380 Information Storage and Neural Control 



of arrival of impulses at points unequally remote from the same 

 source. Excitation across synapses occurs predominantly from 

 axonal terminations to somata. It is still a moot point whether 

 this depends upon irreciprocity of individual synapses or merely 

 upon prevalent anatomical configurations. To suppose the latter 

 requires no hypothesis ad hoc and explains known exceptions, but 

 any assumption as to cause is compatible with the calculus to 

 come. No case is known in which excitation through a single syn- 

 apse has elicited a nervous impulse in any neuron, whereas any 

 neuron may be excited by impulses arriving at a sufficient number 

 of neighboring synapses within the period of latent addition, which 

 lasts less than one quarter of a millisecond. Observed temporal 

 summation of impulses at greater intervals is impossible for single 

 neurons and empirically depends upon structural properties of the 

 net. Between the arrival of impulses upon a neuron and its own 

 propagated impulse there is a synaptic delay of more than half 

 a millisecond. During the first part of the nervous impulse the 

 neuron is absolutely refractory to any stimulation. Thereafter its 

 excitability returns rapidly, in some cases reaching a value above 

 normal from which it sinks again to a subnormal value, whence 

 it returns slowly to normal. Frequent activity augments this sub- 

 normality. Such specificity as is possessed by nervous impulses 

 depends solely upon their time and place and not on any other 

 specificity of nervous energies. Of late only inhibition has been 

 seriously adduced to contravene this thesis. Inhibition is the ter- 

 mination or prevention of the activity of one group of neurons by 

 concurrent or antecedent activity of a second group. Until recently 

 this could be explained on the supposition that previous activity 

 of neurons of the second group might so raise the thresholds of 

 internuncial neurons that they could no longer be excited by 

 neurons of the first group, whereas the impulses of the first group 

 must sum with the impulses of these internuncials to excite the 

 now inhibited neurons. Today, some inhibitions have been shown 

 to consume less than one millisecond. This excludes internuncials 

 and requires synapses through which impulses inhibit that neuron 

 which is being stimulated by impulses through other synapses. 

 As yet experiment has not shown whether the refractoriness is 

 relative or absolute. We will assume the latter and demonstrate 



