In certain conditions of the nervous system convulsions can be readily excited. As most striking among these, may be named poisoning with the alkaloid of nux vomica (strychnia), and the state of the blood which is produced by cessation of the respiratory function (asphyxia). These toxic conditions bring about a peculiar excitable state of the cells or conducting fibres of the spinal cord, in which impulses pass with unwonted facility from one part to another, and give rise to an excessive degree of action even in response to gentle stimulation. A frog poisoned with strychnia is thrown into general spasm by the least touch, which normally would only cause it to withdraw the limb.

On the other hand, there are many poisons which deaden the reflex powers of the cord centres, among which are opium, chloroform, chloral, digitalin, etc. The condition of the blood (apnaea) which may be brought about by very rapid movements during artificial respiration, has also the effect of lowering the excitability of the spinal nerve cells, and slowing respiration.