292 BIOPHYSICAL STUDIES ON NERVE AND MUSCLE 



ment becomes "noisier." In Chapter 2, the effects of shock, blast, sound, 

 and ultrasound were reviewed; and in Chapter 4 the effects of the warming, 

 visible, ultraviolet, and ionizing regions of the electromagnetic spectrum 

 were discussed. In Chapter 7 heat production, and in Chapter 8 heat loss 

 were discussed, as also were changes in our chemical environment (poisons 

 and catalysts — competitors) as they affect the metabolism of the system and 

 its control. Although the details of the complicated processes of control are 

 beyond our means in this book, enough has been introduced to illustrate the 

 mechanisms and the A-B-C's of environmental effects — atomic, biological, 

 and chemical, at least in general terms. 



One further point will be made on the effects of ionizing radiations on 

 the physical apparatus of control — of increasing importance, especially to 

 medical people, in this atomic age. Nerve itself is relatively insensitive to 

 X rays (Chapter 9). Muscle shows good resistance too: it takes thousands 

 of rads to cause detectable damage. The neuromuscular junction, however, 

 is much more sensitive. For instance, consider a nerve-muscle system such 

 as the sciatic nerve-gastrocnemius muscle freshly dissected from a frog, 

 mounted in such a fashion that the nerve can be stimulated electrically by 

 short, square pulses of voltage applied by the platinum wire contacts refer 

 to (Figure 10-8). If the stimulus repetition rate is chosen at about 1 pulse 

 per sec, the muscle will respond faithfully. If now the whole is irradiated, 

 the muscle soon stops, although the nerve continues to transmit, and the 

 muscle will respond to a stimulus given directly to it. 



Further, the neural network in the brain is now known to be affected by 

 only a few rads; and although this radiation does not affect the motor ability 

 of a man, there is reason to believe that short-circuiting in the network and 

 psychological effects accrue. Since it is not likely to be the nerve cells them- 

 selves, it is probably the synapse, or "junction box" which is implicated as 

 radiation-sensitive. 



The parallelism is clear. The neuromuscular junction and the synapse are 

 the most sensitive parts of man's physical control system. Both of these 

 junctions involve production of a chemical or chemicals at one spot in the 

 junction, transport across the junction, and utilization of the chemical(s) at 

 the other end of the junction. With the background of knowledge of the 

 pertinent chemical and physical effects of ionizing radiations discussed in 

 the previous Chapter, and that of the physical apparatus of control given in 

 this Chapter, what do you think is likely to be the first molecular process to 

 fail during irradiation of the control apparatus? 



PROBLEMS 



10-1: If one side of a concentration cell has KC1 at 0.002 equivalents per liter, what 

 must be the opposing concentration so that the "membrane'' potential reaches 

 90 millivolts? Assume restricted diffusion. 



