500 Animal Biology 



The conductors in man are (1) the nervous system^ composed of the 

 brain and cranial nerves, spinal cord and spinal nerves, autonomic nerves 

 (Figs. 16, 17, 246, and 247) and (2) the blood stream (Figs. 237, 240, 

 and 241) and the tissue fluids. The activities of various organs may be 

 aflfected by chemical substances produced in other tissues and organs and 

 carried by the blood, lymph, or body fluids. This is chemical coordina- 

 tion, and the substances, designated as hormones, are produced by duct- 

 less (endocrine) glands discussed later in this chapter. 



Since much of the coordination of the many organs, tissues, and sys- 

 tems of the human body is due to the activities of the nervous system 

 and its so-called nerve impulses, the nature and characteristics of the 

 latter must be understood in order to appreciate the marvels of coordina- 

 tion of the various parts of the body. 



Some of the more important characteristics of nerve impulses are as 

 follows: (1) They are electrochemical phenomena (Fig. 251) in which 

 a stimulus originates electrical changes in one part of a nerve fiber which 

 in turn institutes similar electrical changes in adjacent parts of the fiber 

 as the impulse travels along. According to the polarized membrane 

 theory of nerve im,pulse, the semipermeable membrane surrounding each 

 nerve fiber permits certain ions (charged chemical particles) to penetrate 

 it but prevents others from doing so. Through normal metabolic activi- 

 ties of the nerve, the membrane is polarized (charged) by having an extra 

 number of positive ions on its outer surface and an equal number of nega- 

 tive ions on its inner surface. The positive and negative ions do not neu- 

 tralize each other normally because the membrane is impermeable to 

 them. However, when stimulated at a certain point, the membrane is 

 depolarized (loses the excess positive ions) and its permeability is in- 

 creased so that the ions from an adjacent, nonactivated region pass 

 through this depolarized region to neutralize each other. This results 

 in depolarization (probably because of chemical actions) of this adja- 

 cent region, making it permeable to the movement of the ions from the 

 next region, and so the impulse moves along the surface of the nerve fiber 

 by a series of depolarizations (Fig. 251 ) . After a period of time, a nerve 

 over which an impulse has traveled becomes repolarizcd again with its 

 positive ions on the outer surface of the membrane and negative ions on 

 its inner surface. (2) After a nerve fiber has conducted an impulse, it 

 undergoes certain chemical and physical changes ("recovery") over a 

 definite period of time (0.001 to 0.005 second) and then it can transmit 

 another impulse. The interval between consecutive impulse transmissions 

 is known as the refractory period. (3) When a nerve fiber transmits an 



