Resumo de Physiological Information Transfer

Information transfer
The physiology of animals differs from that of plants in the rapid response of animals to stimuli. René Descartes, responsible for the concept of the reflex that dominated neurophysiology for most of its history, thought a sensory impulse was ?reflected? from the brain to produce a reaction in muscles. Later studies of the effects of ions on nerves suggested that a nerve must be surrounded by a membrane and that a nerve impulse results from a change in the ability of the membrane to allow passage of potassium ions. When it was shown that nerves are made up of thousands of tiny fibres, which are processes that extend from cells located in the brain or spinal cord, the nerve impulse hypothesis was applied to individual nerve fibres rather than to whole nerves. Electronic technology provided the techniques and giant nerve fibres of squids provided the experimental material that enabled two Nobel prize winners for physiology, Alan Lloyd Hodgkin and Andrew Fielding Huxley, to extend this hypothesis into a theory of the excitation of nerve cells in which sodium ions and potassium ions play principal roles.
The reflex concept, however, was not dependent on understanding the molecular basis of excitation, conduction, and transmission. Early in the 20th century the role of interaction of nervous centres in controlling muscle contractions was established. The reflex now is conceived as a unit in which nerve impulses initiated in sensory neurons or nerve cells are conducted to a centre in the brain or spinal cord. In the centre, impulses initiated in motor neurons are conducted to muscles and induce a reflex response. Two processes can occur in the centre; one is associated with central excitatory states, the other with central inhibitory states. The net effect of any stimulus or group of stimuli, therefore, can be interpreted as an interaction of these opposing states in the centre.
After the demonstration that the effects of the vagus nerve in slowing the heart are mediated by a chemical substance, subsequently identified as acetylcholine, the concept of chemical transmission of nervous impulses was extended to the central nervous system. Typically, transmission of excitation from cell to cell is accomplished by the liberation of a chemical transmitter from a nerve ending.
The reflex concept gave rise to premature attempts to develop a psychology based on reflexes. These attempts (behaviourism) were advanced by the Russian I.P. Pavlov's discovery of conditioned responses. Originally known as conditioned reflexes, these responses have been found in most animals with central nervous systems. More complex than simple reflexes, their mechanism has not yet been established with certainty.
The analysis of sensory functions also extends to the cellular level. Sense organs are diverse in structure and sensitivity to specific stimuli. It may be that the common molecular basis for the differences in sensitivity is a change in permeability of a special region of the membrane surrounding a sensory cell. This change in permeability could allow a nerve fibre to become excited and initiate a nerve impulse. Neurophysiology has borrowed from, and contributed to, the information theory used in communications engineering. The function of sense organs is to gather information both from the environment and the organism

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