COORDINATION 



417 



propriate leg movements (Fig. 16-15). The 

 stimulus can be repeated again and again 

 but the response will always be the same. 

 Remember, there is no brain to interpret 

 the message, yet the response is just as ef- 

 fective as if the brain had been intact. 

 Similarly, people with certain regions of 

 their cord injured feel no pain though they 

 respond to the stimulus. It is apparent in 

 these cases that the impulse from the stimu- 

 lus must have traveled only to the cord, 

 returning from it to the appropriate mus- 

 cles for bringing about that response. Var- 

 ious portions of a frog's cord can be de- 

 stroyed but the reflex will persist as long 

 as the region where the sensory and motor 

 nerves join the cord is intact. The complete 

 action, then, must take place in a very local- 

 ized resfion. 



In order to understand how a reflex can 

 take place, a little information about the 

 anatomy of the cord is in order. A spinal 

 nerve bifurcates as it approaches the cord 

 to enter on the dorsal side as the dorsal root 

 (sensory) and on the ventral side as the 

 ventral root (motor) (Fig. 16-16). The dor- 

 sal root bears the dorsal root ganglion, in 

 which the cell bodies of the sensory neurons 

 are located. The cell bodies of the motor 

 nerves are located in the ventral gray mat- 

 ter of the cord (called the ventral horn). 

 The function of these roots can be definitely 

 identified by cutting them and artificially 

 stimulating the stumps. When the ventral 

 root is cut, no impulses reach the muscle 

 from the cord, but when the cut end on the 

 muscle side is stimulated a response will be 

 evoked. Likewise, by severing the dorsal 

 root no impulses will reach the cord, but 

 when the stump on the cord side is stimu- 

 lated a response will take place. The dorsal 

 root thus contains only sensorv fibers and 

 the ventral root only motor fibers. The spi- 

 nal nerve, which is a union of the two, con- 

 tains both, of course. 



A simple reflex begins with an impulse 

 coming from a sense organ and traveling 

 over an afferent nerve fiber to the cord via 



the dorsal root (Fig. 16-16). Here it comes 

 in close contact ( synapse ) with one or more 

 association neurons. The impulse is then 

 dispatched to the proper muscle or gland 

 over an efferent nerve fiber via the ventral 

 root. With few exceptions reflexes involve 

 more than the three neurons just described. 

 The incoming sensory neuron usually con- 

 nects with more than one motor neuron 

 through association neurons in the cord. 

 Thus the impulse may not only elicit the 

 simple response but may also pass along 

 the cord to stimulate other efferent neurons 

 and in turn cause a large group of muscles 

 to contract. Indeed, this is the manner in 

 which it usually happens. Wlien applied 

 to man the simple reflex accounts for many 

 of our daily movements. These actions do 

 not have to be learned, they are innate — 

 one is born with them. 



Not only are different levels of the cord 

 involved but also neurons on the opposite 

 side of the cord. There is a crossing over of 

 the association neurons so that an impulse 

 may travel to both sides of the body ( Fig. 

 16-17). If, for example, acetic acid is placed 

 on the ventral side of a frog whose brain is 

 destroyed, the leg on that particular side 

 will attempt to remove the irritating acid 

 while the leg on the opposite side will ex- 

 tend (Fig. 16-15). Presently, the impulse 

 will spread to the front legs so that eventu- 

 ally all of the legs are moving in such a 

 manner as to rid the animal of the offend- 

 ing substance, and the body also begins to 

 make mass movements as it would in at- 

 tempting to crawl away from the irritation. 

 To be sure, the first pathway is simple, but 

 as the stimulus spreads many other path- 

 ways are involved until the whole animal 

 is thrown into movement. Under these con- 

 ditions the higher centers also are involved 

 (Fig. 16-17). 



In man, when the various parts of the 

 brain are included, the impulse becomes a 

 part of consciousness and one is aware of 

 the reflex. When one steps on something 

 sharp, for example, his foot may be with- 



