There are four neurotransmitter substances identified in vertebrates, these are acetyl choline serotonin, adrenalin and nor adrenaline (nor epinephrine). These are collectivley called as neurohumors that means these substances are secreted by the nerve cells and stimulate or inhibit activity of other cells. Adrenaline and nor adrenaline are secreted by the axon terminals of the sympathetic post ganglionic fibres which are therefore said to be adrenergic.