CENTRAL NERVOUS SYSTEM 



209 



that the nervous system may function as a whole, impulses or 

 disturbance must readily pass from neuron to neuron. The point 

 of junction between two neu- 



nucleus 



^CYTOM 



AXON 



rons is commonly known as a 

 synapse. The sense organs, 

 known as receptors, receive 

 the stimulus which starts 

 the nerve current. The ends 

 of the nerve fibers of these 

 organs are on the outside 

 (skin) of the body. The 

 muscles or glands are called 

 effectors because they bring 

 about activity. An axon 

 that connects with a sense 

 organ is called an in-going, 

 sensory, or afferent axon; 

 one that connects with a 

 muscle or gland is an out- 

 going motor, or efferent axon. 

 The neurons possessing these 

 axons mav be either sensorv 

 or motor neurons. For ex- 

 ample, when the finger is 

 placed on a hot object and 

 immediately withdrawn, a 

 series of actions has taken 

 place in the nervous system. The nerve fiber in the finger re- 

 ceived a stimulus which it carried to the brain. The brain in 

 turn sent out a message to the muscles in the arm and hand, so 

 that the finger was immediately removed from the object. 



When neurons connect sensory with motor neurons, they are 

 called associative neurons. Groups of nerve cells situated outside 



terminoi 

 branch 



The unit of structure of the nervous system is the 

 nerve cell or neuron. The control of all activities 

 in the entire body is maintained by these cells. 



