|Brain: Nucleus of the solitary tract|
|The cranial nerve nuclei schematically represented; dorsal view. Motor nuclei in red; sensory in blue.|
|Transverse section of medulla oblongata of human embryo.|
|Latin||tractus solitarius medullae oblongatae|
In the human brain, the solitary nucleus (nucleus of the solitary tract, nucleus solitarius, nucleus tractus solitarii, NTS) is a series of nuclei (clusters of nerve cell bodies) forming a vertical column of grey matter embedded in the medulla oblongata. Through the center of the NTS runs the solitary tract, a white bundle of nerve fibers, including fibers from the facial, glossopharyngeal and vagus nerves that synapse on neurons of the NTS. The NTS projects to, among other regions, the reticular formation, parasympathetic preganglionic neurons, hypothalamus and thalamus, forming circuits that contribute to autonomic regulation. Cells within the NTS are arranged according to function; for instance, cells involved in taste are located in the higher, more forward ("rostral") part, while those regulating cardio-respiratory processes are found in the lower, more posterior ("caudal") part.
The nucleus is located along the length of the medulla (with a small portion in the lower pons). The solitary tract runs in the middle of the nucleus, creating a speck of white matter (axons of the tract), surrounded by grey matter (the nucleus). This stands out on a stained section, which is where the name solitary comes from.
The solitary nucleus is divided into a rostral (towards the top) gustatory nucleus and caudal (towards the bottom) cardiorespiratory nucleus. The cardiorespiratory nucleus can be further divided into a cardiovascular center, which sits at the midline of the nucleus, and a respiratory center, which is located laterally.
Inputs to the solitary nucleus
In addition to afferent taste information from nerves VII, IX and X, the solitary nucleus also handles primary afferent signals from a variety of visceral regions and organs. These afferents include chemoreceptors in the carotid (via IX) and aortic bodies (via X) as well as stretch receptors from the aorta and carotid arteries called arterial baroreceptors. In addition, chemically and mechanically sensitive neurons with endings located in the heart, lungs, airways, gastrointestinal system, liver and other viscera send axons via cranial nerves (IX and X) chiefly that directly enter the brainstem to form synapses within the caudal third of the solitary nucleus. Neurons that synapse in this nucleus mediate the gag reflex, the carotid sinus reflex, the aortic reflex, the cough reflex, the baroreceptor and chemoreceptor reflexes, several respiratory reflexes and reflexes within the gastrointestinal system regulating motility and secretion. Information about the gut wall, as well as stretch of the lungs and dryness of mucous membranes, also synapses at the solitary nucleus. These first central neurons within the solitary nucleus can participate in autonomic reflexes that may be as simple as two central neurons with the second neuron being an efferent or motor neuron that projects back directly to the organ such as the heart forming some of the simplest reflex pathways in the brain.
Outputs from the solitary nucleus
Information goes from the NTS to a large number of other regions of the brain including the paraventricular nucleus of the hypothalamus and the central nucleus of the amygdala, as well as to other nuclei in the brainstem (such as the parabrachial area and other visceral motor or respiratory networks). The signals projected from the NTS 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 NTS. Some neuronal subpopulations in the NTS, such as the noradrenergic A2 neurons and the aldosterone-sensitive HSD2 neurons project as far rostrally as the bed nucleus of the stria terminalis.
Brain: rhombencephalon (hindbrain)
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