Nervous Systems 795 



high as 0.1 per cent in the crayfish,^^* and in the cockroach.^^'' Injections 

 of large amounts (up to 40 mg.) into crabs CCarcinus and Panulirus^ cause 

 no change in reflex activity,^** although eserine and acetylcholine facilitate 

 autotomy in crabs.^-^ Synaptic transmission in the sixth abdominal ganglion 

 of the crayfish is unaffected by large amounts of acetylcholine, physostig- 

 mine, and prostigmine.^^'* Acet)']choline in a crayfish perfusion saline does, 

 however, maintain the ganglia in a normal state of excitabiUty for potas- 

 sium.^^^ Transmission across the junctions from giant fibers to motor fibers in 

 the crayfish is not affected by a series of drugs, including ACh, adrenalin, 

 prostigmine, and curare, but is blocked by DFP and eserine; nicotine initially 

 facilitates and then blocks transmission.'"^ Absence of effect of a drug may 

 result from lack of penetration rather than from absence of specific action. 

 In cockroaches the anti-esterase DFP causes alternate facilitation and block- 

 ing of synaptic transmission at concentrations which permit fiber conduc- 

 tion, and in the presence of DFP added ACh blocks the synapses.^*^**' ^^^ In 

 the stellate ganglion of the squid, synaptic transmission is blocked by DFP 

 at about the same concentrations at which fiber conduction is blocked.^^ 



The cholinesterase content of central nervous tissues varies as much as 

 the ACh content (Table 77). Ganglia of squid, honeybee, lobster, and 

 Limidus approach mammalian sympathetic ganglia in QChE. The mam- 

 malian cerebral cortex is surprisingly low in QChE. In lower invertebrates 

 it is difficult to separate nervous from muscular tissue as regards cholinester- 

 ase distribution (see also Chapter 16), but there is little doubt that cholin- 

 esterase is present in all nervous tissue. 



In general, the distribution of ACh and ChE seems not well correlated 

 with central nervous activity; acetylcholine is excitatory to some nerv^e cen- 

 ters and inhibitory to others; the sensitivity is high in sympathetic ganglia 

 and in the spinal cord under special conditions, but sensitivity to ACh is low 

 for cerebral cortex and practically zero for arthropod central nervous systems. 

 Liberation of acetylcholine has been demonstrated at axon terminations in 

 sympathetic ganglia. Evidence regarding the meaning of the high concen- 

 trations of ACh and ChE in invertebrate central gangfia might elucidate the 

 role of acetylcholine in synaptic transmission in general. It is becoming more 

 likely that "the transmitter" is electrical but that acetylcholine is one sub- 

 stance associated with the production of the action potential. ^'^^^^ '^^* 



INORGANIC IONIC CHANGES. Synaptic transmission is profoundly affected by 

 the potassium and calcium in the bathing fluids. In the perfused superior 

 cervical ganglion of the cat, potassium is given off on preganglionic stimu- 

 lation and the potassium content of the ganglion is greatly reduced;^^'* potas- 

 sium also moves outward from stimulated nerve fibers (p. 780). Injection 

 of small amounts of potassium increases the ganglionic response to submaxi- 

 mal stimulation.*^'^ Like acetylcholine, potassium excites in low concentra- 

 tions and inhibits in larger amounts. Addition of potassium to the perfusate 

 causes ACh liberation, and ACh injection causes K liberation. Perfusion 

 with a saline deficient in calcium or containing five times the normal con- 

 centration of potassium increases both the number of active cells and the 

 frequency of discharge in sympathetic ganglia.**- Brain potentials are sensi- 

 tive to potassium/calcium balance. Excess potassium causes fast irregular 

 waves, and calcium in excess elicits large slow waves in the isolated frog 

 brain. ^^"^ 



