86 The Conduction of Impulses by Nerves /4 : 5 



One spike potential on a presynaptic fiber may excite one spike 

 potential on the postsynaptic fiber. In some cases, a spike on any one 

 of several different presynaptic fibers may excite a spike on a given 

 postsynaptic fiber. At other synapses, one spike on a presynaptic fiber 

 will produce only a local subthreshold response. In this case, there 

 exists the possibility of adding subthreshold responses from several 

 synapses to produce a spike potential. Thus, the neuron can act as an 

 adder. Likewise, two, three, or more local responses in a short time 

 at one synapse may be necessary to produce a transmitted impulse. 

 Then the synapse is acting as a "divider." If several terminals from 

 one neuron cell synapse with differing time delays at the same second 

 neuron, then the original spike could be "multiplied." 



Neurons can likewise subtract. This is possible because not all post- 

 synaptic membranes are similar. For instance, ACh produces a spike 

 potential at motor end plates but inhibits heart muscles. (This inhibi- 

 tory effect of the ACh secreted by the vagus nerve endings in the heart 

 led to its original discovery.) At inhibitory junctions, the transmitter 

 substance increases the permeability to K + and larger cations but does 

 not alter the Na + or Li + permeability. The net result is a change in the 

 transmembrane potential and an increase in the local response necessary 

 at other synapses to start a transmitted spike. This produces, effectively, 

 subtraction of the impulses from two different incoming neurons. 



At the synapses, then, the arithmetic processes of addition, subtraction, 

 multiplication, and division can occur. Because the local responses 

 exhibit a complex time pattern, the calculus operations of integration 

 and differentiation can also be produced. However, the neurons are 

 not as simple as electronic circuits, and the various numerical processes 

 are also much more complex. This situation may be described in 

 mathematical terms by saying the system is nonlinear. For example, 

 a dividing synapse, if presented with three impulses, may transmit one ; 

 but seven will be necessary for two transmitted spikes and 14 or more will 

 be needed for three transmitted spikes. 



In addition, the synaptic conduction is altered by slow potential 

 fluctuations which are small compared to the membrane potentials and 

 by changes in the ionic content of the intracellular fluid. Aside from 

 the direct effects of K + and Na + , the Ca + + and Mg + + and particularly 

 their ratio alter the synaptic conduction. At the neuromuscular 

 junction, it has been shown that ACh is released in packets of the order 

 of 1,000 molecules from small vesicles in the nerve endings. The 

 probability of a given packet entering the intercellular fluid is a function 

 of the Ca ++ /Mg ++ ratio. 



To summarize this section, transsynaptic conduction usually occurs in 

 one direction. It may be mediated by electrical charge conduction or 



