272 



BIOPHYSICAL STUDIES ON NERVE AND MUSCLE 





Figure 10-5. Electron Micrograph of an Ultrathin Section of Nerve Axon which is Myelin- 

 ated by the Spiral Wrap of the Double-Membraned Schwann Cell. Magnification 90,000 x . 

 (Courtesy of J. D. Robertson, Harvard Medical School, and of Scientific American.) 



ure 10-1) the impulse is forced to skip from node to node, perhaps via semi- 

 conductivity, although it may be by proton transfer ("protochemical" cells 

 have been demonstrated in the laboratory) through the myelin. In any case 

 the skipping mechanism is very fast, and therefore a sheathed nerve nor- 

 mally conducts an impulse somewhat faster than an unsheathed one. 



The Trigger 



To fire the nerve and incite the transmission of an impulse, a stimulus is 

 required. Stimuli are essentially of five kinds: 



(1) Electrical: voltage changes applied directly to the cells of the nerve — 

 in the brain for example. 



(2) Mechanical: pressure changes causing distortion at nerve endings — 

 ear, and mechanoreceptors associated with the sense of touch. 



(3) Electromagnetic: incident radiation absorbed by pigment molecules in 

 cells sensitive to visible light, and by other transducer molecules 

 sensitive to warming (infrared) radiations — eye, and a multitide of 

 closely spaced detectors all over the body's surface. 



