and heart rate of the patient. Conversely, inhibition of this area of the solitary nucleus will result in an increase in blood pressure and heart rate. Therefore, both glutamate (excitatory) and GABA (inhibitory) neurotransmitters play an integral part in regulation of the cardiovascular system. Glutamate and GABA and glycine (inhibitory) neurotransmitters are also involved in regulation of solitary nucleus activity in the respiratory system. The solitary nucleus receives input from stretch receptors and bronchopulmonary nerve fibers of the lungs and airway. They convey information to the pre-Bötzinger and Bötzinger zones (ventrolateral part of the medulla oblongata), bulbospinal and cranial premotor neurons that participate in generating and coordinating respiratory patterns.