CHAPTER XVI 

 REFLEX ACTION 



As pointed out in the preceding chapter, the reflex is to be regarded as the 

 functional unit of the nervous mechanism. Its general nature was described in 

 that place. In exceptional cases, such as the knee-jerk, the reflex arc may, 

 apparently, consist of two neurones only, but, as a general rule, three at least are 

 contained in it, as in the scratch reflex. 



SPINAL REFLEXES 



For the investigation of the characteristic properties of the spinal reflexes, it is 

 clearly necessary to obtain a preparation in which the spinal cord is separated 

 from the higher centres and has recovered from shock. Such an animal is called 

 by Sherrington the spinal animal, and it is by his work that the possibility of 

 maintaining such animals alive and healthy has been demonstrated. Nearly all 

 the results to be described below are due to Sherrington, whose portrait will be 

 found in Fig. 150. 



Sherrington then sums up the chief differences between conduction in nerve 

 trunks and in reflex arcs as follows (1906, p. 14): "Conduction in reflex arcs 

 exhibits (1) slower speed as measured by the latent period between application of 

 stimulus and appearance of end-effect, this difference being greater for weak 

 stimuli than for strong ; (2) less close correspondence between the moment 

 of cessation of stimulus and the moment of cessation of end-effect, i.e., there is a 

 marked 'after-discharge'; (3) less close correspondence between rhythm of 

 stimulus and rhythm of end-effect ; (4) less close correspondence between the 

 grading of intensity of the stimulus and the grading of intensity of the end-effect ; 

 (5) considerable resistance to passage of a single nerve impulse, but a resistance 

 easily forced by a succession of impulses (temporal summation) ; (6) irreversibility 

 of direction instead of reversibility as in nerve trunks ; (7) fatigability in contrast 

 with the comparative unfatigability of nerve trunks ; (8) much greater variability 

 of the threshold value of stimulus than in nerve trunk ; (9) refractory period, 

 ' bahnung ' (or facilitation), inhibition and shock, in degrees unknown for nerve 

 trunks; (10) much greater dependence on blood-circulation, oxygen (Verworn, 

 Winterstein, von Baeyer, etc.); (11) much greater susceptibility to various drugs, 

 anaesthetics." 



These differences are obviously due to the passage through synaptic junctions; 

 perhaps, in some cases, passage through the cytoplasm of the cell body of some 

 constituent neurone may play a part. 



We will consider some of these in a little more detail. 



Latent Period. The measurements of Jolly (1910) in the case of the flexion 

 reflex and the knee-jerk have been given on page 476 above. 



The more intense the stimulus, the shorter the latent period, so that with 

 intense stimuli to the afferent nerve, the delay may scarcely exceed that of the 

 conduction along the nerve trunks alone. The difference between strong and weak 

 stimulation may amount to as much as ten times or more. This delay might be 

 due to the time occupied in setting the synapse into a state of capability of 

 transmission, but experiments by Sherrington (1906, p. 24) did not support 

 this view. A reflex was evoked by a weak stimulus, with a certain latency ; 

 the strength of the stimulus was then suddenly increased, and the latent 



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