Solitary nucleus

The solitary nucleus (also called nucleus of the solitary tract, nucleus solitarius, or nucleus tractus solitarii (SN or NTS)) is a series of sensory nuclei (clusters of nerve cell bodies) forming a vertical column of grey matter in the medulla oblongata of the brainstem. It receives general visceral and/or special visceral inputs from the facial nerve (CN VII), glossopharyngeal nerve (CN IX) and vagus nerve (CN X); it receives and relays stimuli related to taste and visceral sensation. It sends outputs to various parts of the brain, such as the hypothalamus, thalamus, and reticular formation. Neuron cell bodies of the SN are roughly somatotopically arranged along its length according to function.

Neuroanatomy
The nucleus solitarius is a series of purely sensory nuclei forming a vertical column of grey matter embedded within the medulla oblongata. Through the center of the SN runs the solitary tract, a white bundle of nerve fibers, including fibers from the cranial nerves VII, IX, and X.

Cell bodies of the SN are roughly somatotopically arranged along its length according to function: for instance, cells involved in taste are located in the rostral part, while those receiving information from cardio-respiratory and gastrointestinal processes are found in the caudal part.

The SN projects to - among other regions - the reticular formation, parasympathetic preganglionic neurons, hypothalamus and thalamus, forming circuits that contribute to autonomic regulation.

Afferents

 * Gustatory (taste) sensation from the facial nerve (CN VII) via the chorda tympani (from anterior 2/3 of the tongue), glossopharyngeal nerve (CN IX) (from posterior 1/3) and vagus nerve (CN X) (from small area on the epiglottis).
 * Chemoreceptors and mechanoreceptors of the general visceral afferent pathway (GVA) from the carotid body and carotid sinus via (the carotid sinus nerve of) the glossopharyngeal nerve (CN IX), and from aortic bodies and sinoatrial node via the vagus nerve (CN X).
 * Chemoreceptors and mechanoreceptors of the general visceral afferent pathway (GVA) with endings located in the heart, lungs, airways, gastrointestinal system, pharynx, and liver via the glossopharyngeal and vagus nerves. Organ specific regions of neuronal architecture are preserved in the solitary nucleus. Additional minor GVA input from the nasal cavity, soft palate and sinus cavities enters via the facial nerve.

Efferents
The solitary nucleus projects to a large number of other regions of the brain including the paraventricular nucleus of the hypothalamus, the central nucleus of the amygdala, as well as other nuclei in the brainstem (such as the parabrachial area, locus coeruleus, dorsal raphe nucleus, and other visceral motor or respiratory networks).

The stimuli from the SN to the parabrachial area originate in the oral cavity and gastrointestinal tract. The pathways for gastric and gustatory (taste) processes are believed to terminate in different subdivisions of the parabrachial area, but still interact in the SN. Some neuronal subpopulations in the SN, such as the noradrenergic cell group A2 and the aldosterone-sensitive HSD2 neurons project as far ventral as the bed nucleus of the stria terminalis.

Function
Afferents of the SN mediate the gag reflex, the carotid sinus reflex, the aortic reflex, the cough reflex, the baroreflex and chemoreceptor reflexes, several respiratory reflexes and reflexes within the gastrointestinal system regulating motility and secretion.

Neurons which transmit signals about the gut wall, the stretch of the lungs, and the dryness of mucous membranes also innervate the SN. The first central neurons within the SN can participate in simple autonomic reflexes.