Abstract
Evidence from many sides indicates that the primary change in the stimulation of an irritable tissue is a sudden increase in the permeability of the boundary layers or “plasma-membranes” of the constituent cells or elements The resistance to the escape of diffusible substances, including carbon dioxide, is thus diminished, and there results a corresponding acceleration of the energy-yielding oxidations. With increase in the permeability to ions, there is naturally also associated a change in the electrical polarization of the plasma-membrane—hence the characteristic “action-current” of stimulation. The primary and critical change, increase of surface permeability, may be produced by the electric current, by sudden changes of temperature or contact, by mechanical shock, or by the action of various chemical substances.
Chemical stimulation, on this view, results from the action of those substances which affect the constituents of the plasma-membrane in such a manner as suddenly to increase its permeability to the critical degree required. Now the plasma-membrane is primarily a colloidal structure, consisting mainly of prothins and lipoids intimately intermixed, possibly intercombined. We should, therefore, expect its structure or consistency to be altered, and its permeability correspondingly increased or decreased, by substances that influence colloidal aggregation-state; such substances ought, as a class, to show evident relations to stimulation. Again, substances with a specific action on lipoids should also show such relations. These two classes of substances, electrolytes and lipoidsolvents, do in fact show peculiar relations to the stimulation-process; their solutions affect the irritable tissue in two distinct ways; either (1) they stimulate, or (2) without stimulating directly they facilitate or hinder stimulation by other means—in other words, they sensitize or desensitize the tissue.
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