THOUGHTS ON INHIBITION IN THE SPINAL CORD 23 



mental conduction in nerve by Lorente de No and Condouris (1959) serves . 

 to focus attention upon decremental conduction generally and upon the 

 properties of the cell body and dendrites in particular. A large body of 

 evidence (Renshaw. 1942; Lloyd, 1951a, b, 1959) testifies to the nonnal 

 existence of a conduction decrement, at least in the cellulifugal sense, in the 

 dendrites of spinal motoneurons. Assuming that dendritic conduction in the 

 dendrites of spinal motoneurons is decremental also in the cellulipetal sense 

 we have at hand all the necessary requirement for inhibition to take place 

 according to an interference hypothesis of the Keith Lucas type (cf. Lorente 

 de No and Condouris, 1959). Whilst thinking in this wise one should not 

 neglect the potentialities, of which we know little, for decremental conduction 

 in the fine presynaptic structures. 



Recently another use has been made of place theory which accords well 

 with the anatomical observations of Sprague. Frank and Fuortes (1957) and 

 Frank (1959) have found examples of direct inhibition in which a recordable 

 inhibitory postsynaptic potential is not generated in the inhibited moto- 

 neurons. This effect they call "remote inhibition". One of the two explana- 

 tions advanced is that the inhibitory endings concerned may be located so 

 far out on the dendrites that an intracellular microelectrode in the cell body 

 may not "see" the hyperpolarization otherwise presumed to occur. This is 

 imperceptibly different from an hypothesis of the Keith Lucas type. Their 

 other suggestion concerning remote inhibition supposes a presynaptic block, 

 the inhibitory fibers blocking excitatory presynaptic impulses but not them- 

 selves reaching the motoneuron. This latter suggestion, one will appreciate, 

 is a direct descendant of the ideas advanced by Renshaw (1946) and by 

 McCulloch et al. (1952) which were discussed earlier. It is not a part of place 

 theory. 



It is not easy to judge the role that place theory may have in future formula- 

 tions of inhibition. The anatomical evidence of Sprague (1958) is suggestive, 

 but whether or not location of inhibitory endings on dendrites confers some 

 essential quality to their action remains problematical. 



Inhibitory Postsynaptic Potential 



Positive potentials associable with depression in the spinal cord have been 

 known for some time (Gasser, 1937; Bernhard and Skoglund, 1947; Bernhard 

 et al., 1947). Since the discovery by intracellular recording (Brock et al., 

 1952) that a so-called inhibitory postsynaptic potential could, as in other 

 tissues, be recorded from motoneurons of the spinal cord much has been 

 made of this deflection, the time course of which somewhat resembles that 

 of direct inhibition. Several pertinent questions must be raised concerning 

 the role of inhibitory postsynaptic potentials. 



Firstly, is the i.p.s.p. a necessary accompaniment of inliibition in the 



