INHIBITORY PATHWAYS TO MOTONEURONS 



J. C. ECCLES 



Department of Physiology, The AiistraHan National University, Canberra, A.C.T. 



It has now been shown that there are two distinct types of central inhibitory 

 action (cf. Frank and Fuortes, 1957; Eccles, 1961a). One is the conventional 

 type of inhibition that is associated with hyperpolarization of the postsynaptic 

 membrane, and this can appropriately be called postsynaptic inhibition. In 

 contrast, with the other there is no change in the postsynaptic membrane, but 

 instead a depolarization of the excitatory presynaptic terminals that pre- 

 sumably acts by diminishing the output of excitatory transmitter; hence, this 

 type may be called presynaptic inhibition. 



POSTSYNAPTIC INHIBITION 



I propose firstly to discuss the evidence relating to the simplest inhibitory 

 pathways that are concerned in postsynaptic inhibition, and then briefly to 

 consider more complex situations. 



Impulses in the largest afferent fibres (group la from the annulospiral 

 endings) of muscle exert a characteristically brief inhibitory action on moto- 

 neurons supplying antagonistic motoneurons (Lloyd, 1941, 1946a, b; Ren- 

 shaw, 1942). There was a detectable inhibition of a monosynaptic reflex 

 discharge even when the inliibitory volley preceded the excitatory by a small 

 fraction of a millisecond. Thus Lloyd (1946a) and Laporte and Lloyd (1952) 

 were led to postulate that la inhibitory and excitatory actions on motoneurons 

 have identical central latencies ; and hence that inhibition of motoneurons is 

 exerted through a monosynaptic central path, the designation direct inhibition 

 being appUed. However, these investigations on the latency of central inhibi- 

 tory action had the defect that the inhibition was being studied indirectly by 

 relating the interval between the volleys entering the cord to the amount of 

 inhibitory action on the monosynaptic reflex discharge. For our present pur- 

 pose it is important to note that the central latency of inhibitory action was 

 not measured; it was derived from the least volley interval for effective 

 inhibition by a calculation that has proved to be misleading. 



When intracellular recording revealed that inhibitory synaptic action on a 

 motoneuron was associated with a hyperpolarization (the inhibitory post- 

 synaptic potential or i.p.s.p.) wliich had a comparable time course (Brock 



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