Nervous Systems 



781 



The plasma membrane is lipoprotein, as indicated by much indirect evi- 

 dence. Certain electrochemical properties of the lipoprotein membrane seem 

 to be imparted by a substance that is a quaternary ammonium base, the 

 synthesis of which requires sodium. ^"^ Specific aromatic hydrocarbons and 

 other non-polar molecules depolarize, apparently by adsorbing on receptor 

 areas of less than 8 'A;~ calcium stabilizes against such depolarization."*^^ The 

 insecticides DDT, pyrethrin, and naphthalene have non-polar groups which 

 are very active in reducing membrane stability.^-'' 



Acetylcholine (ACh) is involved in neuro-transmission in one or more 

 unknown ways. It is normally bound, presumably to protein, and may be 

 released in excitation. However, the distribution of acetylcholine is not 

 well correlated with nervous function (see Table 77), and its high con- 

 centration in such organs as spleen and placenta can hardly be connected 



A 



B 



Fig. 295. Types of cholinesterase. Enzyme from, A, hum,an serum, B, squid ganglion, 

 and C, nucleus caudatus (beef), h, Activity of enzyme; pS, negative log molar concen- 

 tration of substrate. Substrates: Ach, acetylcholine; PrCh, propionylcholine; BuCh, 

 butyrylcholine; TA, triacetin. Modified from Augustinson.^ 



with propagation. The lowest concentrations of ACh are in those parts of 

 the nervous system which are most sensitive to asphyxia and several bits of 

 evidence suggest that it may be involved in some oxidative system.^-^ An- 

 other hypothesis is that ACh is important in determining permeability and 

 that liberated ACh makes the membrane permeable to potassium and so- 

 dium. ^"^^ This hypothesis is supported by evidence regarding permeability of 

 nerve and red blood cells. A third hypothesis^'^ is based on the fact that ACh 

 has a powerful electrochemical action at certain oil-saline interfaces and sug- 

 gests that released ACh may cause the local electronegativity. 



