INTEGRATION OF ACTIVITIES 



145 



to a muscle, or gland, or some organ capable of responding to a stimulus. 

 This neuron carries the impulse away from the central nervous system, 

 hence is designated an efferent fiber. It is also called an effector neuron, 

 often a motor neuron though the action produced may be something else 

 than movement. 



If the reflex arc consists only of an afferent and an efferent fiber, these 

 two neurons are in contact with one another by a minute surface known 

 as a synapse. The axon of the afferent touches a dendrite of the efferent, 

 and the surface of contact is the synapse. An arc of this simple two- 

 neuron type is represented above, at the right, in Fig. 120. The afferent 







+ 



^ 



X ; 



'ASSOCIATION 





-SPINAL COED 

 ■''0^' AFFERENT 



■^^ 



::i^Ass 



RECEPTOR SENSE 

 Oi3GAN 



ASSOCIATION 



■--EFFERENT 



GLAND 

 > • 



I < 



Fig. 120. — Diagram of simple reflex arcs in the vertebrate nervous system. 



neuron enters the spinal cord through the dorsal root of a spinal nerve, 

 in Avhose ganglion the body of the neuron lies. Within the spinal cord 

 the axon synapses with the dendrite of another cell whose body lies 

 within the cord. The axon of the latter cell passes out through the 

 ventral root of the spinal nerve, and its tip is applied to the responding 

 organ (muscle in the diagram). 



Most reflex arcs consist of more than two neurons. The extra ones 

 are interpolated between the receptor and effector neurons. These con- 

 necting neurons are kno\vn as intermediate or association neurons. The 

 spinal cord is the seat of vast numbers of them. The association fibers 

 are especially useful in carrying the arc over considerable stretches of 

 the central system. In the lower right half of Fig. 120 is a reflex arc 



