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 



GALVANOMETER 

 NERVE FIBER. 



Fig. 11-1. Bioelectric potentials in the axon of a 

 nerve cell. A nerve impulse (the arrow) traveling along 

 a nerve fiber betrays its presence as a rapidly moving 

 wave of negativity (as compared with the positivity of 

 resting, inactive regions of the nerve fiber). Note how 

 the needle of the galvanometer shifts as the impulse 

 travels. 



to an action current, varies between 0.01 

 and 0.1 volt (10 to 100 millivolts), although 

 in animal cells it tends to approximate the 

 higher value. The current generated in one 

 cell, or group of cells, may be strong enough 

 to spread to neighboring cells, thus relaying 

 the excitation (Fig. 11-2). In the heart, for 

 example, the excitation for each beat origi- 

 nates in a small mass of tissue in the wall of 

 the right auricle. From this point the current 

 travels along a highly specialized group of 

 muscle fibers, reaching the other auricle and 

 the ventricles in time to touch off: their con- 

 tractions just at the proper instant. In taking 

 the electrocardiogram, the physician records 

 the strength and pattern of the shifting ac- 

 tion potential as it spreads throughout the 



