IRRADIATION OF IMPULSES WITHIN THE CORD 



521 



are more complex in character. The afferent nerve impulse passes through 

 more than one simple channel in the cord, so that a series of coordinated 

 acts occurs in what may be called a complex reflex. 



The transmission of impulses within the cord occurs over the pathways 

 of least resistance. Increasing the number of synapses, or the number of 

 neurone links in the chain of conduction, increases the resistance so that re- 

 flexes will occur most readily, other conditions being equal, where the least 

 number of neurones is involved, i.e., in the same segment of the cord in which 

 the sensory impulse enters, or in immedi- 

 ately adjacent segments. In addition to the 

 number of synapses in the reflex arc, other 

 factors are of importance in determining reflex 

 reaction; e.g., the intensity of the exciting 

 stimulus; the quality of the stimulus; the 

 rapidity of the recurrence of the stimulus; 

 and the duration of its application. Thus, 

 a strong stimulus will bring about a reflex 

 reaction sooner than a weak stimulus of the 

 same kind. A single weak stimulus which 

 will cause no reflex may do so if often enough 

 and rapidly enough repeated, the phenomenon 

 of summation of stimuli. 



A reflex act once started may result in 

 efferent impulses which continue for some 



time after the exciting cause has been removed. The same phenomenon 

 is observed where groups of nerve cells are stimulated directly. It has been 

 found, by observing electrical changes in nerve fibers by means of the capillary 

 electrometer, that when their cells of origin are stimulated they discharge im- 

 pulses in a rhythmical manner. 



Usually, impulses are transmitted to a nerve cell only over its dendrons, 

 but it must be also assumed that such a conveyance of impulses may take place 

 over the collaterals of its axone near the cell body, or the cell body may be 

 stimulated directly by the afferent neurone. The peripheral fiber of the 

 spinal ganglion cell, although it has the structure of an axone, may be looked 

 upon physiologically as a dendron, since homologues in lower vertebrates and 

 in man himself (olfactory-nerve cells) have this structure, the nerve cell body 

 being situated near the sensory surface from which impressions are received. 



Irradiation of Impulses within the Cord. Taking as an example a 

 frog whose brain has been destroyed, a simple reflex may be demonstrated by 

 irritating the skin of one foot with a weak stimulus. In response to such a 

 stimulus the foot is flexed upon the leg, due to a contraction of the muscles of 

 the reflex arc corresponding to the sensory surface irritated. If the strength or 

 duration of the stimulus be increased, other groups of muscles are involved 



PIG. 366. Showing the Arrange- 

 ment of the Reflex Mechanism, with 

 a Neurone Intercalated between the 

 Sensory and Motor Neurones. 



