The natural divisions of pharmacodynamics

These are further divided into depressants and stimulants. Typical of central nervous depressants are the general anaesthetics such as ether and 

Only depressant drugs are known in this class. These drugs are the well-known local anaesthetics, used also in dental and spinal anaesthesia. In general, they preferentially affect the smaller fibres hence sensory nerves are affected more than motor nerves. They elevate the threshold for excitation and thus block propagation of the nervous impulse without depolarizing the fibre. 

For the effect of the ionization of the drug on these events, see Section 10.5. For a review of local anaesthetics, see Ritchie and Greengard (1966). 

Chemical stimulation at motor nerve-endings, on the contrary, has an immense literature to which Section 7.5 may serve as an introduction. See also Table 7.1. 

Other inhibitors depress the synthesis of acetylcholine in the motor nerve terminals, e.g. triethylcholine 7.43) (Bull and Hemsworth, 1965) and the more complex hemicholinium (Schueler, 1956). These agents interfere with choline transport and hence deprive acetyltransferase of its substrate also their action is reversed by choline (cf. Macintosh, Birks and Sastry, 1956). Glycollic acids bearing liposoluble substituents (e.g. p-phenylmande-lic acid) inhibit this enzyme in the brain (Holan, 1965). 

These are customarily divided into depressants and stimulants, according to their ultimate effect. It might be supposed that the depressant effects are achieved with antagonists and the stimulant effects with agonists. This is not necessarily so, for strychnine which acts as an antagonist for inhibitory nerve fibres, is a violent convulsant. Again, morphine, which is an agonist of the enkephalin receptor, is used clinically for its powerful depressant effects. 

The central nervous system depressants can usefully be divided into the following classes  

Central nervous system stimulants are less used in medicine than are the depressants. They are conveniently divided as follows  

The fact that local anaesthetic action can be partially reversed by high external pressure (Kendig and Cohen, 1977) may point to an additional, non structurally specific, contribution to local anaesthetic action (Metcalfe and Burgen, 1968). This would not be surprising for at least one substance in ten, selected at random, has local anaesthetic action, however slight, usually accompanied by an unfavourable therapeutic index. 

---