Solitary tract and 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. Remember these nerves quickly by using the mnemonic "Non Stop Training" (Ninth, Seventh, Tenth).

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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, auditory 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.

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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).

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