Nervous Systems 805 



Inhibition is essential in the antagonistic action of muscles. An afferent 

 volley which stimulates flexor motoneurones normally inhibits the motoneu- 

 rones of antagonistic extensor muscles, and vice versa. Such reciprocal ex- 

 citation probably occurs in many metazoans. The longitudinal and circular 

 muscles of an earthworm are reciprocally innervated. Even in medusae, 

 where the nervous system is a diffuse nerve net, there are reciprocal con- 

 tractions of circular and radial muscles. ^'^ Whether reciprocal inhibition ac- 

 companies these alternate contractions is not known, but it is likely. 



The term inhibition has also been applied to the cancellation of one wave 

 of activity by another. For example a doughnut-shaped ring can be cut out 

 of a jellyfish and a wave of contraction started in both directions from a 

 stimulus at a point; these waves will cancel when they meet.^^^ Two oppos- 

 ing waves of luminescence in sea fans cancel when they meet.^^^ No evi- 

 dence for periods of depressed excitability has come from the use of paired 

 or repetitive stimulation of coelenterates. The cancellation of colHding waves 

 is probably due to refractoriness of conducting pathways. 



After-discharge. Frequently a single afferent volley sets off a repetitive dis- 

 charge which long outlasts the stimulus. Some reflexes are brief responses, 

 as the knee jerk, others persistent, as the scratch reflex. Three explanations 

 of after-discharge may be considered: 



1. After-discharge may result from the persistence or continued liberation 

 of liminal amounts of a chemical mediator, such as acetylcholine. An eser- 

 inized sympathetic ganglion, for example, may give repetitive response to a 

 single preganglionic volley.^^- ^^'^ 



2. After-discharge may result from delay paths and circus conduction. A 

 message may pass around a ring of neurones, returning periodically to excite 

 the same motoneurone over a side branch. Lorente de No has described such 

 pathways in several centers and has demonstrated their action in the oculo- 

 motor nucleus preparation^"^- ^"^ (Fig. 302, C). A gross example of circus 

 conduction is afforded by cutting a doughnut-shaped ring out of a jelly- 

 f^gjj 184, 285 If 3 wave is started in both directions, then blocked on one side 

 by pressure and allowed to continue on the other side, a circusing self-per- 

 petuating wave can be kept going for many hours; the same units are re- 

 excited repeatedly. 



3. A third explanation of after-discharge is based on the property of many 

 centers of showing spontaneous rhythmicity. Often neurones are capable of 

 rhythmic activity and need only a slight stimulus to trip them off. Sympa- 

 thetic ganglion cells can be made to fire repetitively by slight environmental 

 changes, as with acetylcholine, low calcium, etc.*^- There is a fine balance 

 between rest and activity. 



"Spontaneous" Activity of Nerve Centers. Many nerve centers isolated 

 from incoming sensory messages exhibit activity. In general, integrative cen- 

 ters such as the cerebrum, midbrain nuclei, and the central ganglia of anne- 

 lids and arthropods show spontaneity more than do distributing centers such 

 as sympathetic ganglia and sensory nuclei. There is no final clear-cut proof 

 that any mammalian nerve center is spontaneously active, independent of 

 sensory stimulation or reverberating chains, although synchronized rhythms 

 have been recorded from the spinal cord of curarized cats,^''^ and activity is seen 

 in isolated bits of cerebellum (Snider, personal communication). Probably 



