4:1/ The Conduction of Impulses by Nerves 71 



specific target organs. Similarly, when information is transmitted from 

 one neuron to another, there is often a chemical intermediate. The 

 process differs from the endocrine system only in the length of time 

 involved. The hormones act, in general, over a period of hours or 

 days, whereas the transmission from one neuron to the next takes only 

 milliseconds. Some hormones act faster so that there is no sharp dividing 

 line between the hormones and the neuro-chemical transmitters. 

 Plants also possess chemical transmitters. The distinguishing feature 

 of higher animals is their nervous system, which transmits information 

 far more rapidly than the endocrine systems do. 



Biophysicists have studied both the nervous and the endocrine systems. 

 Both lend themselves to the application of complex physical techniques, 

 and both can be analyzed by the type of reasoning common to physics 

 and electronics. This is particularly true of the interactions between 

 groups of neurons, of interactions between groups of endocrine glands, 

 and also of the neuron-endocrine interactions. In all of these, "feed- 

 back" loops exist in which the effect produced alters the behavior of the 

 neurons or endocrine glands producing these effects. Physicists and 

 electrical engineers refer to these types of control mechanisms as "nega- 

 tive feedback"; physiologists have called many of them "homeostatic" 

 mechanisms because they tend to keep the state of the organism 

 constant. 



In this text, only the actions of the nervous system are discussed. It is 

 the aim of this chapter to present, in so far as possible, a picture of the 

 physical properties of nervous tissues and a description of how nerve 

 fibers conduct spike potentials. Because each reader will have a different 

 background, an attempt has been made first to present the fundamentals 

 of electricity. A more detailed discussion of electrical terminology can 

 be found in Appendix C. The electricity section of this chapter is 

 followed by a brief description of certain salient features of the vertebrate 

 neuron. Details of the physical characteristics of the action potential 

 are then presented. The final section of this chapter deals with con- 

 duction from one neuron to the next, called synaptic transmission. 



Many aspects of the nervous system are discussed in other chapters. 

 Chapter 5 describes the electrical potentials of the brain and contains a 

 discussion of feedback mechanisms. Chapters 6 and 7 deal with the 

 neural aspects of vision and hearing. Chapter 8 includes the stimulation 

 of muscles by nerves, and Chapter 9 the neural control of the heart rate. 

 Perhaps most important of all, from the point of view of the biophysicist, 

 the molecular basis of the action potential is discussed in Chapter 24. 

 A knowledge of the material in Part D (molecular biology) and the 

 other chapters of Part E (thermodynamics and transport systems), 

 makes that chapter much easier to understand. 



