REFLEX ACTION 507 



extraneous stimuli are excluded, is a necessity. On this point, the article by 

 Pavlov (1911) may be consulted. 



Pavlov's address given at the Physiological Congress at Groningen (Pavlov, 

 1913), and his paper (1912) should be read. A research by Orbeli (1909) may 

 be referred to as an example of some of the kinds of work done already in 

 Pavlov's laboratory. 



Many of the facts described above were given me by Prof. Babkin and 

 Dr Anrep, who had taken part in numerous experiments. 



SUMMARY 



The contrast between the effects produced by reflex stimulation of nerves and 

 those produced by direct stimulation of efferent nerves is due to the passage through 

 synaptic membranes in the former case. 



The increased delay in the case of reflexes is not owing to a need of time to 

 make the synapse conducting, nor for " amoeboid " movement of cell processes into 

 contact, but to actual passage through a resistance. 



The discharge does not, as a rule, cease when the stimulus ceases, but lasts for 

 a varying time afterwards. This after-discharge can be cut short by inhibition, 

 and very sharply. 



Repeated subminimal stimuli are capable of evoking a reflex ultimately. In 

 some cases, as that of the scratch reflex, a single induction shock, however strong, 

 will not do so. 



Two reflexes making use of the same final common path may reinforce one 

 another. This, together with inhibition, plays an important part in reactions to 

 " constellations " of stimuli. 



If the skin area for the scratch reflex is stimulated at two different points with 

 subminimal intensity, both stimuli act on the whole centre and produce the reflex 

 by "immediate spinal induction." 



After a period of inhibition, there is frequently an increased excitability, 

 which is not shown when the stimulus is merely removed for a period equal to 

 that of the inhibition. This rebotmd is not, therefore, due to rest alone. 



The synapse between an axis cylinder and the cell body or dendrites of another 

 neurone will only allow impulses to pass in the direction named, and not from 

 the cell body back to the axis cylinder of another neurone. This has been proved 

 experimentally in the case of motor neurones, but there is evidence to show that 

 the synapses of dorsal root fibres (sensory) with cells in the spinal cord is 

 permeable in both directions. 



The phenomenon of the refractory period is well marked in reflexes. In the 

 case of the motor neurones used as final common path by the scratch reflex and 

 also by the flexion reflex, the long refractory period must be situated in some 

 neurone on the afferent side, since the latter reflex is not rhythmic. 



When the contraction of a group of muscles, necessary for a particular reflex, 

 can be opposed by that of an antagonistic group, it is found that along with 

 contraction of the one group there is relaxation, by inhibition of the centre, of 

 the other group. This is the phenomenon known as reciprocal innervation, and 

 was appreciated by Descartes in the case of the eye muscles. The seat of the 

 inhibitory component of the reflex appears to be either at the synapse with the 

 motor neurone of the final common path, or in an intermediate neurone very 

 close to this. 



In some cases, where smooth muscle is the effector, there is reciprocal innerva- 

 tion of peripheral origin. Thus, stimulation of the efferent nerve to the muscles 

 causes contraction of one muscle and inhibition of its antagonist. The claw of 

 the crayfish and the dilatation of the pupil may be mentioned. 



Under natural conditions, a reflex arc is played upon by various afferent 



