Nervous Systems 791 



tain area of a neurone must be depolarized for it to be excited.-''^ The area 

 of the boutons of one axon is insufficient for antidromic excitation of that 

 axon. Hence impulses can summate from various converging axons and ex- 

 cite a motoneurone, but they cannot pass in the opposite direction.^''* 



In synaptic systems which are not structurally polarized, impulses can be 

 conducted in either direction, as across giant fiber septa and nerve net junc- 

 tions. In coelenterate nerve nets, conduction is diffuse and essentially equal 

 in all directions.^^"^' ^^^ If interdigitating lateral or central incisions are made 

 in a jellyfish or if the animal is cut into a zig-zag strip or into a central disc 

 connected by a narrow neck to an outer ring, impulses pass around corners and 

 in both directions, so long as nervous tissue is present. That this diffuse con- 

 duction is nervous is shown by conduction in areas where muscles are ab- 

 sent but nerve fibers present,^^*^ in regenerating areas where conducting but 

 not contracting tissue is present, and in preparations paralyzed with cur- 

 3jrg366 Qj. magnesium. ^■*- -^^' ^^^ Similar conduction across narrow nerve 

 bridges occurs in sea anemones^^^ and from zooid to zooid in colonial coel- 

 enterates such as Renilla.^^^ In sea anemones, however, there is some polar- 

 ity. Responses of the free end and of the central cut end of tentacles dif- 

 jfgj 313, 315 Through conducting tracts occur, particularly in mesenteries, 

 where velocities of 1.2 M./sec. exist, in contrast to local conduction at 

 0.04-0.15 M./sec. The velocity of conduction varies in different regions of 

 an anemone. ^^^ It is possible that polarity is imposed by differences in fa- 

 cilitation for conduction in different directions. 



The transverse septa of giant fibers are not polarized in Lumhricus, nor 

 are the more complex overlapping junctions of giant fibers in Neanihes^^ 

 and Cambarus^^^ It is likely that all non-polarized junctions are monosyn- 

 aptic, and that polarized junctions are polysynaptic. 



The Synaptic Transmitter. Synaptic transmission is a complex series of 

 events and little is really known of the sequence and role of the different 

 agents. Several suggestions have been made as to the mode of transmission of 

 excitation from the axon to the receiving neurone. Several agents and mech- 

 anisms have been proposed, including: (1) acetylcholine; (2) inorganic 

 ionic changes; and (3) electric current. 



ACETYLCHOLINE. In previous chapters it was shown that many cardio- 

 regulator nerve endings and some motor endings on muscles are cholinergic 

 (acetylcholine liberated at the junction), and some others are adrenergic 

 (adrenin liberated at the junction). The only interneuronic junctions at 

 which liberation of acetylcholine has been definitely demonstrated during 

 transmission are those of sympathetic ganglia. The superior cervical gan- 

 glion of the cat can be perfused by a branch of the carotid artery and the 

 perfusate collected from the internal jugular vein. When the preganglionic 

 nerve trunk is stimulated and the ganglion eserinized, acetylcholine is lib- 

 erated into the perfusate in amounts corresponding to the number of im- 

 pulses entering the ganglion. ^^^ Injected acetylcholine stimulates ganglion 

 cells to discharge. Acetylcholine is not liberated in appreciable amounts by 

 antidromic impulses, but it is still liberated on preganglionic stimulation 

 after transmission to the postganglionic neurones is blocked by nicotine, 

 curare, etc. Physostigmine (eserine) protects against destruction by cholin- 

 esterase of the Uberated ACh and sensitizes the ganghon cells to ACh.^^^ 



