INTEGRATION OF ACTIVITIES 



149 



stimulus of some sort) destroys the impermeability of the membrane at 

 one point the polarization there is lost; the ions get together and neutralize 

 one another. Such neutralization could then proceed to adjoining parts 

 of the nerve fiber as rapidly as the impermeability is lost. No material 

 thing moves along the nerve, but a wave of neutralization and permea- 

 bility proceeds at considerable speed. 



Waves of some sort pass over other organs, as over the heart when it 

 contracts, over skeletal muscle, and over glands. It seems likely that 



+ + 



+ + + + 'f ++ + + + + + 



FiG. 123. — Propagation of nerve impulse, a wave of permeability associated with neutraliza- 

 tion of positive and negative ions. Dotted lines, permeable membrane. 



in all these structures essentially the same changes in polarization of 

 surface membranes are taking place. 



Initiation of and Response to Nerve Impulses. — Though the impulses 

 carried by all nerves are the same, no matter where they begin or end, 

 the things that start them and the actions they induce are quite different. 

 The impulse is initiated by a receptor of some kind, that is, a specialized 

 nerve ending which is exceptionally sensitive to some one sort of stimulus. 

 In the retina of the eye the receptors (rods and cones) are sensitive to 



Fig. 124. — Various receptors: left to right, rod and cone of retina of eye, taste bud of tongue, 

 olfactory cells of nasal lining, and cold, touch, and pain endings in skin. 



light, the taste buds of the tongue and the olfactory cells in the lining of 

 the nose (Fig. 124) are sensitive to chemical substances. Certain nerve 

 endings in the skin are sensitive to cold, others to touch, still others lead 

 to pain; the several kinds are structurally different from one another. 

 These receptors are not interchangeable, each does its own work, no other. 

 If a cold spot on the hand is stimulated in some other way than by low 

 temperature — mechanically, for example — the sensation is still that of 

 coldness. 



