PROBLEM 2. Why Co?nplex Behavior Is Possible 



Fig. 263 (above) To which cells vmy cell A send hupulses? 

 From which cells 7nay the dendrites of cell C receive impulses? 



Fig. 262 (left) 

 An impulse 

 can pass from 

 A io B across 

 a synapse. Cell 

 A is shortened 

 {see the dotted 

 lines). Cell B 

 is incomplete. 

 Copy this dia- 

 gram. Com- 

 plete cell B 

 and show how 

 the impidse 

 would get into 

 another cell, 

 C, beyond B. 



rons along a different route. The re- 

 sponse may be different if a different 

 route is taken by the impulse. 



The reflex arc. Now let us trace an 

 impulse from receptor to effector. In a 

 reflex, under normal conditions, the im- 

 pulse always takes the same path and 

 only a few neurons are involved. Let us 

 take as an example your inborn response 



to the touching of a hot stove. A recep- 

 tor in your skin is stimulated by heat, and 

 an impulse travels from receptor to neu- 

 ron. This happens to be one of the neu- 

 rons whose cell body lies in a ganglion 

 outside of the spinal cord. This neuron, 

 too, is different in structure from the 

 neuron we studied above. Instead of hav- 

 ing short, widely branched dendrites it 

 has a long-drawn-out dendrite extending 

 all the way from your finger tip to the 

 cell body which lies in a ganglion, just 

 outside of the cord about on a level with 

 your arm. The impulse travels along this 

 long dendrite into the cell body. From 

 the cell body the impulse continues 

 along the outgoing axon. This axon en- 

 ters the gray matter of the cord (show- 

 ing in cross section as an imperfect "H"). 

 Here the axon ends in an end brush. The 

 impulse travels from the brush into the 

 dendrites of a second cell and out along 

 its axon. This axon extends from the 

 cord along the arm and ends in an end 

 brush. This end brush connects, not with 



