IIAKKV C.KUNDFEST 121 



complex pathways and synaptic connections distribute messages and com- 

 mands to other nerve cells. 



Two general types of excitatory triggers must therefore be distinguished. 

 One is concerned with propagating activity along the extent of the cell in 

 which the bioelectric response is initiated. The other involves transmission, 

 or excitation of one cell (the post-junctional) by the activity of another (the 

 pre-junctional). Propagation or conduction is essentially a point-to-point affair, 

 the electrogenesis in an already active region producing activity in the next 

 neighboring region. This process therefore involves electrical excitation. At 

 each active site the response is a property of local conditions, and when these 

 are uniform, the response is everywhere identical. This type of activity is 

 admirably suited to conduction of messages in a relatively invariant form, or 

 decrement less conduction. 



Transmission, on the other hand, is a more complex phenomenon. Efforts to 

 account for it by electrical action (6i, 135), such as is responsible for conduction, 

 have proved unsuccessful (63, 98-101). The release of a specific transmitter 

 agent at the terminals of the pre-junctional fiber which then acts to excite the 

 post-junctional cell (56, 61, 141) therefore appears to be the sole mode of trans- 

 mission across the synaptic junction. Thus, the excitatory phenomena in post- 

 junctional cells generally involve first a triggering of transmission and then of 

 conduction. Furthermore, transmission may also involve processes of negative 

 excitation, or inhibition. 



VARIETIES OF ELECTROGENIC TRANSDUCERS 



Electrogenic cells may be classified according to the nature of the effective 

 stimulus for their transducer response, effective being defined as that stimulus 

 to which the transducer membrane is most sensitive.^ 



Sensory Receptors 



a) Mechano-Transducers. A mechano-transducer membrane exists in the 

 terminal sensory dendrites of stretch receptors in lobster and crayfish (68, 69, 

 188). The electrogenic response of the receptors (68, 69 and fig. i) is graded in 

 accordance with the degree of stretch, and persists as long as the stimulus is 

 applied. The terminal dendrites in which the mechano-transducer action takes 

 place are apparently incapable of supporting the explosive, all-or-nothing 

 electrical response which the rest of the cell can generate (cf. also 134). The 

 mode of action in the response may be diagrammed as follows: 



Stimulus — > Transducer action — * Electrogenic response 

 Mechanical -^ Membrane jjermeability change -^ Depolarization 



2 Transducer action is not necessarily linked with electrogenesis. Insulin apparently acts 

 at the membrane of some cells to permit entry of glucose (174), but if ionic transports do not 

 occur at the same time this would not involve a changed membrane potential. 



