194 - The Cell 



Electrical stimuli: Changes in strength or direc- 

 tion of electric currents. 



Photic stimuli: Changes in the intensity, color, or 

 direction of light. 



Most cells can be excited by several kinds 

 of stimuli, but usually each kind of cell is 

 especially susceptible to a particular kind of 

 stimulation. Successful stimulation of the 

 retinal cells of the eye, for example, requires 

 an unbelievably small quantity of light, 

 whereas another kind of stimulus, such as a 

 blow on the temple, can arouse sensations of 

 light ("seeing stars") only if the intensity of 

 the stimulus is relatively great. In the case of 

 specialized cells, nonspecific stimuli are rela- 

 tively ineffective; but there is one important 

 exception to this general ride. All cells are 

 easily excited by electrical stimulation, and 

 this fact emphasizes the role of the action 

 potential as a normal agency in propagating 

 the excitation, once a cell has been aroused. 



The rate of the stimulating change has a 

 distinct influence upon its effectiveness, as 

 can be demonstrated with light as an exam- 

 ple. The rapid brightening or dimming of a 

 light is always noticeable. But if the change 

 develops slowly, it may fail to excite the cells 

 of the retina, and consequently it is not per- 

 ceived. Too short a change may also fail to 

 excite the retina. An intermittently (lashing 

 light is perceived to be continuous if the 

 individual flashes begin to follow one an- 

 other at a rate exceeding about 14 per sec- 

 ond. 



Electrical stimulations will also illustrate 

 the foregoing points. If a direct current is 

 passed through a muscle, no contraction oc- 

 curs except at the moment when the current 

 is turned on or off — that is, while a change 

 of current intensity is taking place. But 

 the rate of the change must also be ad- 

 justed properly if the muscle is to be excited. 

 Even with adequate voltage, no excitation 

 occurs if the current rises or falls too slowly 

 or too quickly. 



Receptors, Conductors, and Effectors. In 

 multicellular organisms a typical response 

 invokes not one, but a series of cells, each 



playing a specialized role. Let us take, for 

 example, the closing of the pupil, which 

 occurs when a bright light is directed into 

 the eye. In this response the light does not 

 act directly upon the muscle cells of the iris, 

 which control the size of the pupil opening. 

 Only the cone and rod cells of the retina, 

 deep inside the eyeball, are excited by the 

 light. Thus the retinal cells act as receptors 

 when light is the stimulating agency. But 

 after these cells receive the stimulus, they 

 relay the excitation to a series of nerve cells, 

 which serve as the conductors of the respon- 

 sive system. The nerve cells conduct at high 

 speed, and within about 0.01 second they 

 transmit the excitation to the muscle cells 

 of the iris. Thus the muscles are the effectors 

 of this response, in that the muscles contract 

 and close the pupil. 



A receptor is any part of the organism that 

 displays a special sensitivity to excitation, 

 usually by a specific kind of stimulus. The 

 main function of the receptor is to generate 

 excitations and relay them to the conductor 

 structures. A conductor is any specialized 

 part of the organism that serves to propa- 

 gate excitations toward an effector. And 

 finally, an effector is a specialized part of the 

 organism that executes the final or "active" 

 phase of a response. 



In the higher animals, including man, the 

 receptors are mainly represented by the sen- 

 sory cells and the sense organs; the con- 

 ductors, by the cells of the nervous system; 

 and the effectors, by the muscles and glands. 

 But in many unicellular organisms there are 

 receptor, conductor, and effector structures, 

 differentiated within the single cell. 



Responsive Mechanisms in One-celled Or- 

 ganisms. When the advancing pseudopodium 

 of an amoeba is probed with a microinstru- 

 ment, the amoeba withdraws the pseudo- 

 podium and starts retreating in the opposite 

 direction (Fig. 11-4). Plainly the excitation 

 does not remain localized at the point where 

 the stimulus is received. It is conducted 

 throughout the cell, or otherwise the several 

 pseudopodia, which serve as effectors, could 



