neurophysiology: an integration 



■937 



trip mechanism (174). The more random threshold 

 fluctuations may, like those at the end plate, represent 

 the chance release of presynaptic transmitter packets; 

 but the presence of comparable random fluctuations in 

 the threshold of nerve fibers, where such packets are 

 not involved, leaves the question open. 



If 'pure' inhibition be restricted to hyperpolariza- 

 tion of the synaptic region of the postsynaptic mem- 

 brane, presumably by an inhibitory transmitter (128), 

 then many other types of central inhibition exist. Not 

 only can exciting impulses be cut off from the goal 

 neuron by inhibitory inputs, but also the effectiveness 

 of such impulses might be altered by changes in pre- 

 synaptic terminals (with less transmitter release on 

 activation, for example) or in the postsynaptic mem- 

 brane outside of the receptor site. A more stable mem- 

 brane or one with greater conductance would decrease 

 elcctrotonic spread from the activation site in the 

 dendrite to the axon segment which might fail to re- 

 ceive the minimal current needed to excite it ((>■-;, ti.j 1. 

 Whether GABA, or substance I [or even the epineph- 

 rine group — or histamine, e.g. (194)], is a true inhibi- 

 tory transmitter or is a 'modulator' acting in this 

 fashion is in active debate [see Roberts (247) 



Still other types of 'inhibition' relate to the rebound 

 hyperpolarization seen after repetitive activation 

 (276), or to an excessive depolarization that precludes 

 spike propagation. [-Compare this with striated muscle, 

 which Jenerick & Gerard ( 1 50) found to give only lo- 

 cal responses when depolarized to a fixed level, and 

 with the action of methonium at the motor end plate 

 shown by Paton (229).] Some nerve fibers can either 

 inhibit or excite a given muscle or nerve cell (276, 

 293), depending on local conditions at the junction 

 [cf. Hagbarth & Fex (130)]. 



In any event, the qualitative differences in neu- 

 rons, the existence of excitatory and inhibitory con- 

 nections, the differential sensitivity to drugs of differ- 

 ent neurons and different parts of a neuron, the pres- 

 ence of specific receptor elements within the nervous 

 system with highly developed individual chemical 

 sensitivities, and the generally greater richness possible 

 when a qualitative particularization is added to an 

 indiscriminate quantitation, all indicate the impor- 

 tance and ubiquity of chemical factors in maintaining 

 and manipulating neuron thresholds. 



junction properties. It is often difficult to assign 

 specific aspects of the synaptic mechanism to the ac- 

 tual junction. Changes in presynaptic spikes and eddy 

 currents or in transmitter store and release, as well as 

 threshold changes in the postsynaptic cell body and 



its processes, and in the spatial interaction of manv 

 junctions, all can contribute to the success or failure 

 of a message in crossing a given synapse. Spatial sum- 

 mation and after-discharge effects may well be post- 

 synaptic; temporal summation and more delayed time 

 changes are at least restricted to the particular in- 

 coming pathway — witness the posttetanic potentia- 

 tion effects. Further, the enduring changes of activity 

 or inactivity presumably involve the junctional region, 

 but the evidence on this is not decisive. Moreover, the 

 vigorous movements of the processes of living neurons, 

 as well as the active chemical turnover in these cells, 

 raise problems regarding specific information storage 

 that are not easily answered. The extent to which the 

 modulation of channel transmissivity depends on the 

 junction is also uncertain. Feed-back controls on 

 transmission, chemical alteration of thresholds, and 

 Stead) or transient potential fields and currents prob- 

 .iblv act on the postsynaptic elements rather than up- 

 stream to them. 



Interaction Patterns 



The nervous swnri uses an alphabetic rather than 

 an idiographic language. Large numbers of units of 

 relatively few kinds are combined in different patterns 

 10 give particularized meanings and behaviors. Three 

 sons of mechanisms of interaction can usefully be dis- 

 tinguished: particular synaptic networks < 51, 52, 1 ; ;. 

 201, 203, 204, 212), chemical and electrical fields (97; 

 Fessard, O'Leary & Gold ring), and, a combination 

 of the above, certain statistical properties of large cell 

 aggregates or masses (7, 24, 239, 240). Beyond these 

 general relations .ire the various particular ones de- 

 pending on the major neural structural and functional 

 sv stems. 



Field mechanisms lend themselves to synchroniza- 

 tion of action of masses o! neurons and to mass effects 

 in general. Nerve nets and assemblies lend themselves 

 to specific activity patterns and to reverberation. 

 Both mechanisms, generalized to statistical aggre- 

 gates, can produce slowly moving waves (71, 118, 

 179). It is worth noting that reverberation and pat- 

 terned activity in general demand successive, and 

 therefore nonsv nchronous, activity of separate neu- 

 rons, whereas field effects and synchrony demand the 

 simultaneous activity of masses of neurons. Certainlv 

 a given neuron cannot successfully interact with its 

 fellows both simultaneously and sequentially, except 

 during successive time periods. It is even worth con- 

 sidering that certain neurons may normally be in- 

 volved via the one mechanism, others via the second. 



