Negative Plus Positive Feedback
One striking feature of the vertebrate body is its ability to maintain relatively constant many intensive properties of its internal fluid compartments when the body is exposed to stresses which alter the magnitude of these properties. The regulated intensive properties of the internal environment include temperature, intracellular and extracellular concentrations of certain micro-solutes and colloids (those controlled by more than the law of mass action). This ability to regulate these intensive properties depends upon the nervous system. Each intensive property is transduced by a neural receptor specific for that property into action potentials at a rate (and/or sequence) depending on the magnitude of the intensive property. These are conducted to the brainstem, where they are integrated with other relevant neural activities which may include action potentials from (1) receptors transducing the same intensive property of the brainstem, (2) receptors transducing other intensive properties indirectly affecting the regulated intensive property of the internal environment and (3) neural elements which establish the reference of the regulated intensive property of the internal environment. The reference (and its neural correlate) for a regulated intensive property is optimal for each species and evolved with the species. The reference may be adjusted by intensive properties which affect the internal regulated intensive property directly or indirectly, and it may also vary daily and seasonally. When the magnitude of the regulated intensive property differs from its reference magnitude, the difference between their corresponding neural activities activates neurons, which in turn activate an appropriate response to limit the difference.
KeywordsCore Temperature Cold Exposure Internal Environment Plasma Osmolality Salt Gland
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