. Principles of modern biology. Biology. 190 - TheCel SUCCESSFUL STIMULATION: THE STATE OF EXCITATION Excitability is a universal protoplasmic characteristic. When successfully stimulated, every cell displays a wavelike change of structure and activity that originates at the point of stimulation, spreads throughout all parts of the protoplasm, and then subsides. This abrupt and strictly temporary disturb- ance of the ordinary resting condition of the protoplasm is designated as excitation. The excited state is invariably the forerunner of any definite cellular response, such as the contracting
. Principles of modern biology. Biology. 190 - TheCel SUCCESSFUL STIMULATION: THE STATE OF EXCITATION Excitability is a universal protoplasmic characteristic. When successfully stimulated, every cell displays a wavelike change of structure and activity that originates at the point of stimulation, spreads throughout all parts of the protoplasm, and then subsides. This abrupt and strictly temporary disturb- ance of the ordinary resting condition of the protoplasm is designated as excitation. The excited state is invariably the forerunner of any definite cellular response, such as the contracting of a muscle or the secreting of a gland. Some cells, like muscle cells, perform some visible action each time they are stimulated, ami in such cases the visible action serves to indicate that a stimulation has been success- ful. But for cells like nerve cells, which are incapable of executing any visible act, sev- eral other criteria of successful excitation are available. Invariably there is a propagated change in the electrical potential of the cell membrane, and this action potential spreads in exact synchrony with the excitation. Excitation is also accompanied by the libera- tion of a small quantity of heat, and in most cases at least, excited cells display a tem- porary increase of permeability. Apparently excitation precipitates a temporary flare-up in the metabolism of the cell; and during excitation special enzymes and substrates, which are not used during periods of rest, are utilized by the excited cell. The Action Potential. An action potential is an infallible sign of successful excitation. It has been measured accurately in many kinds of animal and plant tissues and in quite a number of individual cells. In the case of a single cell, difficulty is encountered unless the cell, like a nerve or muscle cell, is long enough to allow for the placement of electrodes leading to a galvanometer (Fig. 1 1-1), or to an oscilloscope. The electrical potential, which gives rise
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