The release of transmitter from noradrenergic nerve endings per unit time de￾pends on the rate of impulse flow on the one hand and the release per impulse
on the other hand. Excitatory and inhibitory neurotransmitters and perhaps
circulating agents, which act on receptors in the soma-dendritic part of the
neurone, regulate the frequency of the action potentials travelling down the
axon. The quantum of noradrenaline released by an impulse is far from being
constant. The causes for its variation are only partly understood. However,
one mechanism has been recognized over the last ten years. It is very likely
that several substances naturally occurring in the body are able to increase
or decrease the release per pulse, and that they act on receptors in the terminal
varicose part of the axon (the "noradrenergic nerve terminal"; HAEFELY, 1972)