150 PRINCIPLES OF ANIMAL BIOLOGY 



The response which a nerve impulse eUcits depends on the nature of 

 the structure to which it is dehvered. An impulse delivered to a motor 

 unit in a muscle causes contraction; an exactly identical impulse carried to 

 a gland causes secretion. It is probable that in each instance a chemical 

 substance is produced at the nerve ending, and that it is this substance 

 rather than the nerve impulse itself which really stimulates the responding 

 organ. At least that is true of responses of some of the internal or 

 visceral organs. 



An impulse from one of the sensory endings in the skin leads to a cer- 

 tain center in the brain, and the appropriate sensation is there produced. 

 The nerve fibers from the retina go to one region of the brain, neurons 

 from the olfactory area in the nose go to another, fibers from the pain 

 endings in the skin lead to a third. These regions of the brain are indi- 

 cated more fully later; the important point nov: is that for each activity 

 there is a special kind of receptor, located at a particular place or places, 

 and a certain organ or region of the nervous system where the appropriate 

 response is given. The nerve impulse which goes from the place of 

 stimulation to the place of response is everywhere the same. 



Direction of Impulse. — When a neuron is stimulated at its receptor 

 ending, the impulse thus started travels toward the other end; there is no 

 place else to go. Experimentally, however, and sometimes in special 

 situations naturally, a neuron may be stimulated in the middle of the 

 length of its axon or dendrite. When this happens, impulses travel in 

 both directions to the limits of the neuron itself; but in one of the direc- 

 tions it goes no farther than the end of that particular neuron. The 

 difference lies in the synapses at the ends . of the axon and dendrite. 

 Each synapse is a one-way conductor. An impulse can go over it 

 from axon to dendrite but never from dendrite to axon. This is the 

 reason why nerve impulses alwa3\s go in one direction over such a chain 

 of neurons. As stated above, when a neuron is stimulated somewhere in 

 its middle, the impulse moves in both directions from that point to both 

 ends of that neuron. In the "forward" direction, arriving at the termi- 

 nus of the axon, it goes over to the dendrite of the next neuron and 

 continues the propagation, Ijocause the synapse there permits passage in 

 that direction. But in the "backward" direction the impulse is blocked 

 when it reaches the tip of the dendrite because the synapse will not carry 

 it over to the adjoining axon. 



What gives the synapse this power of distinguishing direction? While 

 the answer to this question is not certainly kno\\'n, a possibility is sug- 

 gested l)y what is kno^vn of responses to stimuli in general. We are 

 familiar with the control of su(!h organs as the heart by a double innerva- 

 tion, one nerve acting to stimulate, the other nerve to inhibit. Each 

 nei've pi-esumably produces a chemical substance to which the oigan 



