Nervous Systems 807 



lids and arthropods-'*-''- ^^^ (Fig- 304, C). The spontaneous impulses find 

 their way out to body muscles and are tonic in character; incoming messages 

 are superimposed on the spontaneous background (Fig. 304). The spon- 

 taneous activity in the different units originates in the non-axon portion of 

 the neurone, as is shown by the cutting of interganglionic commissures. 



Synchronous Spontaneous Rhythms. In most nerve centers, however, 

 there is some synchrony among the different neurones. A synchronized 

 rhythm may be a coordinated periodic discharge of many units, each unit 

 being active once for each wave, as may hold for "brain waves." Or the 

 rhythm may be slower, each unit discharging several times for each wave, 

 as in the ganglionic pacemakers of arthropod hearts or in respiratory cen- 

 ters. 



The forebrain of fish,i*^ of frogs (Fig. 304, A),-^^ and of mammals,^^ all 

 show rhythmic waves of much longer duration than axon spikes. Just what 

 is the "unit discharge" in a wave is not clear. Probably each cell depolariza- 

 tion lasts much longer than an axon spike but not so long as a full brain 

 wave. Gerard^^"* has suggested that the brain wave represents electrical oscil- 

 lations of soma (non-axon) potentials rather than propagated discharges. The 

 cortical rhythm in several mammals is predominantly lO/sec, with faster 

 and slower components superposed. However, this rhythm can be broken 

 up into short-duration components, can be replaced by irregular asynchron- 

 ous activity, and can change in frequency and pattern as a result of mental 

 activity, sensory stimulation, or the influence of various salts and drugs. 



Electrical activity, as shown by the electroencephalogram (EEG), remains 

 in monkeys after removal of the cortex, but is profoundly altered by lesions to 

 basal ganglia and is abolished if both thalamus and hypothalamus are re- 

 moved. ^^^ 



Synchronized rhythms resembling vertebrate brain waves have been seen 

 occasionally in invertebrates, as in ganglia of the slug, Ariolimax.'''^ The op- 

 tic lobes of the brain of Dytiscus, when the eyes are illuminated, show a 

 rhythm of 20-40 waves per second, whereas in darkness a synchronized 

 rhythm of 7-10 per sec. appears.'^ 



The slow rhythms in which each neurone clearly discharges several times 

 for each wave are best illustrated in respiratory and cardiac pacemakers. Iso- 

 lated nerve cords of some insects show rhythmic discharges corresponding to 

 breathing rate; each axon discharges several times during each respiratory 

 burst. Similar repetitive activity is found in single units of the mammalian 

 respiratory center, as indicated in phrenic nerve axons or muscle units.^"-'^' -"^-^ 

 In the Limulus heart ganglion each unit usually discharges several times 

 per heart beat (Fig. 206, Ch. 15). It is possible to convert the synchron- 

 ous rhythm into continuous asynchronous activity by alterations in salt bal- 

 ance. '^^^ 



Synchronization. The ease with which synchronous waves can be con- 

 verted to asynchronous activity suggests that synchrony is a pattern imposed 

 on a more or less continuous background. Two suggested mechanisms of syn- 

 chronization may be mentioned: 



1. Neurones may be caused to fire together by slow electrical waves, cat- 

 electrotonically. In the Limulus heart ganglion each large pacemaker cell 

 appears to give rise to a slow wave of negativity which tends to coordinate 



