INHIBITORY PATHWAYS TO MOTONEURONS 55 



a latency about 1 msec longer than the monosynaptic excitatory action (Pres- 

 ton and Whitlock, 1960). 



In summary it can be stated that there is no recorded instance of central 

 inhibitory action directly exerted by primary afferent fibres or by volleys in 

 descending tracts. In every case an interneuron with a short axon is inter- 

 polated. It has been suggested that this device is introduced in order to change 

 the chemical transmitter mechanism from an excitatory to an inhibitory type 

 (Eccles, Fatt and Landgren, 1956; Eccles, 1957). It should be noted that, as 

 described in another Chapter (Eccles. 1961b), the specific ionic permeabilities 

 produced by the inhibitory chemical transmitter provide the only hypothesis 

 that can at present account for the membrane hyperpolarization that is 

 characteristic of the i.p.s.p. 



PRESYNAPTIC INHIBITION 



This type of inhibition is characterized by a diminution of tiie mono- 

 synaptically produced e.p.s.p. with no change in the potential or excitabihty 

 of the postsynaptic membrane. It was first described by Frank and Fuortes 

 (1957) and by Frank (1959). who termed it remote inhibition because it was 

 exerted remote from the motoneuron soma, though no decision was made 

 between possible presynaptic or postsynaptic sites of action or of the possible 

 modes of action. It has now been shown (Eccles. Kozak and Magni. 1960; 



A 





-^ E 



J.2 20 



"^^^^ 1 "imimmm.m...... 



1^ "■ 



msec 



msec 



Fig. 6. a-c. Upper traces show /dorsal root records and lower intracellularly 

 recorded e.p.s.p.'s set up by a gastrocnemius afferent volley in a gastrocnemius 

 motoneuron. Note that, compared with the control in a, the e.p.s.p. is diminished 

 when preceded by a biceps-semitendinosus volley, the effect of a group la volley 

 (b) being much less than a la, lb volley (c). In d and e a biceps-semitendinosus 

 volley of la or la -j lb composition, respectively, sets up a dorsal root reflex into 

 the nerve to gastrocnemius, which is much larger for la i lb (e) than for la 

 only (d). Note difference in time scales for the first and second components of 

 D and E (Eccles, Kozak and Magni, unpublished observations). 



