Integration Sherringtons Reflex Centers

Processing the dynamic excitatory and inhibitory patterns of activity within masses of neurons ("reflex centers") was described as "integration" by the famed neuroscientist C. S. Sherrington during the 1930's.

The brain is continually faced with the task of making decisions on the basis of information about the outside world provided by sensory end-organs and information stored in memory. At any one instant, incoming signals from diverse sources in the periphery excite the brain. The mechanisms by which the various types of information are taken into account and assigned priorities is called "integration" which is carried out at all levels of brain organization. In a global example of integration, an animal confronted by danger integrates input towards a binary output decision: "fight or flight." Our higher centers continually receive information arising in a great variety of sources on the surface of the body and in the internal organs. A typical central neuron faces a task similar to that of the brain as a whole. It is a target of converging excitatory and inhibitory signals that it transforms ("integrates") into its own impulses. The general principles of integration were discovered in the early 20th century by Sherrington (1933, 1947) who recorded tension in skeletal muscle by the stretch reflex before electrical recording from individual cells was possible. Integration appears to occur at all levels of nervous systems and among various types of organisms: crustacean, fish, and mammals. Sherrington proposed and cited evidence for integration by groups of neurons which he termed neural masses or reflex centers and suggested that they correlated with anatomically identifiable "nuclei."

A nucleus is a compact region of gray matter of relatively homogeneous neural architecture and recognizable boundaries which contains a high density of neuronal cell bodies and synapses. (White matter connotes a high density of cable-like axon fibers.) A reflex center is an assembly of neurons performing a specific function. A nucleus is purely morphological or structural while a center is functional. Nuclei may coincide with centers, but often do not (Freeman, 1972).

The concept of neural centers may convey an erroneous impression of anatomically specific function, but remains as a vestigial reference to Sherrington's concept of the nervous system and now denotes groups of neurons whose destruction leads to loss of specific function and/or the stimulation of which evoke a certain behavioral or physiological function. Brain functions are clearly not divided among centers in the same way as the work of a large organization or factory is divided among its various offices and workshops. The relation between anatomic regions devoted to specific functions, and the brain-wide distribution of information is perplexing and complicated. For example, the satiety center is located in the hypothalamus; if this general region is stimulated in an animal having a meal, the animal will stop eating as though it has had enough. If the same structure is destroyed, the animal eats too much and gets fat as though it is never satisfied. Thus clearly the satiety center neurons are essentially related to evoking the sensation of fullness or satiety. Feeding behavior, however, is regulated by a much wider range of many neuronal circuits in different regions. The satiety center integrates multiple inputs to a binary output: eat or don't eat. Body representations such as motor and sensory homunculi and other concrete evidence of anatomical localization of neuronal function may also integrate wide sources of distributed input to representations of anatomical sensation and action. Anatomical hardware such as satiety centers, motor and sensory homunculi appear to be evolutionary adaptations necessary for larger and more complex nervous systems.

Sherrington is a key figure in the history of neuroscience. His concept of integration by reflex centers illuminated possible modes of information processing by neural structures. It is now appreciated that information transfer functions occur at all levels of nervous system organization and include functions now used in computers such as summation, ramp triggers, analog/digital conversion, and logic.

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