902 THE CENTRAL NERVOUS SYSTEM 



Characteristic Properties of the Reflex Arc. Conduction in reflex 

 arcs shows certain peculiarities when compared with the conduction 

 in nerve-trunks already studied (p. 781) : (i) The direction of the 

 reflex conduction cannot be reversed. There is an absolute block 

 on the passage of impulses backwards through a synapse. (2) The 

 velocity of conduction over the whole reflex arc is much smaller 

 than over a nerve-trunk of equal length. Both of these differences 

 depend mainly on the fact that the impulses must be transmitted 

 from one neuron to another, and very likely on a fundamental 

 property of the synapse. The delay or ' lost time ' in the discharge 

 of the efferent impulses which constitute the reflex response to the 

 excitation of an afferent path increases with the complexity of the 

 response that is, with the number of neurons and therefore of 

 synapses involved in it. (3) The reflex arc is easily fatigued, easily 

 affected by deprivation of oxygen and by drugs, in comparison with 

 the nerve-trunk. This difference is due to the portion of the arc 

 in the grey matter, including the synapse or synapses. Fatigue 

 expresses itself by an increase in the degree of block or resistance to 

 the passage of impulses along the arc. (4) The reflex end-effect may 

 much outlast the stimulus in other words, "a marked ' after-dis- 

 charge ' is characteristic of reflexes. The more intense the stimulus 

 which liberates the end-effect, the greater is the duration of the after- 

 discharge. For example, the ' crossed extension reflex ' (extension 

 at the knee, ankle, and hip, produced in the spinal dog by stimula- 

 tion of the skin of the opposite or contralateral hind-limb), when 

 provoked by a stimulus of more than a certain intensity, may outlast 

 the stimulation by ten or fifteen seconds, and the after-discharge 

 may be stronger than any other part of the reflex (Sherrington). 

 (5) A succession of impulses may easily passj.lojjg a reflex arc when 

 one of the series would fail to pass (temporal summation). This 

 does not occur in a nerve-trunk. The first stimulus, though itself 

 unable to produce the reflex effect, facilitates the action of succeeding 

 stimuli, so that summation of the impulses occurs in the cord 

 (Stirling). A stimulus e.g., a make-induction shock, far too weak 

 to produce the scratch-reflex when applied once only to a point of 

 that area of skin from which the reflex is normally elicited has been 

 seen to cause the reflex after more than forty shocks had been 

 delivered at the rate of eighteen per second. The facilitation of the 

 passage of an impulse by the previous passage of impulses along the 

 same reflex path recalls a somewhat similar phenomenon already 

 alluded to in connection with the conduction of the propagated 

 disturbance in nerve- fibres (p. 791), although in the case of the reflex 

 arc the effect, it may be supposed, is exerted upon the fields of 

 conjunction, including the synapses, between the different neurons. 

 There is reason to believe that summation in the reflex arc is mainly 

 achieved by the removal of block or resistance. The phenomenon of 



