354 H. MCLENNAN 



Structure; it is not present in peripheral nerve or spinal roots. A more detailed 

 study of its distribution was made by Florey and Florey (1958), who showed 

 that it was largely concentrated in the grey matter of the extrapyramidal 

 centres, with little in white matter other than the superior cerebellar peduncle, 

 optic tract and crus cerebri: however, their conclusion was that "it is not 

 possible to estabhsh a definite correlation between location and function of 

 Factor I". The results were not inconsistent with the role of Factor I as an 

 inhibitory transmitter, although Florey and Florey considered that at some 

 sites it might have excitatory actions as well. 



Topical apphcation of Factor I solutions to the exposed spinal cord results 

 in a prompt and complete inhibition of monosynaptic tendon jerk reflexes 

 (Florey and McLennan, 1955b). This eff"ect is prevented by the administration 

 to the animal of small doses of strychnine (Fig. 3), which Curtis (1959) has 



Fig. 3. Records of the movements of the lower leg of a decerebrate cat in response 

 to tapping the patellar tendon. At the arrows Factor I solution was applied to 

 the exposed spinal cord; a before, b 5 min after the intravenous administration 

 of 008mg/kg strychnine. (Florey and McLennan, 1955b.) (Courtesy of the 

 Journal of Physiology.) 



shown to prevent the action of afferent inhibitory stimulation on the neurons 

 of the cord. Here then is a more direct connection between Factor I and in- 

 hibitory synaptic action, since a drug which affects the action of the physio- 

 logical pathway similarly affects the action of the applied extract. Factor I 

 also blocks synapses in sensory pathways (Honour and McLennan, 1960); 

 but a more disquieting note is introduced by the finding that it likewise 

 prevents the discharge of postganglionic cells in some sympathetic ganglia 

 (Florey and McLennan, 1955a), and in this last instance there is no indication 

 for the presence of a physiological inhibitory nerve supply. It would seem 

 then that Factor I may have properties similar to those of the inhibitory 



