The taste of sodium.

Sodium is crucial to physiological function. The responsibility for detecting it is assumed by the taste system, which devotes perhaps one quarter of its resources to the task. Sodium is transduced by passage into a subset of receptor cells, whose activity is relayed to the brain through a discrete gustatory channel. Responses in hindbrain, thalamus, and gustatory cortex identify the quality and concentration of sodium on the tongue. Coding of reinforcement may begin with the pons and ventral forebrain, particularly the lateral and medial hypothalamic nuclei. When body stores are sufficient, behavioral preference for sodium is mild, encompassing low concentrations and marked by weak avidity. This languid response disappears during sodium shortages. Avidity increases, and hypertonic concentrations are most preferred. This behavioral change may result from altered responsiveness in sodium-specific neurons that offer the sodium signal access to mechanisms of reinforcement. Thus, the taste system detects and recognizes sodium, and accords it a reward value commensurate with the needs of the animal.