216 NERVOUS REACTIONS 



below the knee cap, the tap will cause the knee to jerk. In this 

 experiment, the stimulus is received by a nerve ending in the skin. 

 The stimulus starts an impulse which travels along a sensory 

 neuron into the spinal cord. There, this sensory neuron links up 

 with an outgoing motor neuron by means of a synapse. The out- 

 going neuron conveys the impulse to a muscle in the leg, which 

 causes the knee to jerk. In some way, the in-going sensory stimulus 

 is changed into an outgoing muscular reaction. The sense organs, 

 in this case '* touch spots " in the skin, receive the stimulus and are 

 the receptors. The muscle effects the reaction and is the effector. 

 The pathway" of the receptor, composed of the afferent axon, affer- 

 ent cyton, synapse, efferent cyton, efferent axon, and the effector 

 make up the reflex arc. Any reflex arc involving the central 

 nervous mechanism begins and ends in the outer part of the body. 



Types of responses. The simplest response activities involve at 

 least two neurons. Experiments have been made with frogs 

 whose brains have been severed from their spinal cords. Such 

 frogs lose all conscious activities, but are still capable of making 

 certain reflexes. If the toe is pinched, the leg is withdrawn ; 

 violent pinching causes a distinct jump. If a paper wet with 

 dilute acetic acid is placed on the skin of the leg, the frog makes 

 movements to brush off the paper. 



If the foot of a sleeping baby is tickled, the foot is withdrawn. 

 This is a reaction or reflex of the first level. The impulse, caused 

 by the stimulus, passes into the spinal cord and out again without 

 involving any other connections. The knee jerk is another ex- 

 ample of a reflex of the first level. But, if the blow on the knee 

 is severe enough, the person may gasp or scream or balance him- 

 self to preserve his equilibrium. He may even show an increased 

 heart beat. The centers of respiration and circulation are stimu- 

 lated, and balance and coordination are brought into the action. 

 This response is of the second level. A number of neurons enter 

 into this activity. Some of them are sensory and a great many 



