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J. C. ECCLES 



is produced after synaptic relay of the group I afferent volley in the intermediate 

 nucleus. Provisionally, two alternative explanations, electrical or chemical, 

 may be suggested for the mechanism whereby group 1 muscle impulses produce 

 depolarization of group la presynaptic tenninals. 



According to the chemical hypothesis the intermediate neurons relaying 

 group I impulses make synaptic contact with the presynaptic terminals of 

 primary group la afferent fibres, i.e. axo-axonic synaptic contacts are postu- 

 lated, as shown diagrammatically in Fig. 8a. Impulses discharged from inter- 



FiG. 8. Diagrams showing possible pathways for presynaptic inhibitory action, 

 which is assumed to have an interpolated interneuron (//V). In a the axon of this 

 interneuron is shown making a chemical transmitting synapse on a la afferent 

 fibre close to its termination on a motoneuron {MN). in b electrical transmitting 

 synapses are shown from the la fibre to /N and from the axon of IN to the la 

 fibre near its termination. Arrows indicate path of intracellular current flow 



(see text). 



mediate neurons are assumed to cause the release of transmitter substance at 

 such synapses, which depolarizes the la presynaptic terminals. According to 

 this explanation part at least of the potential field is attributable to depolariza- 

 tion of the presynaptic la fibres in the ventral horn. However, the slow time 

 course of the depolarization tells against this postulate of axo-axonic chemical 

 transmission. Admittedly the effect is but little longer than the synaptic 



