278 



Receptor 



Why Living Things Behave As They Do unit v 



Spinal cord 



Afferent (sensory) neuron 



Fig. 264 The simplest type of reflex arc. No intermediary neuron is shown. Note that 

 the eel! body of the afferent neuroji lies outside of the cord {in a ganglion). It is 

 different fro?n the neurons you have seen before. It has one very long dendrite. Where 

 are its ends? 



another nerve cell, but with a muscle cell 

 in the arm. The muscle cell is the effec- 

 tor. When the impulse reaches it the 

 muscle cell contracts. In this reflex act, 

 there were used, first, a receptor; sec- 

 ond, a transmitter, or nerve cell which 

 carries the impulse to the nerve center 

 (spinal cord); third, another transmit- 

 ter which carries the impulse out of the 

 nerve center (spinal cord); and, fourth, 

 an effector which receives the impulse, 

 and acts. The first neuron, which carries 

 the impulse inward to the cord, is called 

 the afferent, or sensory, neuron. Afferent 

 means leading into. The second neuron, 

 which carries the impulse out of the 

 cord, is called an efferent, or motor, neu- 

 ron. To help you understand what you 

 have read do Exercise 5. 



This description and the diagram are 

 oversimplified. What is shown happen- 

 ing in one receptor and in single neurons 

 happens in a group of receptors and 

 neurons lying side by side. Therefore all 

 the muscle cells in the muscle contract. 

 And the diagram is oversimplified in 

 another respect. In reality there are prob- 

 ably several neurons lying in the cord 



between the afferent and efferent neu- 

 rons. These relay the impulse from the 

 afferent neuron to the efferent neuron. 

 Such neurons are called inter?nediary 

 neurons. But there is little relaying; in 

 other words, there are few synapses and 

 that, no doubt, is one reason why this im- 

 pulse travels with great speed. You pull 

 your hand away almost instantaneously. 



Now in all of us the impulse takes this 

 path and it always takes the same path 

 unless some of the cells have been de- 

 stroyed. But, you will say, "I pull my 

 hand away because the stove feels hot." 

 That is not true. When the impulse takes 

 the path we have just described you 

 could not have felt anything^. No sensa- 

 tions arise from impulses to the cord. 

 You have learned that you feel, you see, 

 and you hear because the cells of the 

 cerebrum receive impulses. 



Becoming conscious of the heat. Evi- 

 dently there must be an impulse reach- 

 ing the cortex of your cerebrum or you 

 would never be aware of "hot stove." 

 How might this impulse reach the brain? 

 When the impulse from the finger tip 

 reaches the cord it jumps across the syn- 



