Mechanisms for Terminating the Sensation of Thirst

Although undoubtedly decreasing osmolality and increasing extracellular volume promote a reduction in the perception of thirst by reactivating inhibitory neuron activity, usually there is a decrease in the perception of thirst and termination of drinking before circulating osmolality, volume, and hormonal levels have returned to predehydration levels. Although it could be argued that receptors in the brain may be responsible for the early cessation of the perception of thirst, the majority of the evidence suggests that it is receptors in the upper gastrointestinal tract that promote the early termination of drinking. Although the nature and neural connections of these proposed receptors have not been fully characterized, most appear to have an inhibitory response. It has been suggested that because much of the thirst and drinking response is behavioral, an individual learns what volume of drink is required to restore a given water deficit. Termination of drinking therefore could be a learned response that anticipates a future fluid deficit or matches a known current level of dehydration. The stimuli for gauging the current level of dehydration may be the same as that which initiates the sensation of thirst.

The mere presence of liquid, particularly cold liquid, in the mouth reduces the perception of thirst. Receptors in the mouth and oesophagus responding to different chemical, tactile, pressure, and temperature stimuli are thought to be part of the mechanism that influences the relative intensity of the perception of thirst. The neural activity involved in swallowing and perhaps oropharyngeal and gastric receptors are thought to be effective in sensing or metering the volume of liquid ingested. Distension of the stomach tends to inhibit drinking due to increased gastric stretch receptor activity, although this response does not always reduce the perception of thirst. Taste and other psychological factors can have a stimulatory effect on consumption of a drink that is considered to be palatable.

The continuation and termination of the acute sensation of thirst are regulated by a series of stimuli that operate before all of the drink consumed has been absorbed and before disturbances in the body water pools have been corrected. A variety of receptors located from the mouth to the upper part of the small intestine, and probably neural control from the higher centers of the brain, appear to monitor and regulate the initial volume consumed. After absorption, if restoration of body water pools does not occur the sensation of thirst is once again initiated, presumably by the same homeostatic stimuli that initially evoked the feeling of thirst, and drinking restarts. The integration of the pre-and postabsorptive stimuli modulates the sensation of thirst and finally the volume of drink consumed.

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