CENTRAL NERVOUS SYSTEM. 



223 



Effects of Afferent Impulses. — Studies on inactivity show that a certain 

 amount of exercise in any given cell is necessary for its proper nutrition, and 

 if the excitation fall below the point which causes this, the responsiveness of 

 the cell is diminished. 



For example, a strychnized reflex frog on being dipped into a solution of 

 cocaine loses in so large a measure its irritability that its responsiveness falls 

 far below that of a normal frog. 1 



In this case the central system is deprived by the action of the cocaine of 

 the impulses which even in the absence of any special form of irritation 

 normally arrive from the skin, and the abolition of these impulses causes a 

 diminution in central responsiveness. Effects which can thus be accom- 

 plished in a few seconds by cutting off the afferent impulses from the skin 

 may of course follow any slow diminution in these impulses, although all 



40- 

 30- 

 20- 

 10- 







10- 



20- 



30- 



\ 



Normal. 



O.I"0.2" 0.4" 



0.7- 



10" 



1.7" 



Fig. 95.=Showing in millimeters the amount by which the "reinforced" knee-kick varied from the 

 normal, the level of which is represented by the horizontal line at 0, "normal." The time intervals 

 elapsing hetween the clinching of the hand 'which constituted the reinforcement) and the tap on the 

 tendon are marked below. The reinforcement is greatest when the two events arc nearly simultaneous. 

 At an interval of 0.4" it amounts to nothing; during the next 0.6" the height of the kick is actually 

 diminished the longer the interval, after which the negative reinforcement tends to disappear; and 

 when 1.7" is allowed to elapse the height of the kick ceases to be affected by the clinching of the hand 

 (Bowditch and Warren). 



such slow changes arc much more likely to be accompanied by some sort of 

 compensation whereby oilier afferent impulses in a measure take the place of 

 those which have been suppressed, 'fhe loss of these impulses which rouse 

 the cells to activity is usually a more important condition than direct nutri- 

 tive change, and must for this reason always be kept in view. 



Inhibition. — Evidence is accumulating to show that all the active tissues 

 of the body may be influenced through their nerves in two opposite ways. 

 That is, stimulation may increase or diminish their activity." Thus the 

 physiological processes in the glands, nerve-centres, or muscles can be so 

 varied. In most cases, nerves which cause inhibition are, except in the 

 central nervous system, distinct from those causing increased activity. The 



1 Poulsson : Archiv fiir experimentelle Pathologie and Pharmakologie, L885, Bd. \xvi. 

 'Meltzer: "Inhibition," JVeto York Medical Journal, May, 1899. 



