808 Comparative Animal Physiology 



the discharge of the smaller motor cells.^^^' ^^^ When certain nerves are cut 

 (crustacean nerves), or when one cut end from each of many types of 

 nerve is dipped into a citrate solution, the injury sets up activity which be- 

 comes synchronized, possibly by electrical means.'' Electrical polarization of 

 pacemaker neurones in the Limulus cardiac ganglion increases the synchron- 

 ization and frequency of discharge.*^^ 



2. Circus conduction in a closed ring with side-branching neurones can 

 maintain a synchronized rhythm (Fig. 302, C). Interruption of subcortical 

 tracts in mammals alters the brain-wave synchrony. Similarly the respiratory 

 center is composed of several centers, and interconnections among these are 

 essential for synchronized respiratory discharge.^^^ 



The significance of "spontaneous" activity in central nervous systems is 

 not clear. It may have a motor effect, although muscle tone is largely re- 

 flex. More probably spontaneity represents a state of "tone" or high excit- 

 ability in the central neurones. 



NERVE NETS 



In the preceding discussion of the properties of synaptic conduction many 

 examples were chosen from coelenterate nerve nets. It is thus evident that 

 coordinative nervous functions evolved very early. Nerve nets, or diffuse ner- 

 vous systems, have been best studied functionally in coelenterates, both polyps 

 and medusae, and in ctenophores. Early histologists^^- ^^^ pictured the nerve 

 net as anastomosing fibers of multipolar and bipolar neurones. Evidence^^- *^^ 

 from vital staining shows that the fibers do not fuse; the junctions are not 

 structurally polarized but the nerve net is probably not a syncytium. 



Conduction in a coelenterate nerve net proceeds freely in all directions; 

 impulses pass around corners and across narrow bridges. Further, a contrac- 

 tile wave dies out with distance of conduction in some cases, and this has 

 been cited as evidence that nerve net conduction is decremental. Actually, 

 the picture of a non-polarized diffuse conducting system is too simple for all 

 the facts. There is condensation of fibers into nerve trunks in sea anemones 

 and in medusae. Conduction in some trunks is fast, also speed of conduction 

 in the subepidermal net in anemones varies greatly in different regions.^^^ 

 Thus definite conduction pathways serve specific functions, even in coelen- 

 terate nervous systems. 



Nerve nets also show facilitation. Repeated impulses are needed to produce 

 threshold excitation (Fig. 227, Ch. 16) in anemones,^^^ and responsiveness 

 is enhanced after a volley in medusae.^* The strength and extent of a con- 

 tractile response depends on the number of impulses delivered to a given re- 

 gion of the nerve net. The decremental appearance of conduction in a nerve 

 net can be explained in terms of decreasing facilitation at junctions, as a 

 wave spreads out from its point of origin. Facilitation may sometimes lead to 

 supernumerary discharges, a sort of after-discharge. 



Two waves of action in a nerve net cancel when they meet, whether in- 

 dicated by blocking of contraction-^^ or of luminescence^^* (see p. 805), 

 probably because of refractoriness. In a jellyfish ring,^"*'' if the absolute re- 

 fractory period of the nerve elements (0.25 sec. at 24.5°, 0.8-0.62 sec. at 

 15°) exceeded the circulation time, the wave stopped. Cancellation of a 

 wave by mechanical pressure raises the possibility that some of the net con- 



