SYNAPTIC AND EPHAPTIC TRANSMISSION 



165 



Interaction of Graded Responses 



Generated and propagated in electrically inexcit- 

 able membrane, p.s.p.'s can spread only by electro- 

 tonus (fig. 2), passively, without evokina; new activity 

 and with considerable decrement. As weak depolariza- 

 tions, p.s.p.'s acting upon adjacent, electrically excit- 

 able membrane may evoke graded local responses 

 (figs. 3, 7, 13). The latter are also decrementally 

 propagated, but the decrement may be smaller than 

 in the case of p.s.p.'s. The depolarizing activity of a 

 graded local response at one site may, in turn, give 

 rise to some degree of active response at other sites. 

 Thus, depending upon the local excitability of the 

 membrane and the amount of initial local response, 

 this graded depolarization may spread only passively, 

 or it may propagate with various degrees of active 

 contribution. The ultimate extent of the latter is that 

 which evokes a spike. This explosive process domi- 

 nates subsequent events since the magnitude of its 

 electrical activity usually far exceeds the require- 

 ments for continued local circuit electrical excitation. 

 In other words, when the spike generator has a high 

 safety factor, decrementlcss propagation is the rule. 



The nature of graded local responses of electrically 

 excitable membrane will be discussed below (p. 167) 

 in conjunction with the mechanisms of gradation of 

 p.s.p.'s. Here, it is desired to stress that the two 

 graded responses provide a pathway for summative 

 gradation as a transition to the all-or-none spike 

 (fig- 3)- 



EVENTS IN SYNAPTIC TRANSMISSIO.V 



Functional Interrelations Within Single Cell 



A generalized schema of the activities within a 

 single unit in a transmission chain is shown in figure 

 16. The input of the cell, the synaptic surface in the 

 present context, but which may also be the receptor 

 surface of a sensory cell (cf. 94, 96, 97), is activated 

 by a specific chemical stimulus and develops an 

 electrical response. Only the depolarizing variety, 

 excitatory for the conductile mechanism, need be 

 considered now. The p.s.p. may ije brief or long and 

 may give rise to a single spike or to a train of impulses. 

 This conductile activity, arriving at the terminus of 

 the cell, causes secretory activity which releases a 

 transmitter agent that can excite another unit of the 

 transinission chain or an effector. 



INPUT , CONDUCTILE | OUTPUT 



GENERATOR 

 ACTIVITY 



CONDUCTILE 

 ACTIVITY 



TERMINAL 

 ELECTROGENESIS 



FIG. 16. Diagrammatic representation of functional com- 

 ponents and electrical responses of a receptor cell or neuron. 

 The electrically inexcitable input produces electrogenesis 

 graded in proportion to its specific stimulus and sustained as 

 long as the latter is applied. The possibility of hypcrpolarizing 

 electrogenesis is shown but is not further considered. The de- 

 polarization at the input, operating upon the conductile elec- 

 trically excitable component, can evoke spikes in the latter 

 coded in number and frequency in proportion to the depolari- 

 zation. These signals, propagated to the output, there command 

 secretory activity, roughly proportional to the information en- 

 coded in their message and sustained as long as the message 

 demands. The transmitter released at the output can initiate a 

 synaptic transfer by operating upon the depolarizing input of 

 another cell. The possibility of a special output electrogenesis 

 is indicated but is not further considered. The lower electrical 

 portion of this diagram may be compared with records from a 

 sense organ (fig. 10). [From Grundfest (97).] 



Evolution of Electrogenic Membrane 



The occurrence of receptor-effector cells in primi- 

 tive metazoa suggested to Parker (154) that the nerv- 

 ous systein ev'olved by parcellation of the two func- 

 tions among separate receptor and effector cells with 

 the interposition of a conductile element extending 

 from the receptor cell. Later in evolution, correlational 

 neuronal cells were presumed to have arisen. This 

 evolutionary schema may also be applied to the 

 individual cells, neurons and muscle fibers as well as 

 receptors (103). The receptor portion of the priinitive 

 unit was probably sensitive to specific stimuli and this 

 characteristic is retained at the electrically inexcitable 

 input of the present nerve cell, mu.scle fiber, gland or 

 receptor (fig. 16). The ouptut likewise inay be con- 

 sidered as representing the primitive effector, frankly 

 so in the contractile muscle fibers or in glands. The 

 terminals of the neurons likewise probably embody 

 the secretory capacity of primitive units adapted to a 

 new function, transmission at close contact. Other 

 neurosecretory cells of more general function are also 



