NEURONAL INTEGRATIVE MECHANISMS 5 



gain in significance if we believe that integrative neurons are typically 

 under tonic subthreshold influence. 



The excitability we have spoken of so far is excitability to artificial stimu- 

 lation of limited kinds. But the outstanding characteristic of synapses is 

 their sensitivity to some consequence of activity in other neurons, and we 

 need not be concerned here whether that is one or more specific chemicals 

 or nonspecific ions. We do need to note : ( 1 ) that the response may be 

 excitation or inhibition; (2) that one and the same input citannel (pre- 

 synaptic fiber) can cause one or the other, depending on the level of the 

 membrane potential of the postsynaptic neuron; (3) that different pre- 

 synaptic fibers can cause response of the same sign but different rates of 

 rise, jacilitation, maximum height, etc., as in crustacean muscle; (4) that 

 these differences may be discontinuous and unequal in the proportional 

 role of the several characters measured ; (5) that summation of the differ- 

 ent kinds of input in the same postjunctional cell may be complex (crusta- 

 cean muscle, crayfish central giant-to-motoneuron synapse) ; and (6) that 

 inhibition is not just the reciprocal of excitation as measured by its effect 

 on various aspects of activity. 



A highly variable property of the utmost importance for integration is 

 the response of the postsynaptic unit to repeated presynaptic impulses. In 

 some cases (e.g., inhibitory escape in the cardiac ganglion of lobsters) the 

 initial eft'ect of a sustained barrage gradually diminishes as measured by 

 the output of the postunit, reaching a plateau at a new level (Fig. 2). In 

 the example referred to this happens in some seconds at a frequency of 

 presynaptic activity of 20-40 per sec. In other cases (e.g., the synapse of 

 pacemaker upon follower cell in the same ganglion) there is actually what 

 may be called de jacilitation — the postsynaptic potential to the second pre- 

 synaptic impulse is less than to the first, the difference being proportional 

 to the frequency. This may be regarded as a consequence of relative refrac- 

 toriness. Its importance lies in the fact that it happens in a frequency range 

 within the normal firing range of these ganglion cells. The more familiar 

 cases are those of jacilitation — which should be distinguished from tem- 

 poral summation by the criterion that each response or increment is greater 

 than the last. The magnitude of facilitation and its rate of growth and decay 

 vary widely. As an example, their consequences can be clearly seen in 

 Wiersma's comparison (1952b) of the responses to the same average fre- 

 quency delivered with alternately long and short intervals and delivered 

 with uniform intervals. Some junctions give the same response (small, 

 slowly decaying facilitation) , others respond enormously more to the paired 

 train (large, rapidly decaying facilitation). 



Another group of properties with profound influence upon output, es- 

 pecially in the formation of patterned bursts, is in the domain of ajtcr- 



