452 - Multicellular Animals, Especially Man 



SKIN- 

 SIMPLE 

 REFLEX 



ARC 



SENSORY 

 NEURON 



EFFECTOR 

 (MUSCLE CELL) 



INTRASEGMENTAL 

 REFLEX ARCS 



-MUSCLE 



NERVE 



CENTRAL NERVOUS SYSTEM 



SYNAPSE 

 DENDRONS 

 MOTOR NEURON 



STIMULUS 



INTERSEGMENTAL 

 REFLEX ARCS 



*BH 



INTERMEDIATE 

 NEURONS 



Fig. 25-6. Sensory-neuromotor mechanisms of the earthworm. Be sure to differentiate be- 

 tween intrasegmental and intersegmental reflexes. 



and the effectors in a given region of the 

 earthworm's body (Fig. 25-6). 



The Reflex Arc. As is indicated in Figure 

 25-6, not one, but a series of neurons takes 

 part in relaying impulses to the effectors, 

 whenever the receptors of an earthworm are 

 stimulated. But regardless of the number of 

 neurons, the route taken by impulses in 

 passing from the receptors to the effectors of 

 an animal is designated as a reflex arc. In all 

 cases impulses pass to the central nervous 

 system via afferent fibers, and pass outward 

 from the CNS via efferent fibers. 



The many degrees of complexity displayed 

 by reflex arcs are shown in Figure 25-6. The 

 simplest arc (Fig. 25-6) is a unilateral intra- 



segmental reflex, which involves just one 

 side of one segment of the animal's body; 

 another simple arc is a bilateral intraseg- 

 mental reflex, which involves both sides of 

 any one segment (Fig. 25-6). Similarly it is 

 possible to recognize intrasegmental reflex 

 arcs (Fig. 25-6), which may be either short 

 or long, and either unilateral or bilateral in 

 nature. 



In the intact animal, each stimulus usually 

 excites many receptors and throws a number 

 of reflex arcs into simultaneous action. More- 

 oxer, each sensory neuron makes contact with 

 several association neurons, as well as with a 

 number of motor neurons. Association neu- 

 rons arc localized entirely within the central 



