CHAPTER CI 



THE INTEGRATION OF SIMULTANEOUS AND SUCCESSIVE RE- 

 FLEXES 



The Principle of the Final Common Path 



A single reflex acting independently of the rest of the central nervous 

 system does not really occur. An afferent impulse on entering the cord 

 spreads so as to involve a large variety of motor neurons, each of which 

 may, however, be excited through other afferent fibers arriving either 

 from other receptors or from higher nerve centers. The motor neuron 

 itself may therefore be a pathway occupied at different times by very 

 different types of nerve impulse. Hence it is appropriately called the 

 final common path, and its activity at any moment must depend on the 

 nature of the various afferent impulses that are transmitted to it through 

 the synapses. That each motor neuron must be at the service of several 

 afferent neurons becomes evident when it is remembered that in the 

 spinal nerve roots there are three times as many afferent fibers as there 

 are efferent fibers, and if the afferent fibers of the cranial nerves are 

 taken into account the proportion of afferent to efferent fibers becomes 

 five to one. Reflex connections involve usually one or more internun- 

 cial neurons, and these may act as a common path connecting several 

 receptors with a common motor mechanism. The propriospinal neurons 

 of the reflex arc for the scratch reflex described on page 93'5 act as a 

 common path for impulses arising from distinctly different parts of the 

 skin. 



The various reflexes which share the final common path leading to a 

 single group of muscles may cause these muscles to respond (1) in some 

 definite way as in the maintained contraction of the flexion reflex; 

 (2) in some different way, as in the rhythmic response of the scratch 

 reflex; or (3) inhibition may be produced so that no contraction can be 

 brought about. Reflexes belonging to any one of these groups are allied 

 reflexes, because they have a common effect on the motor mechanism. 

 Reflexes belonging to different groups, which consequently affect the 

 final common path to different purposes are antagonistic reflexes. Inte- 

 gration in reflex activity depends on the consequences which follow when 

 allied or antagonistic reflexes act upon the final common path, either 

 simultaneously or in rapid succession. 



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