(nucleus tractus solitarius) A nucleus (actually a pair of nuclei) in the MEDULLA that receives and integrates taste sensation (see GUSTATION) as well as visceral sensory signals. The nucleus of the solitary tract belongs to a class of integrative BRAINSTEM nuclei which combine together diverse sources of information from several nuclei of the CRANIAL NERVES. The information is integrated to a degree within the nucleus, and then passed on either to other structures within the brainstem that control reflexive responses, or upwards to the forebrain for further processing.
The solitary tract, in which the nucleus of the solitary tract sits, is a band of fibres near the midline at the top of medulla oblongata. The nuclei themselves are positioned like a pair of cigars laid at the top of the medulla, within the solitary tract, running from front to back. The two nuclei almost converge together at the midline at their most caudal points, but angle outwards slightly as they run forward, forming a V shape together. The front section of the nucleus of the solitary tract is sometimes referred to as the gustatory or taste nucleus. Taste sensory neurons project from the tongue and palate to this section of the nucleus of the solitary tract through two cranial nerves. The FACIAL NERVE (seventh cranial nerve) carries fibres from taste receptors located in the front and middle regions of the tongue. The GLOSSOPHARYNGEAL NERVE (ninth cranial nerve) carries fibres from receptors located at the back of the tongue and the palate. Both nerves project to gustatory neurons within the anterior one-third of the nucleus of the solitary tract. Within this nucleus, an array of neurotransmitters are found: glutamate, GABA, ENKEPHALIN, CHOLECYSTOKININ, and NEUROPEPTIDE Y, among others.
Electrophysiological recording studies have shown that neurons in the anterior nucleus code sweet, salty, sour, and bitter taste qualities in a FUZZY SET fashion. A given neuron is likely to respond to more than one taste quality, and often responds to several. Such a degree of broad tuning for gustatory neurons has been interpreted to mean that taste quality, unlike other sensory features such as auditory pitch or visual space, is not precisely coded within the brain according to a labelled-line coding scheme (see LABELLED-LINE THEORY) in which a particular taste would only activate a narrow set of neurons or ‘labelled lines’ corresponding to its taste quality. However, although each gustatory neuron within the nucleus can be activated by many different tastes, each neuron is activated to a higher degree by certain tastes than by others. There exist sucrose-best, salt-best, sour-best and bitter-best classes of neurons within the nucleus of the solitary tract, in the sense that these classes of neurons are activated to the greatest degree by that particular taste quality. This has given rise to the fuzzy set or ACROSS-FIBRE PATTERN THEORY of taste coding, which suggests that it is the particular pattern of firing across all gustatory neurons which determines the taste that is perceived. Although almost all tastes will activate almost all neurons, the pattern of firing across neurons will be slightly different and specific to each taste. This is a considerably more complex type of sensory code than simple labelled lines, and it is not yet clear how forebrain gustatory structures translate the fuzzy set code, although they must do so.
The nucleus of the solitary tract not only codes taste sensory quality, but may also begin to process taste palatability or pleasantness. There are two types of evidence that indicate palatability is recognized to some degree by neurons in or around the nucleus of the solitary tract. One type of evidence is electrophysiological: neurons in the nucleus of the solitary tract that code a sweet taste may alter their firing patterns to that taste if it has been made less palatable, either by making the animal less hungry by prior feeding, or by producing a conditioned aversion to that taste by pairing it associatively with nausea. The second type of evidence is behavioural: DECEREBRATE animals, which lack all FOREBRAIN structures and can only respond reflexively using their brainstem, including the nucleus of the solitary tract, are still able to discriminate in their behavioural responses to basic palatability. Such decerebrates still swallow a sweet taste and lick their lips if the taste is placed on their tongue, but they violently reject a bitter taste. This behavioural discrimination indicates that neurons within the brainstem, including those within the nucleus of the solitary tract, are still able to decode acceptable from unacceptable taste qualities, and to make a rudimentary decision to respond appropriately.
The caudal parts of the nucleus of the solitary tract receive sensory inputs via the VAGUS NERVE from the visceral organs: intestines, liver, kidneys, heart and so on. For example, illness that produces gastrointestinal distress can activate neurons in the middle portion of the nucleus of the solitary tract. Perhaps most important among the physiological functions of the nucleus, however, are the control of basic functions such as heartbeat and breathing. The respiratory rhythm of ordinary breathing is controlled to a large degree by neural circuits originating in the nucleus of the solitary tract. Lesions of the caudal portion of the nucleus are typically fatal in consequence because they disrupt the rhythm and produce suffocation.
KENT C.BERRIDGE
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