794 Comparative Animal Physiology 



When small amounts of ACh are injected and at the same time a few pre- 

 ganglionic fibers are stimulated, the two stimuli add, producing a response 

 m more ganglion cells.^- There is no doubt that acetylchoHne is liberated 

 from some structure in sympathetic ganglia and that it excites ganglionic 

 neurones, but it does not follow that it is the transmitter. 



In central nervous systems there are suggestions but no proof that acetyl- 

 choline functions in synaptic transmission. The literature is extensive on the 

 effects of ACh in mammalian and amphibian nervous systems^^^ and 

 in invertebrate nervous systems.^^'''- ^'^^ Acetylcholine applied to the cerebral 

 cortex of mammals is excitatory, particularly in high concentrations and 

 after eserine. High amplitude spike-like brain waves are set up, and these 

 may be associated with motor activity.^"- The spinal cord is stimulated to 

 motor discharge when perfused with acetylcholine, particularly after sen- 

 sitization with adrenalin,'^^ and there are numerous instances of potentia- 

 tion of central action of acetylcholine by adrenalin. Acetylcholine potentiates 

 salivary reflexes.^''^ Some spinal reflexes are depressed (e.g., knee jerk), oth- 

 ers are enhanced (e.g., flexors) by eserine.'^^- '^^'^ Eserine is without effect 

 on the potentials associated with synaptic transmissions in the spinal cord, 

 and high concentrations of ACh depress the synaptic potentials. ^-^^ When 

 the lower portion of the spinal cord of a dog is perfused with eserinized 

 Ringer solution, ACh appears on stimulation of the sciatic nerve. ^' When 

 eserine is injected into cats x'\Ch appears in the cerebrospinal fluid, and 

 there is evidence for liberation of ACh when the vagus nerve is stimu- 

 lated.^^ ^ The variable and negative effects of eserine applied to vertebrate 

 nervous systems probably result from the very low permeability of nerve 

 membranes to eserine and the fact, as shown in peripheral nerve, that prac- 

 tically all of the choHnesterase must be inactivated before conduction is af- 

 fected.-^"-'^- ^^ 



Measurements of the acetylcholine content of many nerve centers are not 

 always comparable because of the rapid inactivation of the ester and the 

 uncertainty regarding the manner in which acetylcholine is bound, and be- 

 cause of the variable amounts free or bound. In general (Table 77) there 

 is more ACh in gray than white matter. Regions which are sensitive 

 to hypoxia, such as the cerebrum, contain little ACh.'*-^ The concentra- 

 tion in sympathetic ganglia is higher than in brain, and sensory fibers con- 

 tain much less than do motor fibers.-^'^- '^~'^ The enzvme system which syn- 

 thesizes ACh is very active in cholinergic neurones (such as lower motor and 

 preganglionic sympathetic neurones) and not so active in non-cholinergic 

 nerves (such as sensorv roots and the optic nerve). ^^'^ Among invertebrates 

 the central nervous systems often contain more ACh than mammalian ner- 

 vous systems (Table 77). Crustacean ganglia compare favorably with mam- 

 malian sympathetic ganglia in ACh content, and insect and cephalopod 

 ganglia contain the highest concentrations yet observed. The correlation be- 

 tween ACh content and nervous activity, however, is not good. 



The ganglia of arthropods, molluscs, and annelids are relatively insensi- 

 tive to acetylcholine with or without eserine treatment. In Octopus and 

 Eledone acetylcholine and eserine had no apparent effect on the stellate gan- 

 glion.-" When applied to arthropod central nervous systems acetylcholine is 

 ineffective in causing the discharge of impulses except at concentrations as 



