THE MOLECULAR BASIS OF MUSCLE CONTRACTION 277 



the two optic nerve trunks, a nerve-ending from each fiber carries an im- 

 pulse from a rod or cone to a bipolar cell, thence to the brain. There is evi- 

 dence now that insulation among these strands is not complete, and that 

 parallel signals from two may trigger a third, and so on. This is a mecha- 

 nism which seems to be operative in color vision, as was inferred in the dis- 

 cussion on that subject in Chapter 4. Cross-stimulation seems to be very 

 generally operable, for there is a great deal of psychological evidence that 

 saturation of one sensing organ will have a marked effect on the sensitivity 

 of another. Mentioned earlier was the dentist's new trick of flooding the ear 

 with noise of a suitable frequency so that the pain of drilling cannot be felt! 

 The physical network which accommodates, sorts, and retains certain im- 

 pulses and rejects others is a topic for future study. Furthermore, memory 

 is still a very mysterious phenomenon. One recent proposal about the phy- 

 sical mechanism of memory deserves mention: the "training" of the neural 

 network to store information is done by means of the synthesis of certain 

 ("different") protein molecules. These result from a change in shape of the 

 ribonucleic acid (RNA) effected by a passing stimulus — i.e., the RNA within 

 nerve and neighboring glial (Schwann) cells. Although this does not sound 

 very convincing at first glance, it seems to be the best model yet put forward 

 in the baffling question of what is the physical apparatus of memory; and 

 it certainly is consistent with the known fact that the rate of protein synthesis 

 is very high in active nerve cells. One cannot help thinking that these "dif- 

 ferent" proteins may be imbedded right in the membrane, and exert their 

 effect as "permanent" changes in its permeability. In conclusion, one could 

 say that, from the biophysical point of view, the study of the central nervous 

 system is becoming more and more a study in applied electrochemistry, a 

 study of membrane biophysics. 



THE MOLECULAR BASIS OF MUSCLE CONTRACTION 



By means of nerve, the brain exercises control over both chemical and 

 physical processes in the body. There are good examples of each: for the 

 former, the endocrine gland system; and for the latter, muscle. Of the two, 

 the latter is in many ways inherently less complicated, and only it will be 

 discussed in this attempt to illustrate how control is achieved in a particular 

 case of a physical action. For this we need to know some relevant physical 

 properties of muscle tissue; and, more important still from the biophysical 

 point of view, we need to know the molecular behavior which is at the root 

 of this physical behavior. Fortunately, both electron microscopic examina- 

 tion of muscle-tissue slices, and kinetic methods of analysis of rate data seem 

 to be succeeding with this problem of providing an understanding of mus- 

 cular contraction. On the other hand, a review of muscular contraction from 

 the molecular viewpoint has the added advantage of illustrating the powerful 



