SUPERNORMALITY, A MODIFICATION OF THE RECOVERY PROCESS IN NERVE

Abstract

In a freshly isolated frog nerve, the relatively refractory period lasts 7-10[sigma] after a response (25-29[degree]C) and is not followed by a period of supernormal irritability. On standing (1/2-1 1/2 hr.), the nerve gradually develops the capacity to exhibit supernormality. Simultaneously the relatively refractory period shortens to 1.5 or 2.5[sigma] and max. supernormal irritability now falls where the relatively refractory period ended in the fresh nerve. Normal height of action potential and normal conduction rate are always regained after the same interval as normal irritability; the height rises no farther, but the conduction rate goes through a supernormal period corresponding to that of irritability. The max. degree of supernormality of irritability is usually between 105 and 110% of the resting value; the supernormality of conduction rate is less than that of irritability. The normal level of irritability, height of response and rate of conduction do not change when the nerve acquires the ability to manifest supernormality. Veratrinization increases the resting level of irritability 10-25%; it also increases and prolongs supernormality. Max. supernormality is attained after practically the same interval following a shock as in untreated nerve. The relatively refractory period is not further shortened greatly. The phenomenon of supernormality therefore depends, not on depression of the normal level of irritability, but on a modification of the recovery process consisting essentially of an increased rate of rise of irritability during the latter part of the rise, with a subsequent fall of irritability; it may or may not be accompanied by a change in the resting level of irritability and in the duration of the rise of irritability.