414 



ORGAN SYSTEMS OF MAN 



able membrane, and the membrane becom- 

 ing neutral in the region of the impulse. It is 

 known that the change is chemical as well 

 as electrical because the nerve fiber con- 

 sumes oxygen and gives off carbon dioxide, 

 respiring just as all cells do. The neutral 

 condition is coincident with the refractory 

 period and therefore lasts a very short time. 

 Impulses pass along a nerve in rapid succes- 

 sion and it is highly unlikely that a single 

 impulse brings about a specific action. Im- 

 pulses come in "bursts" like bullets from a 

 machine gun 



cells to muscle cells? Careful and extensive 

 microscopic examination has failed to show 

 any protoplasmic connection either between 

 nerve cells or between nerve cells and mus- 

 cle cells. How, then, does an impulse pass 

 from one unit to another? The best answer 

 today is that the gap is traversed by chemi- 

 cal means. A specific chemical is formed at 

 the termination of a neuron which stimu- 

 lates the dendrites of the next neuron or a 

 muscle fiber. This will be discussed a little 

 more fully under the autonomic system. 

 Divisions of the nervous system. In an at- 





impulse 



I — I 



Fig. 16-14. The nerve is normally polarized, being positively charged on the outside and 

 negatively charged on the inside. This polarity is lost coincident v»^ith the nerve impulse 

 (indicated in black). 



If a sensory nerve fiber is stimulated in its 

 middle, a sensation results which we refer 

 to the usual sense organ of origin. Such sen- 

 sations in the case of pain are called referred 

 pain. For example, if one sits for a consider- 

 able period of time with the legs crossed, 

 he finds that upon rising the crossed leg 

 fails to function properly and prickly sensa- 

 tions seem to come from the toes. The sci- 

 atic nerve ( large leg nerve ) was compressed 

 in the middle during that time so the stimu- 

 lation really came from this region, although 

 the sensation seems to originate in the toes. 



So far, consideration has been given only 

 to the transmission of the impulses within 

 the neuron itself. How does the impulse 

 travel between nerve cells and from nerve 



tempt to understand some of the more sim- 

 ple reactions effected through the nervous 

 system, it is useful to divide it into parts, 

 all of which, however, are very interdepend- 

 ent. The central nervous system is com- 

 posed of the brain and cord while the 

 peripheral system is made up of the nerves 

 which connect the brain and cord to all 

 parts of the body. Impulses enter the cen- 

 tral nervous system through the afferent 

 fibers of the peripheral nerves. Interpreting 

 the incoming messages and subsequently 

 dispatching them is the function of the 

 brain, the cord acting primarily as a relay. 

 The impulses leave the brain and cord for 

 the muscles and glands on the efferent fibers. 

 The simplest type of such action involving 



